Section2 Electricity (1).pdf

International GCSE Physics (Modular) (4XPH1) Unit code 4WPH1/1P* Specification...

International GCSE Physics (Modular) (4XPH1) Unit code 4WPH1/1P* Specification Section 2: Electricity First examination June 2025 The following sub-topics are covered in this section. (a) Units (b) Mains electricity (c) Energy and voltage in circuits (d) Electric charge (a) Units Learners should: 2.1 use the following units: ampere (A), coulomb (C), joule (J), ohm (Ω), second (s), volt (V) and watt (W) (b) Mains electricity Learners should: 2.2 understand how the use of insulation, double insulation, earthing, fuses and circuit breakers protects the device or user in a range of domestic appliances 2.3 understand why a current in a resistor results in the electrical transfer of energy and an increase in temperature, and how this can be used in a variety of domestic contexts 2.4 know and use the relationship between power, current and voltage: power = current × voltage P = I ×V and apply the relationship to the selection of appropriate fuses 2.5 use the relationship between energy transferred, current, voltage and time: energy transferred = current × voltage × time E = I ×V ×t 2.6 know the difference between mains electricity being alternating current (a.c.) and direct current (d.c.) being supplied by a cell or battery 1 International GCSE Physics (9-1 ) ( Modular ) (c) Energy and voltage in circuits Learners should: 2.7 explain why a series or parallel circuit is more appropriate for particular applications, including domestic lighting 2.8 understand how the current in a series circuit depends on the applied voltage and the number and nature of other components 2.9 describe how current varies with voltage in wires, resistors, metal filament lamps and diodes, and how to investigate this experimentally 2.10 describe the qualitative effect of changing resistance on the current in a circuit (c) Energy and voltage in circuits Learners should: 2.11 describe the qualitative variation of resistance of light-dependent resistors (LDRs) with illumination and thermistors with temperature 2.12 know that lamps and LEDs can be used to indicate the presence of a current in a circuit 2.13 know and use the relationship between voltage, current and resistance: voltage = current × resistance V=I×R 2.14 know that current is the rate of flow of charge 2.15 know and use the relationship between charge, current and time: charge = current × time Q = I ×t 2.16 know that electric current in solid metallic conductors is a flow of negatively charged electrons 2.17 understand why current is conserved at a junction in a circuit 2.18 know that the voltage across two components connected in parallel is the same 2.19 calculate the currents, voltages and resistances of two resistive components connected in a series circuit 2.20 know that: voltage is the energy transferred per unit charge passed the volt is a joule per coulomb 2.21 know and use the relationship between energy transferred, charge and voltage: energy transferred = charge × voltage E = Q×V 2 International GCSE Physics (9-1 ) ( Modular ) (d) Electric charge Learners should: 2.22P identify common materials that are electrical conductors or insulators, including metals and plastics 2.23P practical: investigate how insulating materials can be charged by friction 2.24P explain how positive and negative electrostatic charges are produced on materials by the loss and gain of electrons 2.25P know that there are forces of attraction between unlike charges and forces of repulsion between like charges 2.26P explain electrostatic phenomena in terms of the movement of electrons 2.27P explain the potential dangers of electrostatic charges, e.g. when fuelling aircraft and tankers 2.28P explain some uses of electrostatic charges, e.g. in photocopiers and inkjet printers Section 2: Electricity a) Units 2.1 ampere (A), coulomb (C), joule (J), ohm (Ω), second (s), volt (V), watt (W). The SI unit of the electric current is the ampere ( A ). 1 A = 1 C s-1. The SI unit of the electric charge is the coulomb ( C ). 1 C = 1 A.s. The SI unit of the electrical energy is the joule ( J ). 1 J = 1 C.V= 1 W.s. The SI unit of the electrical resistance is the ohm ( Ω ). 1 Ω = 1 V A-1. The SI unit of the time is the second ( s ). The SI unit of the electric potential or the electrical potential difference or voltage or the electromotive force (e.m.f.) is the volt ( V ). 1 V = 1 J C-1. The SI unit of the electrical power is the watt ( W ). 1 W = 1 J s-1. 3 International GCSE Physics (9-1 ) ( Modular ) b) Mains electricity 2.2 understand how the use of insulation, double insulation, earthing, fuses and circuit breakers protects the device or user in a range of domestic appliances Electricity can cause hazards in domestic situations. For example: Hazard Danger frayed cables wiring can become exposed long trailing cables these might cause a trip or a fall damaged plugs wiring can become exposed Water around sockets water conducts, so can connect a person into the mains supply Pushing metal objects into sockets this connects the holder to the mains supply and is likely to be lethal If there is a fault in an electrical appliance, it could take too much electrical current. This might make the appliance itself dangerous, or it could cause the flex between the appliance and the wall to become too hot and start a fire. Physiological effects of an electric current passing through the body Current (approximate) Effect 1 mA Threshold – no pain below this point 5 mA A frightening but harmless shock 10 -20 mA Uncontrolled muscular contractions Loss of muscle control - you cannot let go 50 mA Pain and exhaustion; breathing affected 100 - 300 mA Uncoordinated contraction of the heart leading to death The actual effects experienced vary from one individual to another. The size of the current which flows depends on the voltage and the electrical resistance of your body. Most of the resistance of the human body lies in the skin. Dry skin has a resistance of 100 000 ohms or more. But if the skin is wet, especially with sweat which contains salts or water of good conducting ability, the resistance falls drastically to, perhaps, just a few hundred ohms. Since water ( not distilled water ) can conduct electric current, an electrical appliance should be never operated with wet hands. Switches, plugs, sockets, connecting wires, etc. should always be kept in a dry condition. 4 International GCSE Physics (9-1 ) ( Modular )  The laws for the safe use of electricity are constantly being improved by governments, and electricians learn to work to the latest standards. The most important aids to the safe use of electricity are insulation and fuses or circuit breakers. Insulation is the material that prevents flow of electricity through it. Insulation of these days is usually a plastic such as PVC, which is used to cover the copper wires. This prevents them from touching each other, and also prevents the operator from touching them. In parts of objects where the temperature goes above 100°C, other plastics, glass or ceramic are used. Some household appliances use two-pin plugs instead of three-pin plugs. For such appliances, there is no earth wire. These appliances use double insulation to protect users from electric shocks. Double insulation is a safety feature that can replace the earth wire. Appliances that have double insulation usually use a two-pin plug, as only the live and neutral wires are required. Double insulation provides two levels of insulation: 1. The electric cables are insulated from the internal components of the appliances. 2. The internal components are insulated from the external casing which is made up of non-metallic materials such as plastic. There are usually three wires in a household circuit namely the live ( L ) wire , the neutral ( N ) wire and the earth ( E ) wire. The live wire ( brown ) is connected to a high voltage and delivers current to the appliance. This is the wire to which circuit breakers, fuses and switches are fitted. The neutral wire ( blue ) complete the circuit by providing a return path to the supply for the current. It is usually at zero volt. The earth wire ( green and yellow stripe ) is a low resistance wire. It is usually connected to the metal casing of the appliance. Connection of earth wire and metal casing of an appliance is known as earthing. Metal-cased appliances, such as washing machines or electric cookers, must have an earth wire as well as a fuse. The earth wire and fuse work together to make sure that the metal outer casing of this appliance can never become live and electrocute someone. 5 International GCSE Physics (9-1 ) ( Modular ) Without earthing: An electrical fault results in the live wire touching the metal casing. The metal casing is at high voltage due to the electrical fault. If a person touches the metal casing, a large current flows through the person  the person gets an electric shock. With earthing: An electrical fault results in the live wire touching the metal casing. The large current flows to the ground through the earth wire, which has a much lower resistance than the person. Hence, the person does not suffer an electric shock. The flow of the large current from the live wire, through the metal casing, to the earth wire creates a short circuit. The sudden surge in current exceeds the rated value of the fuse, so the fuse is blown and the circuit is opened. The electricity supply to the appliance is cut off. That makes save to the user. The electric current usually has to pass through a fuse or circuit breaker before it reaches the appliance. There are several ways to make appliances safer to use, and protect the user if a fault should develop. In some countries earth wire the fuse is fitted into the plug of the appliance. The wiring of Fuse a 3-pin plug are given in figure. It is important to check that the live wire three wires in the cable are connected to the correct terminals. neutral wire Follow the colour code as described below: live wire: brown neutral wire: blue earth wire: yellow and green stripes The fuse fits between the live brown wire and the pin. The brown live wire and the blue neutral wire carry the current. The green and yellow striped earth wire is needed to make metal appliances safer. A fuse is a safety device which is made out of a piece of tin-coated copper wire. The fuse does two jobs. It protects the wiring if something goes wrong, and it can also protect us. If there is a sudden surge in the current, the wire in the fuse will heat up and melt- it 'blows'. This breaks the circuit and stops any further current flowing. The thickness ( or strictly speaking, gauge ) varies in different fuses depending on the amount of current which is permitted to flow through them. Rating of the fuse means the current that can blow the fuse, for instance 5A fuse means the fuse will be blown out if 5A current passed through it. So the value of selected fuse for a particular appliance must be greater than maximum operating current but as close as to that maximum value. 6 International GCSE Physics (9-1 ) ( Modular )  If a circuit breaker is used, then the circuit breaker springs open ( trips ) a switch if there is an increase in current in the circuit. This can be happened because increase in current provide stronger magnetic field and this magnetic force pulls the armature and cut off the circuit. This can be reset easily after the fault in the circuit has been corrected. The advantages of using circuit breaker over the use of fuse are: ( i) fast response and ( ii ) easy to reset. Circuit symbol for fuse or circuit breaker is. Some domestic appliances and their power rating a) a vacuum cleaner  360 W b) a television  0.8 kW ( where: 1 kW = 1000 W ) c) a table lamp  100 W d) a kettle  2100 W e) an iron  900 W 2.3 understand why a current in a resistor results in the electrical transfer of energy and an increase in temperature, and how this can be used in a variety of domestic contexts Current is a flow of electric charge. The charges that can flow are mainly electrons. When current flows, the electrons collide with the atoms along the conducting path and transfer their energy to the atoms. So atoms gain kinetic energy and as a result temperature increases. Many household appliances consist of an electrically heated resistor. A resistor is a device that opposes 𝑽𝑽 the flow of current. [𝑹𝑹 = ] As it does so, heat energy is produced. Examples of appliances that have 𝑰𝑰 such a device are electric kettles, electric fires, light bulbs, domestic irons and electric ovens. Even the washing machine, the dishwasher, the tumble drier and the hairdryer consist of an electric heater with an electric motor added. If you touch the electric flex ( the wire that comes out of the device and has the plug on the end of it ) to any of these electric heaters when it is switched on, you will notice that the flex is either at room 7 International GCSE Physics (9-1 ) ( Modular ) temperature or, perhaps, it will be slightly warm to the touch. The electric current is increasing the temperature of the heater, by giving it energy, but it is not having the same effect on the flex. The reason is that the heater has a higher resistance, to make it difficult for the charges to flow through it. The flex contains copper wires to feed the electricity to and from the heater. Copper has a very low resistance - only silver is lower, and silver is seldom used for the obvious reason that it is expensive! So when an electric current passes through a heater ( which is a resistor ), electrical energy that was originally generated in a power station, is converted into internal energy ( potential and kinetic energy of material ) in the heater, and its temperature increases. Note that if two heaters are each connected to the mains power, the one with the lower resistance will allow more current to flow, and will become hotter than the other one. This is why it is so important that the live wire Energy conversion in a heater. and the neutral wire in the flex do not touch. If they do touch, they make a very low resistance circuit, and the flex may burst into flames if the correct fuse is not fitted to break the circuit. 2.4 know and use the relationship between power, current and voltage; and apply the relationship to the selection of appropriate fuses The voltage with which the power station tries to drive electricity through the household appliances is measured in volts ( V ). The voltage is actually potential difference between two terminals and equivalent to 𝑾𝑾 work done in carrying a unit positive charge from one point to another against the electric force. [ 𝑽𝑽 = ] 𝒒𝒒 The voltage between the live socket and the neutral socket on the wall varies from region to region. The most common options are 230 V and 110 V, though there are many other standards depending on what country you are in such as 200 V, 127 V and 100 V. Some equipment can adapt to run on any voltage, but some will be destroyed if it is connected to the wrong voltage, especially if it is too high. All electrical equipment has a power rating, which indicates how many joules of energy are required 𝑾𝑾 to supply each second ( i.e. rate of energy dissipation or generated ). [ 𝑷𝑷 = = 𝑽𝑽𝑽𝑽 ] The unit of power used 𝒕𝒕 is the watt (W). Light bulbs often have power ratings of 60 W or 1 00 W. Electric kettles have ratings of about 2 kilowatts ( 2 kW = 2000 W ). A 2 kW kettle is dissipated 2000 J of energy in each second. The power of a piece of electrical equipment depends on the voltage and the current. The current is a flow of electric charge and it is defined as amount of charge passing a cross sectional area in one 𝒒𝒒 second. Unit of current is the ampere. [ 𝑰𝑰 = ] It is a scalar quantity. 𝒕𝒕 8 International GCSE Physics (9-1 ) ( Modular )  Electron Conventional current Even though current is a scalar quantity normally direction current ⊖ of current is conventionally defined as direction of flow of positive charges. ⊖ In the components or appliances conventional current pass through higher electric potential to lower electric potential. On the other hand current generated by a source of e.m.f ( such battery ) current flows from positive terminal ( higher potential ) to negative terminal ( lower potential ) via the external circuit. The units watt, volt and ampere, are defined as follows. 𝟏𝟏 𝑾𝑾 = 𝟏𝟏 𝑱𝑱 𝒔𝒔−𝟏𝟏 𝟏𝟏 𝑽𝑽 = 𝟏𝟏 𝑱𝑱 𝑪𝑪−𝟏𝟏 𝟏𝟏 𝑨𝑨 = 𝟏𝟏 𝑪𝑪 𝒔𝒔−𝟏𝟏 From these definitions, we can see that 𝟏𝟏 𝑾𝑾 = 𝟏𝟏 𝑽𝑽 × 𝟏𝟏 𝑨𝑨 In other words, 𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑 = 𝒗𝒗𝒗𝒗𝒗𝒗𝒗𝒗𝒗𝒗𝒗𝒗𝒗𝒗 × 𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄 or 𝑷𝑷 = 𝑽𝑽 × 𝑰𝑰 where 𝑷𝑷 = power in watts ( W ), 𝑰𝑰 = current in amperes ( A ) and 𝑽𝑽 = voltage in volts ( V ). WORKED EXAMPLES 1. What is the power of an appliance if a current of 7 A is obtained from a 230V supply? 𝑷𝑷 = 𝑽𝑽 × 𝑰𝑰 = 𝟐𝟐𝟐𝟐𝟐𝟐 𝑽𝑽 × 𝟕𝟕 𝑨𝑨 = 𝟏𝟏𝟏𝟏𝟏𝟏𝟏𝟏 𝑾𝑾. 2. An electric oven has a power rating of 2 kW. What current will flow when the oven is used with a 230V supply? 𝑷𝑷 𝟐𝟐 𝟎𝟎𝟎𝟎𝟎𝟎 𝑾𝑾 𝑰𝑰 = = = 𝟖𝟖. 𝟕𝟕 𝑨𝑨. 𝑽𝑽 𝟐𝟐𝟐𝟐𝟐𝟐 𝑽𝑽 3. What fuse should be fitted in the plug of a 2.2 kW electric kettle used with a supply voltage of 230 V? ( Let 3 A, 5 A and 1 3 A fuses are available.) 𝑷𝑷 𝟐𝟐 𝟐𝟐𝟐𝟐𝟐𝟐 𝑾𝑾 𝑰𝑰 = = = 𝟗𝟗. 𝟔𝟔 𝑨𝑨. 𝑽𝑽 𝟐𝟐𝟐𝟐𝟐𝟐 𝑽𝑽 Since the fuse with the smallest rating bigger than the normal current is required to use, the fuse must be 13 A. 9 International GCSE Physics (9-1 ) ( Modular ) 4. What fuse should be fitted to the plug of a reading lamp which has a 60 W lamp and a supply of 230 V? ( Let 3 A, 5 A and 1 3 A fuses are available.) 𝑷𝑷 𝟔𝟔𝟔𝟔 𝑾𝑾 The required the normal current is : 𝑰𝑰 = = = 𝟎𝟎. 𝟐𝟐𝟐𝟐 𝑨𝑨. 𝑽𝑽 𝟐𝟐𝟐𝟐𝟐𝟐 𝑽𝑽 Since the fuse with the smallest rating bigger than the normal current is required to use, the fuse must be 3 A. 2.5 use the relationship between energy transferred, current, voltage and time: Since power is defined ass rate of energy transferred, energy transferred due to constant power can be calculated as 𝑬𝑬𝑬𝑬𝑬𝑬𝑬𝑬𝑬𝑬𝑬𝑬 = 𝑷𝑷𝑷𝑷𝑷𝑷𝑷𝑷𝑷𝑷 × 𝒕𝒕𝒕𝒕𝒕𝒕𝒕𝒕 i.e. 𝑬𝑬 = 𝑷𝑷 𝒕𝒕 = 𝑽𝑽 𝑰𝑰 𝒕𝒕 where 𝑬𝑬 = the energy transferred in joules (J), 𝑰𝑰 = current in amperes (A), 𝑽𝑽 = voltage in volts (V) and 𝒕𝒕 = time in seconds (s) 𝑽𝑽𝟐𝟐 Since 𝑽𝑽 = 𝑰𝑰 𝑹𝑹 , then 𝑬𝑬 = 𝑷𝑷 𝒕𝒕 = 𝑽𝑽 𝑰𝑰 𝒕𝒕 = 𝒕𝒕 = 𝑰𝑰𝟐𝟐 𝑹𝑹 𝒕𝒕 [where, 𝑹𝑹 = resistance of electrical appliance.] 𝑹𝑹 2.6 understand the difference between mains electricity being alternating current ( a.c. ) and direct current ( d.c. ) being supplied by a cell or battery. A battery produces a steady current. The electrons are constantly flowing from the negative terminal of the battery round the circuit and back to the positive terminal. This produces a direct current ( d.c. ). The sign of the voltage stays constant. So direct current ( d.c. ) is defined as unidirectional current i.e. direction of direct current does not change with time. I I I I pure d.c. pulsating d.c. t t t t fluctuating d.c. fluctuating d.c. The mains electricity used in the home is quite different. The electrons in the circuit move backwards and forwards. This kind of current is called alternating current (a.c.). So, alternating current ( a.c. ) is bidirectional current and direction of current is changing backwards and forwards as time passes. Mains electricity moves forwards and backwards 50 times each second. It has a frequency of 50 hertz ( Hz ). The advantage of using an a.c. source of electricity rather than a d.c. source is that it can be transmitted from power stations to the home at very high voltages, which reduces the amount of energy that 10 International GCSE Physics (9-1 ) ( Modular ) is lost in the overhead cables. The levels of the a.c voltages can be changed by using suitable transformers. Moreover it can be generated more easily than d.c. in large scale. Different countries have different standards for mains supply, reflecting the historical development of the networks across the world. For example, the UK, Sri Lanka, India and Australia use a 230 V supply, China uses 220 V and the USA uses 110 V. I or V I or V I or V t t t The voltage alternates between positive and negative. Where on the voltage axis ( if we refer to the UK value ) should the '230' go? The mean value of the voltage is 0 ( equally spread in positive and negative values ). If we label the highest value ( called the 'peak' value ), that doesn't seem very representative of the voltage, since the supply would only be at that value for very short times. A value called the root mean square ( RMS ) value is calculated in three stages. Firstly, all values of voltage are squared- this makes all the values on the graph positive. Secondly, now that all the values are positive, a mean is calculated- this will not be zero. Finally, since this mean value is in (volts)2, the square root is taken to give a value in volts. It is this final value, the RMS value, that is quoted as 230 V. This root mean square value stands for constant voltage that can produce same heating effect as the a.c. voltage. That is an a.c. supply of 230 V RMS would provide the same heating effect as a 230 V DC battery. So, it becomes an advantage using RMS of a.c. can easily compare with d.c. 𝑽𝑽𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑 𝑽𝑽𝑹𝑹𝑹𝑹𝑹𝑹 = = 𝟎𝟎. 𝟕𝟕𝟕𝟕𝟕𝟕 𝑽𝑽𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑 √𝟐𝟐 𝑽𝑽𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑 c) Energy and voltage in circuits 2.7 explain why a series or parallel circuit is more appropriate for particular applications, including domestic lighting CIRCUIT DIAGRAMS A simple circuit consists of supply ( such as cell , battery, etc. ), components also called load ( such as lamp, motor etc. ) , connecting wires and switch. 11 International GCSE Physics (9-1 ) ( Modular )  There are two different ways of connecting two lamps ( or other components such as LEDs ) to the same battery ( or other power source). Two very different kinds of circuit can be made. These circuits are called series and parallel circuits. The components are connected in such way that exactly the same electric current flows through each of the components in the circuit is called series circuit. The, voltage is shared between the units in the circuit. i.e. same current, 𝑉𝑉𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 = 𝑉𝑉1 + 𝑉𝑉2 + 𝑉𝑉3 + …... The components are connected in such way that exactly the same electric potential difference appears across each of the components in the circuit is called parallel circuit. The current splits, with part of it going through each component. i.e. same voltage, 𝐼𝐼𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 = 𝐼𝐼1 + 𝐼𝐼2 + 𝐼𝐼3 + …... The two great advantages of the parallel arrangement are that each appliance can be designed to work with the same voltage supply, and that the appliances can be switched on and off individually. So all of the appliances in a house are connected in parallel to the mains supply and each one sees the full 11 0 V or 230 V of the mains supply when it is switched on. Series Parallel Circuit diagram Appearance of Both lamps have the same brightness, both Both lamps have the same brightness, both lamps are dim. lamps are bright. the lamps The battery is having a hard time pushing the The battery pushes the charge along two Battery same charge first through one bulb, then alternative paths. This means more charge another. This means less charge flows each can flow around the circuit each second, so second, so there is a low current and energy is energy is transferred quickly from the transferred slowly from the battery. battery. The lamps can be switched on and off Switches independently by putting switches in the The lamps cannot be switched on and off independently. parallel branches. Advantages/ A very simple circuit to make. The battery The battery will not last as long. will last longer. If one lamp 'blows' then the If one lamp 'blows' the other one will keep disadvantages circuit is broken so the other one goes out too. working. 12 International GCSE Physics (9-1 ) ( Modular ) Current in a series circuit The current in a circuit can be measured using an ammeter. The ammeter must be connected in series with the component through which amount of current passes is wanted to know. In a series circuit, the current is the same no matter where the ammeter is put. Resistance of ammeter must be usually much smaller than resistance in the circuit. For ideal case resistance of ammeter can be assumed to be zero. The voltage across a component can be measured using a voltmeter. The voltmeter must be connected in parallel with the component across which amount of potential difference is wanted to know. Resistance of voltmeter must be usually much larger than resistance in the circuit parallel to it. For ideal case resistance of voltmeter can be assumed to be infinite 2.8 understand how the current in a series circuit depends on the applied voltage and the number and nature of other components For a series circuit, the current in the circuit depends on the applied voltage and on the number and the nature of the components in the circuit. In a series circuit amount of current passes through the circuit is directly proportional to the supply voltage and inversely proportional to the total effective resistance along the circuit. Resistance: Resistance is a property of a given conductor limits the current flow. The resistance ( R ) of a conductor is defined as the ratio of potential difference across its ends ( V ) to the current passing through it ( I ). 𝑽𝑽 𝑹𝑹 = , its SI unit is the ohm, ( Ω ), 1 Ω = 1 V A-1. 𝑰𝑰 Note : The origin of resistance arises from imperfections in the crystal lattice which causes frequent scattering of the drifting electrons under an electric field. The scattering of electrons are due to: Lattice ions located some distance from its equilibrium position because of its thermal vibrational motion, and the presence of impurity atoms.  In metal, the numerical density of the charge carriers is not effected by temperature change, however the amplitude of the lattice ions vibration increases with temperature. Thus resistance of metal increases with increase in temperature.  In semiconductor, the numerical density of the charge carriers increases with increase in temperature, that may cause lower resistance. However the amplitude of the lattice ions vibration increases with temperature that may cause higher resistance. The first overcomes the second effect, thus resistance of semiconductor decreases with increase in temperature. 13 International GCSE Physics (9-1 ) ( Modular ) The resistance ( R ) of a conductor at a given temperature depends on : ( i ) its length , 𝒍𝒍 ( ii ) its cross-sectional area, A and (iii) the type of material. 𝒍𝒍 𝑹𝑹 = 𝝆𝝆 𝑨𝑨 where: ρ is the resistivity of that conductor and is a property of the material. The SI unit of resistivity is Ω m and it is defined as the resistance of that conductor having one unit length and unit cross-sectional area. Ohm’s Law : The current flowing in a metallic conductor ‘ I ’ is directly proportional to the potential difference ‘ V ’ applied across it, provided that the physical situations ( such as temperature, stress, etc. ) are constant. i.e. 𝐈𝐈 ∝ 𝐕𝐕 ( at constant physical situations ) 𝟏𝟏 𝐈𝐈 = 𝐕𝐕 ( where R is the resistance of the conductor and is constant under steady 𝐑𝐑 physical conditions. ) If a material is obeyed Ohm’s law the nature of V vs. I graph or I vs. V graph is straight line passes through origin. V I ∆𝑽𝑽 ∆𝑰𝑰 𝟏𝟏 Slope = = 𝑹𝑹 Slope = = ∆𝑰𝑰 ∆𝑽𝑽 𝑹𝑹 I V 2.9 describe how current varies with voltage in wires, resistors, metal filament lamps and diodes, and how this can be investigated experimentally Determination of relation between voltage ( potential difference ) and current through a resistor ( component ) To measure how current varies with voltage, the associated circuit can be used. The component is placed in a circuit with an ammeter to measure the current through the component, and with a voltmeter to measure the voltage across it. The battery is used to power the circuit and variable R resistor is used to adjust the voltage across the component ( resistor ) and current passing through it. To take readings, the circuit is switched on, and readings are made of the voltage and the current. Then a graph of voltage against current can be plotted. 14 International GCSE Physics (9-1 ) ( Modular )  Note that the readings may change a little over the first few seconds. If so, this is probably because the component is heating up and its resistance is changing. If this happens, it is required to decide whether to take how current varies with voltage the readings before the component has heated up, and so measure the resistance at room temperature, or to wait until the readings have stopped changing. By that way, the steady- state resistance ( the resistance of the component under normal operating conditions, once it has warmed up to room temperature ) with the component at its usual running temperature can be obtained. 2.10 describe the qualitative effect of changing resistance on the current in a circuit The associated voltage vs. current graph is plotted for a component where the resistance remains constant, as shown by the constant gradient of the voltage vs. current graph. This would be the graph for an 'ohmic' resistor, such as carbon. For such a resistor, Ohm's law applies and the voltage is directly proportional to the current - a straight line is obtained. However, the resistance of most conductors becomes higher if the temperature of the conductor increases. As the temperature rises, the particles in the conductor vibrate more and provide greater resistance to the flow of electrons. For example, the resistance of a filament lamp becomes greater as the voltage is increased and the lamp gets hotter. Ohm's law is not obeyed because the heating of the lamp changes its resistance. The associated figure shows the voltage / current graph for such a component. It can be seen that the gradient of the graph increases with increasing voltage. That means resistance increases as voltage increases. It does not matter which way the current passes through a lamp, but for some components or device, say a pocket calculator , could be destroyed if the battery is not inserted correctly. One way to prevent this is to add a diode to the circuit. A diode is a circuit component that only allows current to pass through it in one direction. The associated figures in the next page show the a device is protected by a diode. The arrow on the diode shows the way that conventional current can flow. When the battery is inserted the wrong way round, as shown in the right -hand part of figure, no current can flow. 15 International GCSE Physics (9-1 ) ( Modular )  device device EFFECT ON CURRENT OF CHANGING RESISTANCE As mention former, using more voltage across a component increases the current in it. The amount of current in a circuit can also be controlled by changing the resistance of the circuit using a variable resistor or rheostat. Adjustment of the rheostat changes the length of the wire the current passes through. Since the wire resists the flow of current, changing the length of the wire alters the resistance in the circuit..Variable resistors are often used, for example, to change the brightness of the lighting in a car or volume control of radio. 2.11 describe the qualitative variation of resistance of light dependent resistors ( LDRs ) with illumination and of thermistors with temperature As mention in 2.8 some substances, increasing the temperature actually lowers the resistance. This is the case with semiconductors such as silicon. A semiconductor material is a material that does not conduct electricity as well as, for example, a metal, but conducts electricity better than an insulator, such as plastic. Silicon has very few free electrons ( free electrons are necessary for an electric current to flow through a material ) and so behaves more like an insulator than a conductor. However, if silicon is heated, more electrons are removed from the outer electron shells of the atoms, which produces an increased electron cloud. The released electrons can move throughout the structure, allowing an electric current to pass more easily. This effect is large enough to outweigh the increase in resistance that might be expected from the increased vibration of the silicon ions in the structure as the temperature increases. ( already explained in page 19 ). 16 International GCSE Physics (9-1 ) ( Modular )  Semiconducting silicon is used to make thermistors, which are used as temperature sensors, and light- dependent resistors ( LDRs ), which are used as light sensors. Thermistor is made up of semiconductor and its electrical resistance 𝑹𝑹𝒕𝒕𝒕𝒕 decreases as temperature increases. Its circuit symbol and variation of resistance with temperature are shown in figure. temperature LDR is made up of semiconductor and its electrical resistance decreases as intensity of light increases. This can be happened light energy that removes electrons from the silicon atoms, increasing the 𝑹𝑹𝑳𝑳𝑳𝑳𝑳𝑳 electron cloud. So LDRs have a very high resistance in the dark, and a very low resistance in the light. LDRs are used in street lamps that switch on automatically at night, and in the type of burglar alarm that sets a light beam ( usually an infra-red beam so Light intensity that it is invisible ) across the path of the burglar. Its circuit symbol and variation of resistance with light intensity are shown as in figure. 2.12 know that lamps and LEDs can be used to indicate the presence of a current in a circuit Lamps ( incandescent lamps ) are constructed based on heating effect of current when current passes through resistance wire ( especially tungsten filament ). Circuit symbol of a lamp is shown in figure. LEDs ( Light Emitting Diodes ) are made up of semiconductors and they become glow when current passing through them. Since they are one of the types of diodes they allow current only in one direction. Circuit symbol of an LED is shown in figure. Both lamps and LEDs can be used to indicate the presence of current in the circuit. 2.13 know and use the relationship between voltage, current and resistance: The relationship between voltage, current and resistance in electrical circuits is given by the following equation. 𝒗𝒗𝒗𝒗𝒗𝒗𝒗𝒗𝒗𝒗𝒗𝒗𝒗𝒗 = 𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄 × 𝒓𝒓𝒓𝒓𝒓𝒓𝒓𝒓𝒓𝒓𝒓𝒓𝒓𝒓𝒓𝒓𝒓𝒓𝒓𝒓 𝑽𝑽 = 𝑰𝑰 × 𝑹𝑹 where V is the voltage in volts ( V ), I is the current in amps ( A ) and R is the resistance in ohms ( Ω ). This equation is often called Ohm’s law, it is also the definition of the resistance of an object as mentioned in section 2.8. 17 International GCSE Physics (9-1 ) ( Modular ) 2.14 understand that current is the rate of flow of charge Electric current: Electric current is a flow of electric charge. Electric current I is defined as the rate of flow of electric charge Q through a given cross-sectional area of a conductor. Q N e For steady current: I = = t t Where: N is the total number of electrons, e is the charge of electron or proton. Current is a scalar quantity. Its SI unit is the ampere ,( A ). [ 1 A = 1 C s-1 ] Effects of electric current: ( i ) Heating effect ; ( ii ) Chemical effect ; ( iii )Magnetic effect. 2.15 know and use the relationship between charge, current and time: The unit of electric charge: the coulomb ( C ). The coulomb is the quantity of electric charge that passes through a given cross- section in a current when a steady current of one ampere flows for one second. Q = I t, and 1 C = 1 A s. 2.16 know that electric current in solid metallic conductors is a flow of negatively charged electrons Conductor is a material which is composed of plenty of free electrons. The free electrons are loosely bounded by the respective nucleus and as a result it can move freely through the conducting material depending upon the external applied electric field ( potential difference across the material ). Since current is flow of electric charge, electric current in solid metallic conductors is a flow of negatively charged electrons. So electrons will flow from part of lower potential to the part of higher potential. However as mention as in section 2.4, direction of conventional current is opposite to the direction of flow of negatively charged electrons. That means conventional current must flow from higher potential to lower potential through the conducting material. 2.17 understand why current is conserved at a junction in a circuit At a junction in a circuit, conservation of electric charge requires charges flow into the junction must be the same as charges flow out from that junction. Since current is flow of electric charge, to obey the law of conservation of electric charge the currents flow into the junction must be the same as the currents flow out from that junction. That means the algebraic sum of the currents at a junction must be zero. i.e. ∑ 𝑰𝑰 = 𝟎𝟎. That is why current is conserved at a junction in a circuit. 18 International GCSE Physics (9-1 ) ( Modular ) 2.18 know that the voltage across two components connected in parallel is the same If components are connected in parallel all components have same common junction on each end. That means their ends are connected to two separate junction points. Since the potential of the junctions are not the same there will be potential difference so all the components connected in parallel have same potential difference. 𝑰𝑰𝟏𝟏 𝑹𝑹𝟏𝟏 𝑰𝑰 𝑰𝑰𝟐𝟐 𝑹𝑹𝟐𝟐 𝑰𝑰𝟑𝟑 𝑹𝑹𝟑𝟑 + 𝑽𝑽 - 𝑰𝑰 = 𝑰𝑰𝟏𝟏 + 𝑰𝑰𝟐𝟐 + 𝑰𝑰𝟑𝟑 , 𝑽𝑽 = 𝑰𝑰𝟏𝟏 𝑹𝑹𝟏𝟏 = 𝑰𝑰𝟐𝟐 𝑹𝑹𝟐𝟐 = 𝑰𝑰𝟑𝟑 𝑹𝑹𝟑𝟑 In parallel combination  Same potential difference.  𝑰𝑰 = 𝑰𝑰𝟏𝟏 + 𝑰𝑰𝟐𝟐 + 𝑰𝑰𝟑𝟑 ( obeys conservation of electric charge ) 𝟏𝟏 𝟏𝟏 𝟏𝟏 𝟏𝟏  = + + , 𝑅𝑅𝑝𝑝 < 𝑅𝑅𝑚𝑚𝑚𝑚𝑚𝑚 𝑹𝑹𝒑𝒑 𝑹𝑹𝟏𝟏 𝑹𝑹𝟐𝟐 𝑹𝑹𝟑𝟑 𝑅𝑅1 × 𝑅𝑅2 𝑅𝑅𝑝𝑝 = = product by sum , only for two resistors. 𝑅𝑅1 + 𝑅𝑅2 2.19 calculate the currents, voltages and resistances of two resistive components connected in a series circuit 𝑹𝑹𝟏𝟏 𝑹𝑹𝟐𝟐 𝑰𝑰 𝑹𝑹𝟑𝟑 𝑰𝑰 𝑰𝑰 𝑰𝑰 𝑽𝑽𝟏𝟏 𝑽𝑽𝟐𝟐 𝑽𝑽𝟑𝟑 + 𝑽𝑽 - Since potential difference is defined as work done ( required energy ) to carry a unit positive charge from one point to another against the electric force, potential difference across the series combination of resistances is equal to the sum of potential difference across each resistance. 𝑽𝑽 = 𝑰𝑰 ( 𝑹𝑹𝟏𝟏 + 𝑹𝑹𝟐𝟐 + 𝑹𝑹𝟑𝟑 ) and 𝑽𝑽 = 𝑽𝑽𝟏𝟏 + 𝑽𝑽𝟐𝟐 + 𝑽𝑽𝟑𝟑 In series combination  Same current  𝑽𝑽 = 𝑽𝑽𝟏𝟏 + 𝑽𝑽𝟐𝟐 + 𝑽𝑽𝟑𝟑 ( obeys conservation of energy )  𝑹𝑹𝒔𝒔 = 𝑹𝑹𝟏𝟏 + 𝑹𝑹𝟐𝟐 + 𝑹𝑹𝟑𝟑 , 𝑅𝑅𝑠𝑠 > 𝑅𝑅𝑚𝑚𝑚𝑚𝑚𝑚 19 International GCSE Physics (9-1 ) ( Modular ) 2.20 understand that: Voltage and its unit Voltage across a component is the energy transferred per unit charge to pass through that component. If amount of energy 𝑬𝑬 is required to transferred to charge 𝑸𝑸 to pass through a component then potential difference 𝑽𝑽 across that component can be written as, 𝑬𝑬 𝑽𝑽 =. 𝑸𝑸 SI unit is: the volt and the volt is equivalent to joule per coulomb. 𝑽𝑽 = 𝑱𝑱 𝑪𝑪−𝟏𝟏. 2.21 know and use the relationship between energy transferred, charge and voltage: energy transferred = charge × voltage E = Q × V 𝑽𝑽𝟐𝟐 Since 𝑄𝑄 = 𝐼𝐼 𝑡𝑡, 𝑎𝑎𝑎𝑎𝑎𝑎 𝑉𝑉 = 𝐼𝐼 𝑅𝑅, 𝑬𝑬 = 𝑸𝑸 𝑽𝑽 = 𝑽𝑽 𝑰𝑰 𝒕𝒕 = 𝑰𝑰𝟐𝟐 𝑹𝑹 𝒕𝒕 = 𝒕𝒕 𝑹𝑹 d) Electric charge 2.22P identify common materials which are electrical conductors or insulators, including metals and plastics CONDUCTORS AND INSULATORS ( Electrical ) Conductor: Material having plenty of free electrons is known as electrical conductor. (or ) The material having low electrical resistance is called electrical conductor. e.g. metals such as copper, silver, gold, aluminium and graphite ( nonmetal). ( Electrical ) Insulator : Material having no or few free electrons is known as electrical insulator. ( or ) The material having high electrical resistance is called electrical insulator. e.g. most plastic such as polythene, perspex, etc. , rubber, acetate, glass, teflon and dry air e.t.c.. 2.23P practical: investigate how insulating materials can be charged by friction Atomic or molecular size : ∼ 10-9 m , ∼ 10 -10 m ; Nucleonic size ∼ 10-15 m. Atoms are composed of nucleus and electrons. Nucleus are made up of nucleons ( protons and neutrons ). For neutral atoms number of electrons in the orbit is equal to the number protons in the nucleus. Particle Charge Mass Electron - 1.6 x 10-19 C 9.11 x 10-31 kg Proton + 1.6 x 10-19 C 1.673 x 10-27 kg Neutron Nil 1.675 x 10-27 kg mp ≈ mn ≈ 1840 me Note: Mass of electron is small enough to compare that of proton, transfer of charges can occur mainly by transfer of electrons. 20 International GCSE Physics (9-1 ) ( Modular ) Charged body: A body in which the electrical balance is upset is called charged body. i.e the condition of excess of electrons ( negative charges ) or deficiency of electrons ( positive charges ). Positively charged body means it is under deficiency of electrons. e.g. Perspex ( rubbed with wool ), in this case Perspex loses electrons whereas wool gains electrons. Glass ( rubbed with silk ), in this case Glass loses electrons whereas silk gains electrons. Negatively charged body means it is under the condition of excess of electrons. e.g. Amber ( rubbed with fur ) , in this case amber gains electrons whereas fur loses electrons. Rubber ( rubbed with fur ) , in this case rubber gains electrons whereas fur loses electrons. Polythene ( rubbed with wool ), in this case polythene gains electrons whereas wool loses electrons. Charging by friction is mostly suitable for insulators because any electrons that get transferred tend to stay where they are. Illustration of charging by rubbing ( friction ) 2.24P explain how positive and negative electrostatic charges are produced on materials by the loss and gain of electrons If an atom gains extra electrons, then it becomes negatively charged. If an atom loses electrons, then it becomes positively charged. An atom that becomes charged by gaining or losing electrons is called ion. Electric field, flames( thermal energy ), air movements, and radiations can all remove electrons from atoms and molecules so that ions can be formed. 2.25P know that there are forces of attraction between unlike charges and forces of repulsion between like charges Laws of Electricity: Like charges repel, unlike charges attract; The closer the charges, the greater the force between them; The more the charges, the greater the force between them. 21 International GCSE Physics (9-1 ) ( Modular ) 2.26P explain electrostatic phenomena in terms of the movement of electrons Charging by induction ( Suitable for conductors ) Induction is the production of electric charge on the surface of a conductor under the influence of an electric field. The advantage of charging by induction is that it can be repeated many times without any loss of charge from the inducing specimen like the negatively charged rod in the example shown. Illustration of charging by induction ( Note: Same amount of charges but opposite in sign appear on respective spheres. ) Charging by induction through earthing process ( alternative way ) ( Note: The kind of charge induced on the conductor opposite to that on the charged rod. ) ( Note: Charged object can attract the neutral object by induction since attractive force between unlike charges are greater than repulsive force between like charges. ) To obtain negative charges by induction To obtain positive charges by induction 22 International GCSE Physics (9-1 ) ( Modular ) 2.27P explain the potential dangers of electrostatic charges, e.g. when fuelling aircraft and tankers Electrostatic Hazards Sometimes objects get dangerously charged due to the build up of electrostatic charges. The rotating tyres of a moving truck acquire negative charges by friction from the road. Parts of the metal body of the truck near the tyres then become positively-charged by induction and sparks may be produced. This, in turn, might ignite any highly flammable load and cause a fire. That is why trucks which transport petrol or other highly flammable liquids usually have a metal chain or conductive strip at the rear end, dangling to the ground. This chain conducts electrons from the ground to neutralize the positive charge on the metal body of the truck, before it can build up and cause sparks. An aircraft in flight may become charged by “ rubbing ” the air. Its tyres are made of conductive rubber which lets the charge pass harmlessly to the ground on landing, otherwise an explosion could be “ sparked off ” when the air craft refuels. If tyres are not conductive it must be necessary to earth the plane with a conductor as soon as it lands and be fore refueling commences. Many synthetic fibres ( like nylon and acrylic ) used in clothing are good insulators and are easily charged by rubbing. People may also pick up charges as they walk on carpets made of synthetic fibres. In some situations like a dry environment sparks may be produced and the clothing may catch fire. Electrostatic discharge can also damage electronic equipment, such as circuit boards and hard drives. To protect these equipment, they are usually packed in antistatic packing. Antistatic materials have a thin layer of metalized film, which acts as an electrostatic shield for the equipment placed inside. 2.28P explain some uses of electrostatic charges, e.g. in photocopiers and inkjet printers Photocopier Office photocopiers operate on the principles of electrostatics. One of the commercial processes makes use of a rather unusual property of the metal selenium ( a photoconductor ) which conducts when it is in the light and is an insulator when it is in the dark. Inside the copier is a drum coated with a thin layer of selenium. Here are the steps involved in the photocopying process: Step 1: The whole surface of the drum is charged positively by rotating it near a highly charged metal wire. Step 2: When a printed page is photocopied, light is reflected off the page and projected onto the drum. The white parts of the page reflect intense light to some parts of the drum, making these areas conducting and thereby lose their charge. Other parts of the drum, correspond to the black parts of the page and receive no light at all. They remain insulating and hold on to their charge. So the drum ends up with a pattern of a positively charge of area which is an exact copy of the pattern of printing on the original page. 23 International GCSE Physics (9-1 ) ( Modular ) Step 3: Fine particles of carbon powder ( toner ), those are negatively- charged, are then attracted to the positively-charged areas of the drum. Step 4: The toner is transferred onto the photocopy as the drum rotates and presses against the copy paper. Finally heat is supplied to melt the toner powder and fix it onto the paper surface. Laser Printer The steps for producing a document on a laser printer are similar to those used in a photocopier. Steps 1, 3 and 4 are essentially the same. The difference between the two techniques lies in step 2, the way the image is formed on the selenium -coated drum. In a laser printer, the command to print a letter is sent to a laser from the memory of a computer. As the drum revolves, a laser beam is shone across the surface to discharge certain points. In this way, the laser 'draws' the letters and images to be printed as a pattern of electrical charges. After the pattern is set, the printer coats the drum with toner. The toner is then transferred to the copy paper. 24 International GCSE Physics (9-1 ) ( Modular ) Inkjet printer In this printer, a nozzle continuously produces a jet of droplets at a very high speed (50 m/s). The droplets are collected up by the printer by using the fact that each droplet is given a small charge by friction as it passes through the nozzle. The printer can then steer the droplet by applying voltages to metal plates fixed near to the jet. Electrostatic Paint Spraying As the droplets of paint emerge from the spray gun, they are charged. As the droplets all carry the same charge they repel and spread out forming a fine spray. The metal body wanted to spray has a wire attached to an electrical supply giving the body the opposite charge. The paint droplets are therefore attracted to the surface of the body. There is the added benefit that paint is attracted into places, such as tight corners, that might otherwise not receive such a good coating. Electrostatic precipitators Many heavy industrial plants, such as steel-making furnaces and coal-fired power stations, produce large quantities of smoke. This smoke carries small particles of ash and dust into the environment, causing health problems and damage to buildings. One way to removing these pollutants from the smoke is to use electrostatic precipitators. As the smoke initially rises up the chimney it passes through a mesh of wires that are highly charged. ( The wires are at a voltage of approximately - 50 kV. ) As the passes through the mesh, the ash and dust particles become negatively charged. Higher up the chimney these charge particles are attracted by and stick to large metal earthed plates. The cleaner smoke is then released into atmosphere. When the earthed plates are completely covered with the dust and ash, they are given a sharp rap. The dust and ash fall into collection boxes, which are later emptied. 25 International GCSE Physics (9-1 ) ( Modular ) Distribution of charge on a conductor The excess charge on a conductor collects only its outside surface. The concentration of a charge on a conductor is greatest where the surface is most sharply curved. The action of a point If an upturned drawing pin is placed on top of a van de Graaff generator, any charge which collects on the dome immediately 'leaks ' away from the sharp point. The charge seems to be carried away by a stream of air, sometimes called an electric wind. The action of the point is illustrated in figure. The metal surface is so sharply curved, and the charge so highly concentrated, that the forces on nearby air molecules are strong enough to strip electrons from them. Some of these electrons become attached to other molecules. As a result, the point is surrounded by a large number of molecules which have either lost or gained electrons. Because of these charged molecules the air is said to be ionized. The charge on the dome falls rapidly as positive ions are attracted to the point, strike it, and collect electrons to replace those which they have lost. At the same time, a fast-moving stream of air is produced as negative ions are pushed rapidly away from the point. A sharp point with a positive charge on it loses charge by a similar process. In this case however there is a flow of positive ions away from the point, which is gaining electrons rather than losing them. The lightning conductor Tall buildings usually have a strip of copper called a lightning conductor. One end of the strip is fixed to a metal plate buried in the ground, the other end is attached to a sharp spike or spikes which point upwards above the highest part of the building. Thunderclouds carry electric charges. If, say, a negatively charged, as in figure, thundercloud passes over a building, positive charges are induced on the roof, and the force of attraction between these opposite charges may be strong enough to produce a sudden flow of electrons from cloud to roof. In other words, the roof may be struck by lightning. The lightning conductor reduces the risk to the building in two ways: 1. The flow of ions from the spikes lowers the induced charge on the roof and cancels out some of the charge on the cloud. This reduces the chances of lightning striking. 26 International GCSE Physics (9-1 ) ( Modular ) 2. If lightning does strike, the lightning conductor provides a route for electrons to pass into the ground without damaging the building. The ground or earth has an almost infinite capacity for absorbing extra electrons. SPARKS Normally, air does not conduct electricity. The electrons in each atom in the air remain bound to that atom and so there are no 'free' electrons available to move in an organized way ( an electric current). However, if the nearby charge becomes strong enough- if there is a large enough electric field - then the force of attraction or repulsion on an electron may become so large that the attraction to the nucleus of the atom may not be enough to hold it. This then ionizes the atom: that is, the electron is separated off from the 'main' part of the atom. As the atom is now missing an electron it has an overall positive charge and is called a positive ion. Ionized air behaves very differently to 'normal' air. The presence of the free electrons and the positive ions means that an electric current can now pass through the air: the air becomes a conductor. As an electric current passes through it, heat is generated ( since the air still has quite a high resistance ) and the air emits light. This is the effect we would notice as a spark. As the spark ( electric current passing through the air ) heats up the air, it causes the air to expand rapidly. This expansion causes a vibration to pass through the air as the particles from the expanding air collide with nearby particles. This energy transfer is the sound wave we associate with a spark- it is the crackle we hear. If you have a piece of clothing that crackles when you take it off, then the crackles are caused by tiny sparks moving between fibres in the cloth. If you take off such a piece of clothing in the dark, you can see the sparks. Sparks are dangerous in places such as petrol filling stations. It is the heating effect that poses the major risk. To ignite any fuel vapour in the air a source of heat is required and it the heat generated by the electric current in the air, the spark, that provides the heat source. 27 International GCSE Physics (9-1 ) ( Modular )