Electricity and Magnetism - Section 4 - PDF

Section 4 Electricity and Magnetism By: Ms. Valensa Yossyana  4.2 Electrical Quantities Positive and Negative Charges 4.2.1 Electric Charge When a strip of polythene is rubbed with a...

Section 4 Electricity and Magnetism By: Ms. Valensa Yossyana  4.2 Electrical Quantities Positive and Negative Charges 4.2.1 Electric Charge When a strip of polythene is rubbed with a cloth it becomes charged. If it is hung up and another rubbed polythene strip is brought An atom as being made up of a small central near, repulsion occurs. nucleus containing positively charged particles Attraction occurs when a rubbed strip of called protons, surrounded by an equal number cellulose acetate is brought near. of negatively charged electrons. Every nucleus This shows there are two kinds of electric except hydrogen also contains uncharged charge, positive (+) and negative (-). particles called neutrons. Like charges (+ and +, or – and –) repel, while The charges on a proton and an electron are unlike charges (+ and –) attract. equal and opposite so an atom as a whole is The production of charges by rubbing can be normally electrically neutral, i.e. has no net explained by supposing that friction causes  4.2 Electrical Quantities Positive and Negative Charges IGCSE Question: Markscheme: 1. rods repel OR move apart / away (1) like / same (type of) charges on both rods (1)  4.2 Electrical Quantities Positive and Negative Charges IGCSE Question: Markscheme: B Explanation: Static electricity is the build-up of electric charge on the surface of objects. This occurs when certain materials are rubbed against each other, causing electrons to be transferred from one material to the other. Concept of Charge Transfer: When two different materials come into contact and are then separated, electrons may transfer from one material to the other. The material that loses electrons becomes positively charged. The material that gains electrons becomes Gold-Leaf Electroscope An electroscope can be used for (read The device is used for detecting electric charge Coursebook pg 186): and can also identify its polarity Detecting a charge Charging by contact Determine conductors and insulators Youtube Electroscope Induction: https://youtu.be/dwJ-MM7yu4E https://youtu.be/76tFmbbnkH0 Electrons, Insulators and Conductors Insulator In an insulator all electrons are bound firmly to their atoms; (Good insulators include plastics such as polythene, cellulose acetate, Perspex and nylon) An insulator can be charged by rubbing because the charge produced cannot move from where the rubbing occurs, i.e. the electric charge is static. Conductor In a conductor some electrons can move freely from atom to atom. (All metals and carbon are good conductors) A conductor will become charged only if it is held with an insulating handle; otherwise electrons are transferred between the conductor and the ground via the person’s body. Electric Fields The electric field lines radiating from an isolated Electric field is the region of space where positively charged an electric charge experiences a force due conducting sphere and a to other charges. point charge are shown If the electric force felt by a charge is the in Figures 4.2.7a and b: same everywhere in a region, the field is the field lines again uniform; a uniform electric field is emerge at right angles to produced between two oppositely charged the conducting surface. parallel metal plates (see Fig 4.2.6). An electric field is a vector quantity as it has both magnitude (strength) and direction. The direction of an electric field at a point, denoted by arrows, is the direction of the force on a small positive charge placed in the field  2. Lightning happens when charged particles Dangers of Static Electricity travel from the clouds to earth. Lightning See Youtube Video: has potential to damage. The Dangers of Static Electricity 3. To prevent this damage, so tall buildings has https://youtu.be/SAhrPa0XB88 lightning conductors (or thick metal rods) at The Dangers Of Electrostatic Electricity the top of the building. This rods allow the https://youtu.be/XKAhx4NdJTs lightning to discharged safely through them Sparks occur between and flow through a metal rod straight electrostatic charges Uses ofthe through Static Electricity building and into the ground. when the electric See Youtube Video: field is strong enough. The Uses of Static Electricity Solution: https://youtu.be/gVeRoaakRp4 Providing an easy path for electrons to Applications Static electricity: flow safely to and 1. Laser Printer from the Earth. 2. Filters in Chimneys 3. Electrostatic Sprayer for Crops Mindmap 4. Electrostatic Paint Sprayer 1. Ground is 5. etc positively charged relative to the clouds. Because of this there is now a huge potential  4.2 Electrical Quantities Dangers of Static Electricity IGCSE Question: Markscheme: A 4.2.2 Electric Current How the Van De Graaff engine works: 1. The table-tennis ball shuttles rapidly An electric current consists of moving electric backwards and forwards between the plates charges. and the very sensitive meter records a small current. The van de Graaff machine is working, it 2. As the ball touches each plate it becomes produces a continuous supply of charge which charged and is repelled to the other plate. In produces an electric field between the metal this way charge is carried across the gap. plates to which it is connected. 3. This also shows that static charges, produced by friction in the van de Graaff machine, cause a deflection on a meter just as current electricity produced by a battery does. Effects of a Current The Ampere and the Coulomb An electric current has three effects that reveal An electric current is defined as the charge its existence passing a point per unit time and can be 1. Heating and lighting: The lamp lights written in symbols as because the small wire inside (the filament) is made white hot by the current. 2. Magnetic: The plotting compass is deflected when it is placed near the wire because a Current (I) with unit is Ampere (A). Charge magnetic field is produced around any wire (Q) with the unit is Coulomb (C). carrying a current. 3. Chemical: Bubbles of gas are given off at the wires in the acid because of the chemical action of the current. Conventional Current Before the electron was discovered scientists agreed to think of current as positive charges moving round a circuit in the direction from positive to negative of a battery. This agreement still stands. Arrows on circuit diagrams show the direction of what we call the conventional current, i.e. the direction in which positive charges would flow. Electrons flow in the opposite direction to the Worked Example Ammeter An ammeter is used to measure currents. It should always be placed in series in a circuit with the positive terminal on the ammeter connected to the positive terminal of the supply. When making a measurement on either type of ammeter a suitable range must be chosen.  The pointer of an ammeter for measuring d.c. Direct and Alternating Current is deflected one way by the direct current. In a direct current (d.c.) the electrons flow in Alternating current makes the pointer move one direction only. back and forth about the zero if the changes are slow enough; otherwise no deflection can be seen. Batteries give d.c.; generators can produce either d.c. or a.c In an alternating current (a.c.) the direction of flow reverses regularly. Frequency of a.c Frequency is the number of complete alternations or cycles in 1 second. The unit of frequency is the hertz (Hz). In Figure 4.2.15: The frequency of the a.c. is 2 Hz, which means there are two cycles per second, or one cycle lasts 1/2 = 0.5 s. Youtube : Alternating and Direct Current 4.2.3 Electromotive force and potential difference 1. The chemical action inside a battery produces a surplus of electrons at one of its terminals (the negative) and creates a shortage at the other (the positive). It is then able to maintain a flow of electrons, i.e. an electric current, in any circuit connected across its terminals for as long as the chemical action lasts. 2. Work is done by the battery in moving charge around the circuit. 3. Electromotive force (e.m.f.) is defined as the electrical work done by a source in moving a unit charge around a complete circuit. Electromotive force is measured in volts (V). 4. Potential difference (p.d.) is defined as the work done by a unit of charge passing through a component. Like e.m.f., potential difference between two points is measured in volts (V). The term voltage is sometimes used instead of p.d.  4.2 Electrical Quantities e.m.f and p.d IGCSE Question: Markscheme: Energy Transfers and p.d. ((an electric Energy current)) Energy (Energy store ex: a battery) (Energy into the sorroundings ex: a lamp) Mains lamp with big potential difference (230 V) gives much more light and heat than the car Evidently the12p.d. lamp with V across a device affectsthe rate at which it transfers energy. This gives us a way of defining the unit of potential Model of a Circuit Imagine that: The current in a circuit is formed by ‘drops’ of electricity, each having a charge of 1 coulomb and carrying equal-sized bundles of electrical energy. Mr Coulomb represents one such drop. Mr Coulomb travels round the circuit and unloads energy as he goes, most of it in the lamp. He receiving a fresh bundle every time he passes through the battery, which suggests he must be travelling very fast. The greater the e.m.f. of a supply, the larger is the bundle of energy given to each coulomb and the greater is the rate at which energy is transferred from a lamp. The Volt How to find e.m.f or E (with the unit is volts) If W (joules) is the energy transferred (i.e. the work done) when charge Q (coulombs) moves around a complete circuit, the e.m.f. E (volts) of the supply is given by Or How to find p.d or V (with unit is volts) If W (joules) is the work done when charge Q (coulombs) passes between two points, the p.d.V (volts) between the points is given by or Remember that so that  4.2 Electrical Quantities e.m.f and p.d IGCSE Question: Markscheme: Voltmeter Voltmeter A voltmeter is used to measure potential differences; it should always be placed in parallel with the component across which the p.d. is to be measured. The positive terminal on the voltmeter should be connected to the side of the component into which current flows. When making a measurement on either an analogue or digital voltmeter a suitable range must first be chosen. For example, if a voltage of a few millivolts is expected, the 10 mV range might be selected and the value of the voltage (in mV) read from the display; If the reading is off-scale, the sensitivity should be reduced by changing to the higher, perhaps 100 mV, range.  4.2 Electrical Quantities e.m.f and p.d IGCSE Question: Markscheme: 4.2.4 Resistance Resistor Resistance is the opposition of a conductor to Conductors intended to have resistance are current. A good conductor has a low resistance called resistors and are made either from and a poor conductor has a high resistance wires of special alloys or from carbon. Those (see slide 4). used in radio and television sets have values If the current in a conductor is I when the from a few ohms up to millions of ohms. voltage across it is V its resistance R is defined Circuit symbol for a by resistor Fixed resistor Resistor Variabe resistor (potentiometer) These consist of a coil of constantan wire (an alloy of 60% copper, 40% nickel) wound on a tube with a sliding contact on a metal bar above the tube. There are two ways of using such a variable resistor. 1. As a rheostat for changing the current in a circuit; In Figure 4.2.26a moving the sliding contact to the left reduces the resistance and increases the current. 2. As a potential divider for changing the p.d. applied to a device; In Figure 4.2.26b any fraction from the total p.d. of the battery to Resistance of a metal wire The resistance of a metallic wire will increases as its length increases increases as its cross-sectional area decreases depends on the material Resistance of a metal wire I–V graphs: Ohm’s law The variation of current with voltage is shown for various conductors in Figure Youtube: Linear and Non Linear https://youtu.be/9l8PwEPaGW4 I–V graphs: Ohm’s law I–V graphs: Ohm’s law Metallic conductors Filament lamp They are called ohmic or A filament lamp is a non- linear conductors and since I ohmic conductor at high ∝ V, it follows that V/I = a temperatures. For a constant (obtained from the filament lamp the I–V slope of the I–V graph). The graph curve flattens as V resistance of an ohmic and I increase. That is, conductor therefore does not the resistance (V/I) changelaw, Conductors obey Ohm’s when the p.d. stated does. as follows: increases as I increases “The current in a metallic conductor is directly and makes the filament proportional to the p.d. across its ends if the hotter. Thermistor temperature and other conditions are The resistance of constant.” semiconductor thermistors Semiconductor diode decreases if their Figure shows that current temperature rises, i.e. their passes when the p.d. is I–V graph bends upwards, applied in one direction but as in Figure 4.2.29d. is almost zero when the p.d. is applied in the opposite direction. It conducts in one direction only and is a non- ohmic conductor. 4.2.5 Electrical working Electric power is the rate of transfer of electrical energy within a circuit. Its SI unit is the Watt. Worked Example If a lamp on a 240 V supply has a current of From the definition of p.d. that 0.25 A in it, its power (see slide 20) so, This means that 60 J of energy are transferred You can use Ohm’s Law to find another to the lamp each second. formula of power or Joulemeter To measure the electrical energy transferred to an appliance, a joulemeter can be used to measure it directly in joules. The circuit connections are shown Paying for Electricity Electricity meters, which are joulemeters, are marked in kWh: the latest have digital readouts Electricity supply companies charge for the like the one in Figure amount of energy they supply. The kilowatt- hour (kWh), is used. A kilowatt-hour is the electrical energy used by a 1 kW appliance in 1 hour. How to calculate electricity 1. Calculate the power P of an electrical appliance 2. Convert unit of power become kW (kilowatt) 3. Calculate energy used by appliance in hour Unit of 4. Calculate total costs (see WB pg 60; number 17) DONE 4.2 Next subtopic 4.3  4.3 Electric Circuits 4.3.1 Circuit Diagrams & Components Some of the symbols used for the various parts of an electric circuit are shown in Figure 4.3.2 Series & Parallel Circuits Series Circuit Parallel Circuit The current at every point in a series circuit is The current splits: some goes through one the same lamp and the rest through the other Electric current in a circuit cannot be stored. This means that when circuits join or divide, the total current going into a junction must be equal to the total current leaving the junction. The total p.d. across the components in a The p.d. across devices in parallel in a circuit series circuit is equal to the sum of the are equal. individual p.d.s across each component. 4.3.2 Series & Parallel Circuits Series Circuit Parallel Circuit A battery consists of two or more electric cells. If two 1.5V cells are connected in parallel, as in Greater e.m.f.s are obtained when cells are Figure, the e.m.f. at terminals P, Q is still 1.5V joined in series, i.e. + of one to − of next. but the arrangement behaves like a larger cell and will last longer. 𝜀 𝑇𝑜𝑡𝑎𝑙=1 ,5 𝑉 +1 ,5 𝑉 =3 𝑉 𝜀 𝑇𝑜𝑡𝑎𝑙=1 ,5 𝑉 − 1 , 5 𝑉 =0 𝑉 4.3.2 Series & Parallel Circuits Series Circuit Parallel Circuit Resistance in series circuit. The same current I Resistance in parallel circuit. The voltage V flows through each and the total voltage V between the ends of each is the same and the across all three is the sum of the separate total current I equals the sum of the currents voltages across them, i.e. in the separate branches, i.e. Dividing both sides by V, Dividing both sides by I, Properties of parallel circuits (i) the current from the source is larger than the current in each branch (ii) the combined resistance of two resistors in parallel is less than that of either resistor by itself. Lamps are connected in parallel (Figure 4.3.5) rather than in series in a lighting circuit. The advantages are as follows: The p.d. across each lamp is fixed (at the supply p.d.), so the lamp shines with the same brightness irrespective of how many other lamps are switched on. Each lamp can be turned on and off independently; if one lamp fails, the others Resistor Colour Code Resistors have colour-coded bands as shown in Figure 4.3.12. In the orientation shown the first two bands on the left give digits 2 and 7; the third band gives the number of noughts (3) the fourth band gives the resistor’s ‘tolerance’ (or accuracy, here ±10%) So the resistor has a value of 27 000 Ω (±10%). 4.3.3 Action and Use of Circuit Components Increase in resistance of a conductor In a metal the current in a circuit is carried by free electrons. When the temperature of the metal increases, the atoms vibrate faster and it becomes more difficult for the electrons to move through the material. This means that the resistance of the metal increases. From Ohm’s law V = IR, so that if R increases then if a constant current I is to be maintained, the p.d. V across the conductor also increases. Variable Potential Divider The resistance of materials other than metals does not necessarily rise when their temperature increases. For example, in a semiconductor thermistor. The voltage across is Characteristics of thermistor: If a thermistor is part of a potential divider The voltage across is circuit then its resistance decreases when the external temperature rises. The combined resistance of the two resistors Also the ratio of the voltages across the two then decreases, resistors is so if the supply voltage remains constant, the current in the circuit will increase. Relays Small to Big Power: The relay lets a small current (from a weak signal) control a much bigger current (for a powerful device). Switching Appliances: Relays can also be used to turn on household appliances. Safety: The relay also keeps the small, safe circuit separate from the dangerous high- voltage power. In short, relays are like a bridge that lets a small signal control big power safely. Light-dependent resistor (LDR) How the LDR work? 1. Light on the LDR: When light shines on the LDR, its resistance decreases, making it easier for electricity to pass through. This causes less voltage across the LDR and more The action of an LDR depends on the fact voltage across the other components, like the that the resistance of the semiconductor relay. cadmium sulfide decreases as the intensity of the light falling on it increases (and the 2. Relay Activation: As the voltage across the current increases). relay increases, it reaches a level where the relay turns on, acting like a switch. This LDRs are used in photographic exposure switch allows electricity to flow to the bell, meters and in series with a resistor to provide making it ring. an input signal in switching circuits such as a light-operated intruder alarm. 3. When Light is Gone: If the light is removed, the LDR's resistance increases, causing the voltage across the relay to drop. If the voltage Thermistor A negative temperature coefficient (NTC) thermistor contains semiconducting metallic oxides whose resistance decreases markedly when the temperature rises. The temperature may rise either because the thermistor is directly heated or because a current is in it. 1. When the temperature rises, the resistance of the thermistor falls, and so does the p.d. across it. 2. The voltage across resistor R and the relay increases. When the voltage across the relay reaches its operating p.d. the Figure 4.3.15b shows the symbol for a normally open contacts close, so that the thermistor in a circuit to demonstrate how the circuit to the bell is completed and it thermistor works. When the thermistor is rings. heated with a match, the lamp lights. 3. If a variable resistor is used in the circuit, the temperature at which the alarm Semiconductor diode A diode is a device that lets current pass in one direction only. The wire nearest the band is the cathode and the one at the other end is the anode. The diode conducts when the anode goes to the + terminal of the voltage supply and the cathode to the − terminal (Figure 4.3.20a). It is then forward-biased; its resistance is small and conventional current passes in the direction of the arrow on its symbol. If the connections are the other way around, it does not conduct; its resistance is large Light-emitting diode (LED) LEDs are used as indicator lamps on computers, radios and other electronic equipment. Each segment is an LED and, depending on which have a voltage across them, the display lights up the numbers 0 to 9. LEDs are small, reliable and have a long life; their operating speed is high and their current requirements are very low. An LED, is a diode made from the semiconductor gallium arsenide phosphide. When forward biased the current in it makes it emit red, yellow or green light. No light is emitted on reverse bias. If the reverse bias voltage exceeds 5 V, it may cause damage. DONE 4.3 Next subtopic 4.4  4.4 Electrical Safety Dangers of electricity How to avoid electric shocks: Electric shock Turn off the power before fixing any Electric shocks happen when electricity electrical appliance. Use plugs with an earth pin and make sure flows through your body to the ground. This can occur if there's damaged insulation or the plug has a cord grip; rubber or plastic faulty wires. cases are safer. Keep appliances and cables away from water The strength of the shock depends on how —don’t use things like hairdryers with wet much electric current flows and how long it hands, and keep electrical devices away from lasts, not the voltage. baths or pools. Don’t let cables trail across rooms or under The path the current takes through your carpets where they can get damaged. Be body affects how dangerous the shock is. careful not to cut cables when using electric tools like hedge cutters. Wet conditions make shocks worse because water lowers resistance, making it easier for If someone gets an electric shock: current to flow. Wearing rubber-soled shoes Turn off the power if the person is still in or standing on a dry surface increases resistance and helps protect you. contact with the equipment. Call for medical help right away. Start CPR (chest compressions) if the person isn’t breathing or their heart has stopped. Fire risks How fires can start from electrical appliances: If flammable materials, like paper or fabric, are too close to a hot appliance (like a heater), they can catch fire. Wires in the walls can overheat and cause a fire. Wires heat up when electrical current flows through them—the bigger the current, the hotter the wire gets. To prevent fires from overheated wires: Use the correct fuse in plugs and appliances. Don’t plug too many devices into one socket or extension lead. Avoid overloading circuits by using too many adapters. Don’t plug high-power devices like heaters into circuits meant for low power (like lighting circuits). Thicker wires handle more current and are safer for high-power devices. Damaged wires or faulty insulation can cause large currents to flow through flammable Electric lighting Heating elements LED lights: In appliances like electric heaters, cookers, LEDs are becoming more common in homes. kettles, and irons, the heating part is made They are 40–50% efficient at turning from Nichrome wire, an alloy of nickel and electrical energy into light. chromium. This wire doesn't rust or break Older filament bulbs were only about 10% when it gets very hot. efficient. In radiant electric heaters, the element heats up to about 900°C and gives off heat. The Fluorescent lamps: heat is directed into the room by shiny Fluorescent strip lamps last long and are reflectors. energy-efficient. When switched on, they In convector heaters, the element heats up give off ultraviolet (UV) light inside the tube, to around 450°C and warms the air, which is which makes the powder inside glow as moved around the room by convection. visible light. Storage heaters use off-peak electricity Compact fluorescent lamps (CFLs) can fit (cheaper at night) to heat special bricks. The into regular light sockets and save energy. bricks then release the heat slowly during the day. Three-heat switch: High setting: A three-heat switch is used to control heating Both heating elements are connected in appliances with three settings: high, medium, parallel. This means that both elements get and low. the full supply of voltage. It works with two heating elements: As a result, they both heat up fully, providing 1. On high, both elements are connected in the maximum heat. parallel, meaning they both heat up (Figure 4.4.3a). Medium setting: 2. On medium, only one element heats up Only one element is turned on, so only that (Figure 4.4.3b). element heats up while the other remains 3. On low, the elements are connected in series, off. so they share the current and heat up less. This provides medium heat because only half of the heating power is being used. Low setting: The two elements are connected in series, meaning the voltage is shared between them. Since each element gets only part of the voltage, they heat up less and provide low heat. How electricity comes to our homes: Electricity comes to our homes through an underground cable with two main wires: the live (L) and the neutral (N). The neutral wire is connected to the ground (earth) at the local sub-station, so it has no voltage compared to the ground. A third wire, the earth (E), connects the top socket in your home to the ground for safety. In many countries, electricity is a.c. (alternating current), meaning the live wire switches between positive and negative. The three main wires in your home: 1.Live wire (L): This wire carries the electrical current into your home from the power station. It is dangerous because it has high voltage and switches between positive and negative in an alternating current (a.c.) system. 2.Neutral wire (N): The neutral wire completes the circuit by carrying the current back to the power station. It is connected to the ground (earth) at the local substation, so it usually has no voltage. Must watch!!! 3.Earth wire (E): Youtube Plugs and Wires The earth wire is a safety feature. It https://youtu.be/2g8SusMrX_o?si=2gxdld5bSo QyeGOe Switches Fuses Switches and fuses are always connected to the A fuse protects a circuit by being placed in the live live wire. wire. If they were connected to the neutral wire instead, It's made of a thin wire that melts and breaks the the device would still have power, even if the circuit if too much current flows. switch was off or the fuse was blown. Too much current can happen because of a short This could be dangerous because you might get an circuit (damaged wires) or too many devices using electric shock by touching something like an power. electric heater, even when it's switched off. Without a fuse, the wires could overheat and cause a fire. Ring Main Circuit Thicker wires can carry more current, but each wire A ring main connects live and neutral wires in two has a limit. loops around the house, powering all sockets. Always switch off the power before replacing a fuse, Each socket can handle 13 A. and use the same fuse value as recommended by the Current flows from both directions, allowing manufacturer (e.g., 3A for devices up to 720W, 13A thinner wires. for higher-power devices). The ring is protected by a 30 A fuse to prevent overheating. Function: The ring main distributes electricity efficiently across multiple sockets, ensuring power is shared evenly throughout the house. Watch This Video First!!! Youtube Fuses and Earthing https://youtu.be/S8lB2kxT1n0?si=oggMZme_h O8mXf2x Trip Switches (Circuit Breakers) Trip switches (or circuit breakers) are used instead of fuses to protect circuits. They use an electromagnet to break the circuit when too much current flows. This happens quickly, and they can be reset by pressing a button, unlike fuses that need to be replaced. Like fuses, circuit breakers must be set for a current just above the normal level for the device. RCD (Residual Current Device) RCDs are special circuit breakers that stop the circuit if there’s a problem with the earth connection. They detect differences in the current between live and neutral wires, and if something is wrong, they cut off power fast. RCDs are important when using things like electric lawnmowers, where the risk of electric shock is higher. Earthing Double Insulation Electrical sockets have a third wire called the Some appliances, like hairdryers or vacuum earth wire for safety. cleaners, have double insulation for safety. This wire is connected to the ground, either They have plastic cases and no earth wire. through a metal pipe or the power supply. The plastic keeps the internal electrical parts The earth wire protects you from electric separate from anything you touch, so even if shock if an appliance has a fault. something breaks, you won't get a shock. In a three-pin plug, the earth pin connects the metal parts of the appliance to the ground. If something goes wrong inside, like a wire touching the metal case, the current flows to the ground, blowing the fuse and making it safe. Without earthing, touching a faulty appliance could give you a dangerous shock, especially if you're in a damp place. DONE 4.4 Next subtopic 4.1  4.1 Simple Phenomena of Magnetism Properties of Magnets Magnetic Poles Poles are the areas of a magnet where the Magnetic Materials Some materials, like iron, steel, nickel and magnetic force is strongest. You can find the poles near the ends of a cobalt, can become magnets. We call these magnet. ferromagnetic materials. Magnetic poles always come in pairs: North When these materials are not magnets, they (N) and South (S) poles. are still attracted to magnets. North and South Poles A magnet has two poles: a North pole (N) and a South pole (S). If you let a magnet hang freely, the North pole will always point towards Earth’s North Pole. That’s why magnets can be used as compasses. Law of Magnetic Poles Like poles repel each other: N repels N, and S repels S. Opposite poles attract: N attracts S, and S Youtube What Are Magnets? How to Draw attracts N. Magnetic Field Lines : The further apart two magnets are, the https://youtu.be/3elpPfyHV0E?si=gUPViM0Yy- weaker the force between them. Induced Magnetism When a magnetic material (like iron) comes near or touches a permanent magnet, it becomes a magnet for a short time. This happens because the magnet induces (creates) magnetism in the material. Induced Magnetism Illustrations: If the iron chain is removed by pulling the Temporary magnets made of soft iron, lose top nail away from the magnet, the chain their magnetism easily Permanent magnets made of steel, retain collapses, showing that magnetism induced in iron is temporary. their magnetism When the same is done with the steel chain, it does not collapse; magnetism induced in Soft vs. Hard Magnetic Materials: Iron is called a soft magnetic material. It steel is permanent. becomes a magnet easily but loses its magnetism quickly. Youtube Permanent & Induced Magnets Steel is a hard magnetic material. It is https://youtu.be/bOZ2Hk2hKLE?si=XemYYfEKn harder to magnetize, but once it becomes a Magnetic Fields Magnetisation and Demagnetisation Magnetic field is the space surrounding a Magnetisation: magnet where it produces a magnetic force. 1. You can make a magnetic material (like iron) A magnetic field has a direction and to into a magnet by placing it inside a solenoid represent the field by lines of force. It has (a coil of wire). been decided that the direction of a magnetic When you slowly increase the direct current field at a point should be the direction of the (d.c.) in the solenoid, the magnetic material force on a N pole. becomes magnetized as the magnetic field To show the direction, arrows are put on the gets stronger. lines of force and point away from a N pole If you reverse the current, the magnetic towards a S pole. field changes direction, and so does the Strength and Interaction of Magnetic Fields magnet's poles (polarity). A magnetic field is stronger in regions where the field lines are close together than where 2. ‘Stroking’ a magnetic material several times they are further apart. in the same direction with one pole of a magnet will also cause it to become magnetized. Demagnetisation: A magnet can be demagnetised by placing it Youtube Methods of Magnetisation and inside a solenoid through which an Demagnetisation: alternating current (a.c.) is passed and https://youtu.be/Dka-cROHxBY?si=gUAIRVq3r-_ gradually reduced. Electromagnets Uses of permanent magnets & electromagnets An electromagnet is made by passing an Permanent Magnet Electromagnet electric current through a coil of wire. The strength of the magnet can be changed Can be used in Electromagnets are by adjusting the current. applications where temporary and are Unlike permanent magnets, electromagnets the magnetic field used where one does not need to be wants to be able to can be switched on and off, making them varied. vary the strength of temporary magnets. Examples: Compass, The coil surrounds a soft iron core, which the magnetic field Computer hard (by varying the becomes magnetized only when there is disks, Electric current) and switch current in the wire. motors, Electricity it on and off. generators, Examples: Cranes to The strength of an electromagnet increases if Microphones and lift iron and scrap the current in the coil increases loudspeakers, Credit metal, Electric bells, the number of turns on the coil increases and debit cards Magnetic locks, the poles are moved closer together. Advantage: does not Relays, Motors and require a current to generators maintain its magnetism Youtube Electromagnetism: https://youtu.be/79_SF5AZtzo?si=4JAjmiEKC-Q RrVJk DONE 4.1 Next subtopic 4.5 Magnetic effect of a current Field due to a straight wire Hans Oersted’s Discovery: In 1819, a scientist named Hans Oersted If the current direction discovered something surprising by accident: is known, the direction of the field can be Electricity creates a magnetic effect. predicted by the right- You can do a simple experiment to see this: hand screw rule Hold a wire over a compass, which points North-South. When you turn on the current (electricity) in the wire, the compass needle moves! If you reverse the direction of the current, the needle moves the other way. Magnetic Field Around a Straight Wire: This shows that a wire carrying current Imagine a straight wire going through the middle creates a magnetic field around it, just like a of a card. When an electric current flows through magnet has a magnetic field. the wire, you can see a cool effect: Sprinkle iron filings on the card, and they will form circles around the wire when the card is gently tapped. These circles show the magnetic field around the wire. If you place small compasses on the card, the needles will line up with the magnetic field and show its direction at different points.  Finding the North and South Poles: Field due to a circular coil To find which end is north and which is south, At the centre of the you can use the right-hand grip rule: coil the field lines Grip the solenoid with your right hand so are straight and at your fingers point in the direction of the right angles to the current (the flow of electricity). plane of the coil. Your thumb will point toward the north (N) pole of the solenoid. You can also use the right-hand screw rule for a small part of the solenoid to find the magnetic field direction. Field due to a solenoid Visualizing the Magnetic Field: A compass can be used to show the magnetic field lines around the solenoid, just like we saw with a bar magnet. Magnetic Field of a Solenoid: A solenoid is a long coil of wire shaped like a cylinder. It creates a magnetic field similar to a bar magnet. One end of the solenoid acts like the Variation of magnetic field strength The magnetic field inside a solenoid can be made even stronger by: Magnetic Field Strength Around a Wire: Using more turns of wire in the coil. The strength of the magnetic field around a Increasing the current. current-carrying wire changes with distance: The further away you are from the wire, Permanent magnets can be created by cooling molten ferromagnetic metals in these strong the weaker the magnetic field. You can magnetic fields. see this because the magnetic field lines are spread farther apart. If you increase the current (more electricity), the magnetic field gets stronger, and the field lines move closer together. When the direction of the current changes, the magnetic field also reverses direction. Magnetic Field Inside a Solenoid: Inside a solenoid (a long coil of wire), the magnetic field is stronger than outside: The field lines inside are closer together, showing a stronger magnetic field. When the current direction changes, the magnetic field also flips direction. Applications of the magnetic effect of a current What is a Relay? A relay is a type of switch that uses an electromagnet to control one circuit with another circuit. It is useful when we want a small current in one circuit to control a larger current in another circuit. Example: How a Relay Works: If the coil resistance, R, of a relay is 185 Ω and 1. A coil in the first circuit (AB) creates a its operating p.d. V is 12 V, then the pull-on magnetic field when current flows through current it. 2. This magnetic field magnetizes a soft iron core, which pulls on an L-shaped armature (a moving part). 3. The armature moves and closes the contacts in a second circuit (DE), turning it on. Pull-On and Drop-Off Current: The pull-on current is the current needed to turn the relay on. Applications of the magnetic effect of a current What is a Reed Switch? A reed switch is a type of switch controlled by a magnetic field. It has two thin strips of metal (called reeds) inside a glass tube. Reed Switch in a Burglar Alarm: Components: Reed Switch: A small glass tube containing two ferrous (iron-based) metal contacts. How a Reed Switch Works: Magnet: Positioned near the reed switch. When current flows through a nearby coil, it Alarm Circuit: Connected to the reed creates a magnetic field. switch and the alarm system. This magnetic field makes the reeds magnetic, and the opposite poles of the How It Works: reeds attract each other, closing the 1. Closed Circuit When Door Is Shut: In switch and completing the circuit (AB). most alarms, the reed switch stays closed When the current is turned off, the (circuit complete) when the door or window magnetic field disappears, and the reeds is closed because the magnet keeps the separate, opening the switch. switch closed. A reed switch can also be operated by a 2. When the Door Opens: The magnet moves permanent magnet (without current). away from the reed switch, opening the circuit. This break in the circuit triggers the  Electric Bell Applications of the magnetic effect of a current 1.Starting the Bell: When you press the bell push, the circuit closes. Current flows through the coils of the electromagnet, making it magnetized. Loudspeaker 2.Making the Sound: 1.How It Works: The electromagnet Varying electric currents from a radio or attracts a soft iron CD player pass through a coil. bar called the This coil is placed in a magnetic field armature. This pulls a created by a magnet. 3. Breaking hammerthe Circuit:that 2.Magnetic Interaction: When the hammer hits the gong, it breaks the strikes the gong, The magnetic fields from the coil and the circuit.a sound. making magnet interact. The electromagnet loses its magnetism and This causes the coil to vibrate at the same can’t attract the armature anymore. frequency as the changing electrical 4. Resetting the Bell: A spring pulls the armature back into place. signal. This reconnects the circuit, and the cycle starts 3.Creating Sound: again. A paper cone is attached to the coil. 5. Continuous Ringing: As the coil vibrates, the cone moves, As long as the bell push is pressed, the bell Force on a Current-Carrying Conductor The Motor Effect When a wire carrying current is placed in a magnetic field, it feels a force and moves. If the wire is free to move, it will move when current flows. Demonstration Imagine a wire loosely placed between the poles of a strong C-shaped magnet. When the switch is turned on, current flows through the wire, and the wire jumps up. If you reverse the current or the direction of the Youtube The Motor Effect: magnetic field, the wire moves down. https://youtu.be/ltpPhpi-CC4?si=LMRgYdJZ1FN-MHO_ The force on the wire becomes stronger if: To find direction of the force using 1. The magnetic field gets stronger. 2. The current increases. Motor Rule If the wire is not at right Explanation angles to the field, the The wire creates its own magnetic field, shown by force is smaller and is circles around the wire. zero if the wire is The magnet also has its own magnetic field (moving parallel to the field. from one pole to the other). When these two fields combine, they create a new field. There are more magnetic lines below the wire Force on beams of charged particles in a Force on Beams of Charged Particles in a magnetic field Magnetic Field In the diagram, the crosses (×) show a magnetic field going into the paper. A beam of electrons (negatively charged) enters this magnetic field at a right angle. The electrons feel a force because of the motor effect. We use Fleming’s left-hand rule to find the direction of this force. The force pushes the electrons sideways, making them move in a circular path. The beam of negatively charged electrons is treated as being in the opposite direction to conventional current. Practice!!! WB pg 71 (5), 72 (10), 74-75 (15-16) The d.c. motor Practical motors Practical motors have the following features: The motor effect shows that a straight current- A coil of many turns wound on a soft iron carrying wire in a magnetic field experiences a force. cylinder or core which rotates with the coil. If the wire is wound into a coil, forces act on both This makes it more powerful. The coil and core sides of the coil and a turning effect results when the coil carries current in a magnetic field. together are called the armature. Several coils each in a slot in the core and each Watch this video! having a pair of commutator segments. This Youtube How the Electric Motor Works: gives increased power and smoother running. https://youtu.be/evWpDrRAyCc?si=wSmuT6Qf36STTA An electromagnet (usually) to produce the field tQ in which the armature rotates. Most electric motors used in industry are induction motors. They work off a.c. (alternating current) on a different principle from the d.c. motor. The turning effect increases if: (i) the number of turns on the coil increases Moving-coil galvanometer Current enters and leaves the coil by hair springs above and below it. A galvanometer detects small currents or small When there is a current, a turning effect acts p.d.s, often of the order of milliamperes (mA) or on the coil (as in an electric motor), causing millivolts (mV). it to rotate until stopped by the springs. The greater the current, the greater the deflection which is shown by a pointer attached to the coil.  4.5 Electromagnetic Effects Electromagnetic Induction Bar Magnet and Coil A bar magnet is pushed into a coil of wire. Straight Wire and U-Shaped Magnet The meter shows a current when the magnet A wire is placed between the poles of a U- is moving. shaped magnet. When the magnet moves into the coil, the When the wire is still, nothing happens. current flows in one direction. When the wire moves up or down When the magnet is pulled out, the current (directions 1 or 2), the meter shows a flows in the opposite direction. deflection. This means a current is induced If the magnet is still, no current is detected. in the wire. The same thing happens if you move the coil The direction of the current is different instead of the magnet—what matters is the depending on whether the wire moves up or relative movement between them. down. The current only appears when the wire is moving, not when it is at rest. Key Idea A current is induced in a wire (or coil) when it moves through a Youtube Generator magnetic field or Effect / Electromagnetic when the Induction magnetic field https://youtu.be/pkzY7QfT around it  4.5 Electromagnetic Effects Faraday’s Law Direction of Induced e.m.f Faraday explained electromagnetic The induced e.m.f. (voltage) always opposes the induction by saying: change that caused it. A voltage (e.m.f.) is produced in a conductor Magnet and Coil Example: whenever it cuts across magnetic field lines When the north pole of a magnet approaches a coil: (moves through them). The coil becomes like a magnet with its top as a No voltage is induced when the conductor north pole. moves along the field lines or when it is at This repels the approaching magnet, opposing its rest. movement. If the conductor is part of a complete circuit, an induced current will flow. When the magnet is pulled away: The coil’s top becomes a south pole, trying to pull back the magnet. Three factors increase the size of the This follows the principle of conservation of energy induced e.m.f.: —the current works against the magnet’s movement, 1. Faster movement of the magnet or coil. so energy isn’t created out of nothing. The faster they move, the bigger the voltage. 2. More turns in the coil. Adding more loops of wire increases the voltage. 3. Stronger magnet. A stronger magnetic field also produces a Fleming’s Right-Hand Rule (Dynamo Rule) For a straight wire moving at right angles to a To find the direction of the induced current in a magnetic field the direction of the induced straight wire moving through a magnetic field, use current can be found from Fleming’s right- Fleming’s Right-Hand Rule: First Finger = Direction of the Field. hand rule (the ‘dynamo rule’) ThuMb = Direction of the Motion of the wire. SeCond Finger = Direction of the Current. This helps predict which way the current will flow when a wire moves across a magnetic field. Youtube: https://youtu.be/3HyORmBip-w?si=XYIt7lVXdKkZp The A.C Generator Youtube: https://youtu.be/Ylgb8FFMgd4?si=vlovIphznqn _ShmU A basic a.c. generator consists of a rectangular coil placed between the poles of a C-shaped magnet. The coil is connected to two slip rings on an axle, and carbon brushes press against the rings to allow the current to flow. How it works: When the coil rotates, it cuts through the magnetic field lines, causing an e.m.f. (voltage) to be generated. As the coil turns through different positions, the e.m.f. changes: No voltage is produced when the coil is vertical because the coil sides are moving along the field lines (not cutting them). Alternating Current (a.c.) The e.m.f. increases as the coil rotates and a.c. is current that flows first in one direction, then in the reaches its maximum when the coil is horizontal, opposite direction. where it cuts through the field lines at the fastest rate. The frequency of a.c. is how many times the current switches In the second quarter turn, the e.m.f. decreases direction each second, measured in hertz (Hz). For example, if the coil rotates twice per second, the again to zero as the coil becomes vertical. As the coil continues to rotate, the direction of the frequency is 2 Hz. The a.c. mains supply is 50 Hz, meaning it alternates 50 Practical generators Practical generators 5. Exciter: 1. The exciter is a small generator that How a Thermal Power Station Works helps the rotor have a strong magnetic 1.Fuel Source: field. It ensures that the rotor spins 1. The process starts with fuel (like coal or effectively to generate more electricity. gas) being burned in a boiler to produce 6.Condenser: heat. 1. After the steam passes through the 2.Boiler: turbine, it enters a condenser, where it 1. This heat turns water into steam. The cools down and turns back into water. steam is under high pressure. This water goes back to the boiler to be 3.Turbine: heated again. 1. The steam flows to a turbine, making it 7.Transformer: spin. This changes the heat energy into 1. The electricity produced is sent to a mechanical energy. transformer, which increases its voltage 4.Generator: for long-distance travel. Higher voltage The turbine is connected to a generator, means less energy loss. which has three main parts: 8.Power Lines: 1. Rotor: The rotor spins inside the 1. Finally, the electricity travels through generator when the turbine turns. power lines to reach homes and 2. Stator: The stator is the stationary part businesses. that contains wires. When the rotor spins, it creates a magnetic field. 3. Electricity Generation: This moving magnetic field makes electricity in the The transformer Youtube IGCSE Physics - How Transformers Work: https://youtu.be/7RtBUEZbKmI?si=5ytvXDXDu E2XL1LM Transformer Calculation: https://youtu.be/IxqUjM8cOcU?si=UPeo9670v KyG8UJH A transformer transforms (changes) an alternating voltage from one value to another of greater or smaller value.