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KS4 Physics Uses of Electromagnetism 1 of 20 32 © Boardworks Ltd 2005 2004 Contents Uses of Electromagnetism Electromagnetic devices Magnetic force of a current Electromagnetic induction Transformers Summary activities 1 2 of 20 32 © Boardworks Ltd 2005 2004 What is a relay? A relay allows one circuit to control another circuit by turning it on and off. What will happen when the switch in circuit A is closed? N coil steel switch M S A 1 3 of 20 32 B As electricity flows in circuit A, the coil becomes an electromagnet. The coil attracts the steel switch in circuit B, which then closes and completes this circuit. Electricity then flows in circuit B and operates the motor. © Boardworks Ltd 2005 2004 How does a circuit breaker work? A circuit breaker is based on the same principle as the relay. Current flows through a coil of wire. If the current becomes dangerously high, the coil creates a magnetic field strong enough to attract the iron catch. This iron catch separates from the upper contact and so breaks the circuit. 1 4 of 20 32 © Boardworks Ltd 2005 2004 Using electromagnets in switches Electromagnets can be used to open switches or to close switches. Is there a circuit breaker like this in your laboratory? 1 5 of 20 32 © Boardworks Ltd 2005 2004 How a loudspeaker uses an electromagnet Varying an a.c. through the wire makes the loudspeaker cone vibrate back and forth. The vibrations cause sounds. 1 6 of 20 32 © Boardworks Ltd 2005 2004 Uses of electromagnets 1 7 of 20 32 circuit breaker Uses one circuit to turn another circuit off and on. relay Converts electrical energy into sound energy. loudspeaker Used to break a circuit if the current is too high. © Boardworks Ltd 2005 2004 Contents Uses of Electromagnetism Electromagnetic devices Magnetic force of a current Electromagnetic induction Transformers Summary activities 1 8 of 20 32 © Boardworks Ltd 2005 2004 A current-carrying wire in a magnetic field A wire carrying a current can be made into an electromagnet and will behave like a magnet. If this wire is placed near a permanent magnet, it will experience a force. Fleming’s left-hand rule can be used to predict the direction of this force. This rule can also be used to predict the direction of any one of the following (if the the direction of the other two is known): the direction of flow of conventional current; the direction of the magnetic field (from north to south); the direction of the force exerted on the wire. 1 9 of 20 32 © Boardworks Ltd 2005 2004 Fleming’s left-hand rule To use Fleming’s left-hand rule, hold the thumb and the first two fingers of your left hand at right angles to each other. direction of force (thumb) direction of magnetic field (first finger) direction of current (second finger) 1 10ofof20 32 © Boardworks Ltd 2005 2004 Using Fleming’s left-hand rule 1 11ofof20 32 © Boardworks Ltd 2005 2004 Increasing the strength of the force Which of the following would increase the strength of the force on a wire carrying a current? 1 12ofof20 32 more current TRUE/FALSE more resistance TRUE/FALSE stronger magnetic field TRUE/FALSE © Boardworks Ltd 2005 2004 The DC motor A DC motor works using the principle of the motor effect. When a DC current flows in the coil, the sides of coil are pushed in opposite directions because of the motor effect. N S coil This results in the coil rotating. d.c. current What happens to the coil if the direction of the current or the magnetic field is reversed? The coil will spin in the opposite direction. 1 13ofof20 32 © Boardworks Ltd 2005 2004 Contents Uses of Electromagnetism Electromagnetic devices Magnetic force of a current Electromagnetic induction Transformers Summary activities 1 14ofof20 32 © Boardworks Ltd 2005 2004 Electromagnetic induction What happens if a magnet is moved into a coil of wire? A current is induced in the coil when the magnet is moving. 1 15ofof20 32 © Boardworks Ltd 2005 2004 Stationary magnet What happens when the magnet is inside the coil? Nothing! A current is only induced when the magnet is moving. 1 16ofof20 32 © Boardworks Ltd 2005 2004 Electromagnetic induction What happens if the magnet is moved out of the coil of wire? A current is induced in the coil in the opposite direction. If the direction of movement is reversed, then the direction of the induced current is also reversed. 1 17ofof20 32 © Boardworks Ltd 2005 2004 Inducing current What will happen if a magnet is moved in and out of a coil of wire repeatedly? A current will be induced in the coil. When the magnet is moved into the coil, the current will flow in one direction. Then, when the magnet is moved out of the coil, the direction of the current is reversed and flows in the opposite direction. The induced current is constantly changing direction. What type of current is this? alternating current 1 18ofof20 32 © Boardworks Ltd 2005 2004 Increasing the size of the induced current What are the four ways in which the (induced) current from an AC generator can be increased? faster movement 1. _________________________ stronger magnetic field 2. _________________________ 3. _________________________ more coils 4. _________________________ larger area of coils 1 19ofof20 32 © Boardworks Ltd 2005 2004 Contents Uses of Electromagnetism Electromagnetic devices Magnetic force of a current Electromagnetic induction Transformers Summary activities 1 20ofof20 32 © Boardworks Ltd 2005 2004 Transformers What is a transformer? A device used to increase or decrease voltage. Where are transformers used? In the national grid and household appliances. What do we call a transformer that increases voltage? A step-up transformer. What do we call a transformer that decreases voltage? A step-down transformer. 1 21ofof20 32 © Boardworks Ltd 2005 2004 How does electricity get to your home? power station national grid step-up transformer homes, factories, businesses, etc. 1 22ofof20 32 step-down transformer © Boardworks Ltd 2005 2004 Why are transformers useful? riction is generated When electrical current flows, f______ eat in the conductor which in turn generates h___. This heat is wasted e_____. nergy More wasted energy is oltages than high voltages. created at low v_______ The voltage of the current is increased before it is transferred to the n______ ational g___, rid so that less energy is lost. Then, when the current gets to homes, schools or factories, the voltage is stepped d____. own 1 23ofof20 32 © Boardworks Ltd 2005 2004 Which type of transformer? Is this a step-up or a step-down transformer? secondary primary coil coil This a step-down transformer because there are less turns in the secondary coil than the primary coil. 1 24ofof20 32 © Boardworks Ltd 2005 2004 Which type of transformer? Is this a step-up or a step-down transformer? secondary primary coil coil This a step-up transformer because there are more turns in the secondary coil than the primary coil. 1 25ofof20 32 © Boardworks Ltd 2005 2004 Transformer calculations The size of the output voltage from a transformer depends on the ratio of the turns on the primary and secondary coils. This can be calculated using the following formula: V1 V2 = N1 N2 In this formula, V is the voltage in a coil and N is the number of turns in that coil. 1 26ofof20 32 © Boardworks Ltd 2005 2004 Transformer question (1) A transformer has 200 turns on its primary coil and 50 turns on its secondary coil. The input voltage is 920 V. a) Is this a step-up or step-down transformer? step-down a) What is the output voltage? V2 V1 = V2 = V2 = = 1 27ofof20 32 N2 N1 N2 x V1 N1 50 x 920 200 230 V © Boardworks Ltd 2005 2004 Transformer question (2) A transformer has 100 turns on its primary coil. It has an input voltage of 35 V and an output voltage of 175 V. a) Is this a step-up or step-down transformer? step-up a) How many turns are on the secondary coil? N2 N1 = N2 = N2 = = 1 28ofof20 32 V2 V1 V2 x N1 V1 175 x 100 35 500 turns © Boardworks Ltd 2005 2004 Contents Uses of Electromagnetism Electromagnetic devices Magnetic force of a current Electromagnetic induction Transformers Summary activities 1 29ofof20 32 © Boardworks Ltd 2005 2004 Glossary circuit breaker – An electromagnetic switch which breaks the circuit if too much current flows. Fleming’s left-hand rule – The rule used to work out the direction of the force when a current is at right angles to a magnetic field. induction – Generating a voltage in a wire by moving a magnetic field near the wire. motor effect – The effect by which a wire carrying a current in a magnetic field moves. relay – An electromagnetic switch which allows one circuit to control another. transformer – A device which increases or decreases the voltage of alternating current electricity. 1 30ofof20 32 © Boardworks Ltd 2005 2004 Anagrams 1 31ofof20 32 © Boardworks Ltd 2005 2004 Multiple-choice quiz 1 32ofof20 32 © Boardworks Ltd 2005 2004