Level 2 Notes 2024 - 07Electricity PDF

Catholic High School Level 2 Lower Secondary Science Topic 7: Electrical Systems Part 1: Current of Electricity Learning Outcomes Students should be able to: 1. Elect...

Catholic High School Level 2 Lower Secondary Science Topic 7: Electrical Systems Part 1: Current of Electricity Learning Outcomes Students should be able to: 1. Electric Circuits a) Draw and interpret circuit diagrams and set up circuits containing electrical sources, switches, lamps, resistors, ammeters and voltmeters. b) Distinguish the differences between circuits with parallel and series arrangements of fixed resistors, and state their advantages and disadvantages. c) Understand that a short circuit is an alternative path of no resistance and hence, all current will flow through it instead of the other paths. 2. Electric Current a) State that current is a rate of flow of charge and that it is measured in amperes. b) Distinguish between conventional current and electron flow. c) Recall and apply the relationship charge = current x time to new situations or to solve related problems. 3. Electromotive Force and Potential Difference a) Show a simple understanding of electromotive force (e.m.f.) as the sum of all the potential differences (p.d.) across all circuit components in series within a complete circuit. b) Calculate the total e.m.f. when several electrical sources are arranged in series. c) State that the e.m.f. of a source and the potential difference (p.d.) across a circuit component is measured in volts. d) Show a simple understanding of p.d. across a component in a circuit using analogy of ‘water pressure’. 4. Resistance a) State the definition that resistance = p.d. / current. b) Apply the relationship R = V/I to new situations or to solve related problems. c) Recall and apply the formulae for the effective resistance of a number of resistors in series and in parallel to new situations or to solve related problems. 1. What is Electricity? Energy can be transferred electrically by the flow of electric charge (e.g. electrons and ions) through an electric conductor. A positive ion A negative ion Metals are considered good conductors of electricity. Why is that so? 2. Electric Circuits When there is a flow of electric charge, there is an electric current. The path through which electric charge flows is called an electric circuit. In a closed circuit, electric charge can flow in an uninterrupted path. In an open circuit, electric charge do not flow in the circuit at all. This electric circuit is a closed circuit as it provides a continuous path for the electric charges to flow through. 2.1 Circuit Diagrams With reference to the earlier diagram, it is evident that drawing of electrical circuits in colour and 3-D forms are too time-consuming. Instead, we use circuit diagrams. The diagram below shows the circuit symbols for some common electrical components. 2.2 Switch A switch is used to open and close a circuit safely and conveniently. When the switch is Different types of opened, no electric switches commonly charges flow at all and available in the the electric bulb does science lab not light up. 2.3 Electric Cells An electric cell has energy in its chemical potential store, which is transferred electrically by the flow of electric charge to other components in the circuit such as resistors and lamps. When two or more electric cells are connected in a circuit, they form a battery. Electric cells may be connected in series or in parallel. series parallel   Try it out #1!  Can you use symbols to represent the following connections? 2.4 Series and Parallel Circuits Series Circuit In a series circuit, electrical components are connected in a single continuous path i.e. the current is the same throughout the circuit. The same current flows through each of the components. When one of the bulbs in a series circuit is removed or damaged, the circuit is open and no electric charge flow through the other bulbs. All bulbs will not light up. a series circuit Parallel Circuit In a parallel circuit, the electric circuit consists of more than one path. The current splits itself (not necessary equally) into the different branches and combines again before it flows back to the cell. When one of the electric bulbs in the parallel circuit is removed or damaged, the other bulbs will continue to light up. This is because the light bulbs are connected a parallel circuit to the cell by separate paths, so electric charge can still continue to flow through the other paths in the circuit. Which of the following diagrams represent series or parallel circuits? Short Circuit An alternative path of zero resistance is present. Hence, all the current flows through wire X instead of the bulb. The bulb will not light up since no current passes through it due to the short circuit. 3. Electric Current Electric current is defined as the rate of flow of electric charge (through a given cross- section of an electrical conductor). Q I= t where I = current flowing through the circuit (measured in ampere, A), Q = amount of electric charge flowing in the circuit (measured in coulomb, C), t = time taken (measured in seconds, s) The SI unit of electric current is ampere (A). Smaller units such as milliampere (mA) and microampere (µA) are sometimes used.   Try it out #2!  A current of 0.5 A flows for 2 minutes through an electrical heater. Calculate the total amount of charge moving through the heater. The direction of a conventional current is from the positive to the negative terminal of the cell. The direction of electron flow is in the opposite direction (ie from the negative to the positive terminal of the cell). In the past, scientists thought that current was due to the flow of positive charge from the positive terminal to the negative terminal of a cell. Subsequently, it was discovered that current was due to the flow of electrons from the negative to the positive terminal of a cell. To avoid further confusion, it was decided that circuit diagrams would be labelled with the direction of the conventional current rather than that of electron flow. Direction of conventional current flow Direction of electron flow 3.1 Measuring Electric Current An ammeter measures the amount of electric current flowing in the circuit. The ammeter must be connected in series with the circuit. The positive (red) and negative (black) terminals of the ammeter must be connected to the positive and negative terminals of the electric cell or battery respectively (Remember, positive to positive and negative to negative). During laboratory practice, you will observe that there is a reflective stripe on an ammeter (as well as voltmeter). This stripe is to prevent parallax error by observing that the pointer and its image are aligned before taking any readings. Reflective strip The precision of the reading from an ammeter is half the smallest interval that can be read on the meter (that is, 0.01 A (2 dp) for the diagram above). 4. Electromotive force (e.m.f) and Potential difference (p.d.) 4.1 Electromotive force (e.m.f) To explain the concept of e.m.f, an analogy is made between a battery and a water pump. A water pump does work (providing energy) to drive the water around the pipe. The water pump does not supply the water in the pipe. Similarly, a battery does work to drive electrons around the circuit. It does not supply electrons to the circuit, as electrons are already present in the circuit components (eg wires, lamps). Pipe = electrical circuit Water = charges Water pump = e.m.f. source Definition The electromotive force (e.m.f) of an electrical source (e.g. cell) is defined as the work done by the source in driving a unit charge around a complete circuit. The SI unit of e.m.f. is the volt (V). The millivolt (mV) and the kilovolt (kV) are commonly used. Note: electromotive force is not a force, as its unit is V and not N! Also, a unit charge refers to one coulomb (C) of charge, not one charge! 4.2 Potential Difference (p.d.) Current flows from a point of high electric potential to a point of low electric potential. The difference in the electric potential between two points in an electric circuit is known as the potential difference (p.d.). Definition: Potential Difference (p.d.) is the work done in driving a unit charge across two points in a circuit. The SI unit of p.d. is the volt (V). Just like water which always flows from higher to lower ground, positive electric charge also flows from a point of higher potential to a point of lower potential. (In reality, it is electrons that flow from a point of lower potential to a point of higher potential, as mentioned in Section 3.) Electromotive force Potential difference Associated with an electric source (e.g. a Associated with two points in an electric cell). circuit. It is the work done by the source in It is the work done to drive a unit charge driving a unit charge around a complete through two points in the circuit. (unit is circuit. (unit is volt). volt). 4.3 Measuring potential difference The voltmeter measures the potential difference between two points. The voltmeter is connected in parallel with an electric component. Just like an ammeter, the positive (red) and negative (black) terminals of the voltmeter must be connected to the positive and negative terminals of the electric cell or battery respectively. A voltmeter The precision of the reading from a voltmeter is half the smallest interval that can be read on the meter, (that is, 0.05 V (2 dp) for the diagram above). 4.4 Electric cells in series When electric cells are connected in series, the e.m.f.s of the cells add up. Below is an example of four 1.5 V cells connected in series. ε 1.5 V ε = 1.5.+ 1.5 + 1.5 + 1.5 = 6.0 V ε  Try it out #3!  What is the effective e.m.f. of the following connections? + - + - 2.5 V 1.5 V 3.5 V 5 RESISTANCE An electrical component resists or hinders the flow of electric charges when it is connected in a circuit. Simply put, electrical resistance measures the difficulty of an electric current to pass through the conductor. The resistance arises from the collision of the electrons with the particles in the conductors. The higher the resistance, the more difficult it will be for an electric current to flow. Energy transferred electrically by an electric current causes an increase in the internal energy of a resistor as it becomes hotter. 5.1 Resistance in conductors All materials have electrical resistance. Materials with high resistance are called electrical insulators whereas materials with low resistance are called electrical conductors. Different conductors have different electrical resistance. For the same cross-sectional area (ie. thickness of wire), Copper has a very low resistance, allowing current to flow easily. Silver has a lower resistance than copper but it is more expensive. Nichrome has a higher resistance than copper. It is an alloy of 2 metals: nickel and chromium. Nichrome wires are often called resistance wires. 5.2 Fixed resistors Resistors are made to create some resistance in the circuit, so as to limit the current flow. Resistors can be found in electrical appliances such as televisions, radios and personal computers. Circuit symbol for a fixed resistor Resistors with fixed resistance are called fixed resistors. They are usually made of carbon encased in plastic casing and their resistances can range from a couple of ohms to thousands of ohms. The coloured bands on the resistors above serve to indicate their resistances. 5.3 Rheostats - Variable resistors Resistors with variable resistance are called variable resistors or rheostats. Do you know in what situation does varying the resistance become necessary and useful? A sliding rheostat A rheostat in radio Application of rheostat as a dimmer When the slider is on the left, the short resistance wire creates a smaller resistance. The current flowing through the circuit is large. Hence, the light bulb lights up brightly. When the slider is on the right, the long resistance wire creates a larger resistance. The current flowing through the circuit is smaller. Hence, the light bulb dims. 5.4 Electrical Resistance Definition Definition Electrical resistance is the ratio of the potential difference across a circuit component to the current which flows through it. V R is the resistance across the circuit component R= V is the potential difference across the circuit component, in volts (V) I is the current flowing though the circuit component, in amperes (A) I The SI unit for resistance is the Ohm ( Ω )  Try it out #4!  1. The potential difference across a lamp is found to be 1.2 V when the current in the lamp is 200 mA. What is the resistance of the lamp? 2. For the same lamp, when a potential difference of 2.0 V is applied, calculate the current in the lamp. 5.5 Effective Resistance The total resistance caused by the whole network of resistors is known as the effective resistance. For calculation of effective resistance in a network of resistors (i.e. more than 1 resistor connected to the circuit), we can consider it to be a single resistor that can be used to replace all the resistors in the network without changing the current flowing in the circuit. Calculation of effective resistance Series In a series connection, the effective resistance increases as more resistors are added. Consider the following circuit with resistors connected in series. For a circuit in series, the current flowing through each resistor is the same. V = V 1 + V 2 + V3 IR = IR1 + IR2 + IR3 (I)R = (I)(R1 + R2 + R3) R = R 1 + R 2 + R3 Effective resistance is sum of individual resistances when resistors are connected in series. Parallel Consider the following with resistors connected in parallel. The current flowing into the resistors is equal to the current flowing out of the resistors. For a circuit in parallel, the potential difference across each resistor is the same. I=I +I +I 1 2 3 V V V V = + + R R1 R2 R3 1 1 1 1 = + + 𝑅 𝑅1 𝑅2 𝑅3 In a parallel connection, the effective resistance decreases as more resistors are added.   Try it out #5!  Find the effective resistance of the following network of resistors. Part 2: DC Circuits Learning Outcomes Students should be able to: a) State the current at every point in a series circuit is the same and apply the principle to new situations or to solve related problems. b) State that the sum of the potential differences in a series circuit is equal to the potential difference across the whole circuit and apply the principle to new situations or to solve related problems. c) State the current from the source is the sum of the currents in the separate branches of a parallel circuit and apply the principle to new situations or to solve related problems (restricted to only two parallel branches). d) State that the potential difference across the separate branches of a parallel circuit is the same and apply the principle to new situations or to solve related problems. e) recall and apply the relevant relationships, including R = V/I and those for current, potential differences and resistors in series and in parallel circuits, in calculations involving a whole circuit. 1. Series Circuit 1.1 Current In a series circuit, the components are connected one after another in a single loop. A series circuit has only one path through which electric charge can flow. In a series circuit, the current at every point is the same. I1 = I2 = I3 1.2 Potential Difference and Electromotive Force The potential difference across a few components is the sum of the potential differences across each individual component. Hence, for a circuit in series, when we sum up the individual potential differences across each component, the total will be equal to the e.m.f. of the cell. Vε = V1 + V2 2. Parallel Circuit 2.1 Current The current flowing from the cell splits at junction x and recombines at junction y. The ammeter reading for A is equal to the sum of the readings for ammeters A 1 and A. 2 I = I1 + I2 2.2 Potential Difference and Electromotive Force In a solely parallel circuit (ie. where all the circuit components are connected in parallel and not in series with other circuit components), the e.m.f. Vε of the cell is equal to the potential differences across each of the resistors. Vε = V1 = V2 Summary Series Parallel Current Same throughout I = I1 + I2 Voltage V1 = V2 (= e.m.f. only if circuit is Vε = V1 + V2 + V 3 solely parallel) Effective Re = R1 + R2 + R3 Resistance Example 1 7.0 V a) Find voltmeter reading V1. b) Given that the current reading in the ammeter L2 A is 2.0 A, find the resistance of L2. L1 V1 V2 c) Find the resistance of L1. 3.0 V d) Find the effective resistance of the circuit Example 2 a) Find the potential difference across (i) L1 (ii) L2 What can you conclude about the e.m.f. of the cell? L1 A 2.0 A 5.0  1.0 A 10.0  b) Find the ammeter reading. L2 c) Find the effective resistance of the circuit. d) Using the formula V = RI and your answers to part (b) and (c), find the e.m.f. of the cell. Compare this to the answer in part (a). Part 3: Practical Electricity Learning Outcomes Students should be able to: Electrical Power and Energy (a) Describe the use of the heating effect of an electric current in appliances such as electric kettles, ovens and heaters. (b) Explain what is meant by power, relate it to an output of an electrical system, stating its SI unit. (b) Recall and apply the relationship E = P x t to new situations or to solve related problems. (c) Calculate the cost of using electrical appliances where the energy unit is the kWh. Dangers of Electricity (d) State the hazards of using electricity in the following situations: i) damaged insulation ii) overheating of cables iii) damp conditions Safety Features in Home Circuitries (e) Explain the use of fuses and circuit breakers in electrical circuits and of fuse ratings (f) Explain the need for earthing metal cases and for double insulation (g) State the meaning of the terms live, neutral and earth. (h) Describe the wiring in a mains plug. (i) Explain why switches, fuses, and circuit breakers are wired into the live conductor. 1 Uses of Electricity Energy can be transferred electrically by an electric current to increase the temperature of an object which can have useful applications. Electric Heating Most household appliances that are used for heating purposes (see examples below) have heating elements that are made of nichrome. Nichrome is used because it has a large resistance (for a fixed length and cross- sectional area) compared to other metals, and can withstand high temperatures without melting. When an electric current passes through these heating elements, they heat up rapidly. The Filament Lamp Energy transferred electrically to a filament lamp increases energy in its internal store as it heats up. Some of this energy is also transferred by light and infra-red waves to the surrounding air. The filament in the lamp is a tungsten wire. This wire has a small cross-sectional area and is coiled. When an electric current flows through it, it is heated to about 2500 °C. This intense heating effect generates light. Tungsten is used because of its high resistance (for a specific length and cross- sectional area) and high melting point (3400 °C). 2 Electrical Power We define power as the rate of work done or rate of energy transferred. W P = Power (W) or E P= P= t t E = Energy transferred (J) power as rate of power as rate of W = Work done (J) work done energy transferred t = Time taken (s) The SI unit of power is the Watt (W). One watt is equal to one joule per second. Example 1 If a 20 W bulb operates for 10 s, how much energy is consumed? Example 2 A 5.0 kW immersion heater is used to heat water for a bath. It takes 40 minutes to heat up the water. How much energy has been transferred from the heater to the water? Example 3 A dry cell will deliver 3 000 J of energy to a 2 W electric motor before the cell is exhausted. What duration of time (in minutes) will the motor run? Calculating the Cost of Electricity Consumption In normal households, the unit for electrical energy is measured in kilowatt-hours (kWh) instead. The cost of electricity consumed is based on the number of kilowatt-hours (kWh) of electrical energy consumed. 1 kWh = the energy consumed by a 1 kW device in 1 h = 1 kW × 1 h = 1000 W × (60 × 60) s = 3.6 × 106 J Electrical Energy (in kWh) = Power (in kW) x Time (in h) Cost of electrical energy = Energy Consumption (in kWh) x Cost per kWh Example - Monthly PUB bill: Electricity consumption is measured in kilowatt-hours (kWh). Example 4 An air conditioner rated 1500 W is used for 180 minutes. a) Calculate the amount of energy used in units of kWh. b) Calculate the cost of electricity consumed if the unit cost is 24 cents. 3 Dangers of Electricity There are 2 main dangers that we need to worry about when using electricity. They are: Electric Shock Electric Fire Electricity can be a hazard in the following situations: Damaged Insulation When the insulating material covering the conducting wires gets worn or damaged, the conducting wires may be exposed. Exposed conducting wires can cause electric shock if touched. Overheating of Cables exposed wires Overheated cables can lead to electric fire. Two common causes of overheated cables are: - Overloaded power sockets When a power socket is overloaded, an unusually large current flows through the wires. - Use of inappropriate wires Appliances that require high power to function need thicker wires. Damp Environments Water that comes in contact with exposed electrical wires provides a conducting path for current. This can lead to electric shock. Electrical appliances should be kept in dry places and handled with dry hands. Resistance of Human Body when Damp (For information only) Situation Resistance when Dry Resistance when Damp Wire touched by finger 40 kΩ to 1 MΩ 4 kΩ to 15 kΩ Wire held by hand 15 kΩ to 50 kΩ 3 kΩ to 5 kΩ Emergency Response to Electric Shock (For information only) If you see someone lying unconscious, the very first thing to do is shut off the power by activating the appropriate disconnect switch or the circuit breaker. If someone touches another person being shocked, there may be enough potential difference across the body of the victim to shock the would-be rescuer, thereby shocking two people instead of one. Don't be a hero. Electrons don't respect heroism. Make sure the situation is safe for you to step into, or else you will be the next victim, and nobody will benefit from your efforts. If the power disconnect switch cannot be located quickly enough, it may be possible to dislodge the victim from the circuit they are touching by prying them or hitting them away with a dry wooden board or piece of non-metallic conduit, common items to be found in industrial construction scenes. Another item that could be used to safely drag a shocked victim away from contact with power is an extension cord. By looping a cord around his torso and using it as a rope to pull them away from the circuit, his grip on the conductor(s) may be broken. Bear in mind that the victim will be holding on to the conductor with all his strength, so pulling them away probably won't be easy! Effects of Electric Current in the Human Body (For information only) Example 5 The diagram shows four electric kettles plugged into a 4-way adaptor. An extension lead connects the adaptor to a single mains plug. Why is this use of the adaptor dangerous? A The heating elements in the kettles will overheat. B The extension lead will overheat. C The water will overheat. D The wires connecting the kettle will overheat. Example 6 An electric motor is connected to the mains supply. The insulation covering the wires becomes damaged. Why is this damage dangerous? A It might not be possible to turn the motor off. B The motor might overheat and catch fire. C The motor may not have enough power to start working. D The user of the motor might receive an electric shock. Example 7 Explain why it is hazardous for a boy to switch on the TV after bathing without drying his hands. 4 Safety Features in Home Circuitries Safety features that can be found in our homes include: 1. Circuit breakers 2. Fuse 3. Switches 4. Earthing (Earth Wire) 5. 3 pin plug 6. Double Insulation 4.1 Circuit Breakers Circuit breakers are safety devices that can switch off the electrical supply in a circuit when large currents flow through them. Without circuit breakers, a surge of current can damage home appliances or even cause an electric fire. Circuit breakers are connected to live wires (see Section 4.5). 4.2 Fuses Fuses have the same function as circuit breakers. They prevent excessive current flow. However, unlike a circuit breaker that can be reset after it trips, a fuse must be replaced after it blows. Fuses are connected to live wires. A fuse consists of a short, thin piece of wire. If the current flowing through it is too large, the wire heats up and melts. This causes the circuit to be opened (so current no longer flows in the circuit). Example 8 Which rated fuse should be connected to an appliance that uses a 4 A current? A 3A B 4A C 5A D 20 A Example 9 Which circuit diagram shows the correct way of connecting a fuse? Why? appliance A live wire neutral wire main s appliance B live wire neutral wire main s 4.3 Switches Switches are designed to break or complete an electric circuit. They should be fitted to the live wire of the appliance. Due to an electrical fault, the live wire touches the metal casing of the appliance. 1 However, the switch fitted on the live wire 2 disconnects the metal casing from the live No current flows through the person wire when it is opened. touching the casing. The metal casing is at low voltage (almost 0 V). 4.4 Earthing Earthing can prevent electric shocks to the user through the Earth wire. When there is a fault in the circuit such that the live wire touches the casing of the appliance, the Earth wire provides a conducting path for the current to flow to the ground instead of the person touching the appliance. The Earth wire is able to do so as it is of very low resistance and zero potential. Without Earth wire: Imagine that an electrical fault causes the live wire to be in contact with the washing machine’s metal casing. What will happen if you touch the washing machine’s metal casing? You will get an electric shock! Note that the increase in current will not be sufficient to cause the fuse to melt. With Earth wire: Even though the live wire is in contact with the metal casing, a large current (due to the low resistance of the Earth wire) will flow to the ground through the Earth wire and not the person. This large current will blow the fuse, thus breaking the circuit. 4.5 Three-Pin Plug Earth wire Neutral wire There are usually three wires in a home circuit: Live wire (colour: brown) – connected to a high potential and delivers current to the appliance Live wire – is the wire to which circuit breakers, fuses and switches are connected Neutral wire (colour: blue) – completes the circuit by providing a returning path to the supply – at a potential of 0 V Earth wire (colour: yellow and green) – low resistance wire connected to the metal casing of appliances The fused plug used in Singapore is a three-pin plug. It is also known as a safety plug. Safety features: The fuse protects the appliance when there is an electrical fault. The Earth wire protects the user from electric shocks in the event that an electrical fault causes the appliance to be live. 4.6 Double Insulation Do you notice that not all electrical appliances have a three-pin plug? For some of them, their plugs have only two pins each. These appliances do not have an Earth wire. Instead, they have double insulation to protect users from electric shocks. So what exactly is double insulation? The electric cables are insulated from the internal components of the appliance and the internal components are insulated from the outer casing. Appliances with double insulation typically have non-metallic casings (e.g. plastic).  How are airplanes protected from lightning? (adapted from https://monroeaerospace.com/blog/did-you-know-how-airplanes-are-protected-from- lightning/) The outer shell of a typical airplane is made of aluminium. Aluminium offers several benefits when used in aerospace engineering: it is strong, lightweight, readily available, and predictable. Moreover, it is an excellent conductor of electricity, making it ideal for use in lightning-prone aircraft. When a bolt of lightning strikes the exterior of an airplane, current travels into the aluminium shell and out through another point on the aluminium. Most airplanes still avoid lightning storms when possible, but if a lightning strike occurs, everyone on board can be assured knowing that it is not going to cause damage. However, there is also the possibility of damage to electronics systems and flight instruments from lightning strikes. To guard against this phenomenon, airplanes are equipped with shielding and suppressors, while all vital components must meet the Federal Aviation Administration’s (FAA) regulations. This further protects the airplane from potential damage caused by lightning strikes. To recap, lightning does in fact strike airplanes in mid-air. It typically causes no damage, however, because the lightning simply travels into and out of the airplane’s aluminium shell.   Try it out! #7  1. The figure below shows a toaster with a power rating of 2000 W and has two resistors of 1500 Ω each arranged in parallel. metal casing of toaster fuse Live wire 1500 Ω 1500 Ω mains plug Neutral wire (a) State the colour of the neutral wire. …………………… (b) Complete the figure by (i) connecting the live wire and neutral wire to the correct pins on the mains plug, (ii) connecting the earth wire from the mains plug to the toaster. 2. The diagram below shows the circuitry in a household where a refrigerator and a light bulb is connected to the mains of 240 V. A B live 20 W 5000 W neutral 240 V (a) Is A or B the refrigerator unit? Explain your choice. (b) Suggest a suitable fuse rating for the fuse if A draws a current of 0.08 A and B draws a current of 20.8 A. (c) If the circuit is switched on for 20 hours and the cost of electricity is 20 cents per kWh, what is the cost of electricity used? (d) Where should a switch be inserted in the circuit? How should more light bulbs be connected in this circuit?