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Download KS4 Electricity – The Uses of Electromagnetism
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KS4 Electricity – The Uses of Electromagnetism © Boardworks Ltd 2003 Teacher’s Notes A slide contains teacher’s notes wherever this icon is displayed - To access these notes go to ‘Notes Page View’ (PowerPoint 97) or ‘Normal View’ (PowerPoint 2000). Notes Page View Normal View Flash Files A flash file has been embedded into the PowerPoint slide wherever this icon is displayed – These files are not editable. © Boardworks Ltd 2003 Relays A relay allows one circuit to control another circuit – turning it on and off. What will happen when the switch in circuit A is closed? N Coil The coil becomes an electromagnet as electricity flows in circuit A. Steel switch M The coil attracts the steel switch in circuit B and completes the circuit. S A B Electricity flows in circuit B and the motor now works. © Boardworks Ltd 2003 Circuit Breakers Circuit breakers are also based on this principal. The current flows through a coil of wire. If the current becomes dangerously high, the coil creates a magnetic field strong enough to… …cause the iron or steel switch to open… …which breaks the circuit. © Boardworks Ltd 2003 Using electromagnets in switches Electromagnets can be used to OPEN switches or to CLOSE switches. There could be one of these circuit breakers in your laboratory! © Boardworks Ltd 2003 Loudspeakers Varying an AC through the wire makes the cone of the loudspeaker vibrate back and forth. The vibrations cause sounds. © Boardworks Ltd 2003 Using electromagnets Circuit breakers Relays Loudspeakers Convert electrical energy into sound energy Using one circuit to turn another circuit off and on Used to break a circuit if the electrical current is too high © Boardworks Ltd 2003 A current carrying wire in a magnetic field We know that a wire carrying a current can be made into an electromagnet. Consequently, such a wire will behave like a magnet. If this wire is placed near a permanent magnet, it will experience a force. We can use Flemings left hand rule to predict the direction of this force. In fact, we can also use it 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. © Boardworks Ltd 2003 Flemings Left Hand Rule Force (thumb) Field (first finger) Current (second finger) © Boardworks Ltd 2003 Animation of a current carrying wire in a magnetic field © Boardworks Ltd 2003 How could you increase the strength of the force? More current TRUE/FALSE More resistance TRUE/FALSE Stronger TRUE/FALSE magnetic field © Boardworks Ltd 2003 The DC Motor Uses Motor effect principle. How does it work? Each side of coil pushed in opposite direction due to motor effect. This results in the coil rotating. N S What happens if you reverse the direction of The coil spin in current orwill magnetic opposite direction. field? © Boardworks Ltd 2003 Electromagnetic induction What will happen if you move a magnet into a coil of wire? A current is induced in the coil. Only when the magnet is moving. © Boardworks Ltd 2003 Stationery magnet What happens when the magnet is in the coil? Nothing!! A current is only induced when the magnet is moving. © Boardworks Ltd 2003 Electromagnetic induction What will happen if you move the magnet out of the coil of wire? A current is induced in the coil. This time in the opposite direction. If you reverse the direction of the movement, then you reverse the direction of the current. © Boardworks Ltd 2003 AC or DC? What will happen if you move a magnet in and out of the coil repeatedly? A current will be induced in the coil, the current will flow in one direction and then reverse and flow in the opposite direction. What type of current is this? Alternating current © Boardworks Ltd 2003 Increasing the size of the current How can you increase the size of the current? more coils stronger magnetic field faster movement © Boardworks Ltd 2003 AC generator What are the four ways you can increase the current from an AC generator? 1. 2. 3. 4. Faster movement _____________________ Stronger magnetic field _____________________ More coils _____________________ Larger area of coils _____________________ ? © Boardworks Ltd 2003 Transformers What is a transformer? A device used to increase or decrease voltage Where are transformers used? On the National Grid, household appliances What do we call a transformer that increases voltage? STEP-UP Transformer What do we call a transformer that decreases voltage? STEP-DOWN Transformer © Boardworks Ltd 2003 How electricity gets to your home…. Power station Step-up transformer National Grid Step-down transformer Homes, businesses and factories etc © Boardworks Ltd 2003 Why bother with transformers? riction in When electrical current flows you get f_____ eat the conductor. With friction you always get h___. nergy You get more wasted This heat is wasted e____. oltages than high voltages. So the energy at low v______ current is increased in voltage before it is ational G__. rid In this way less transferred to the N_____ energy is lost. Then, when it gets to your home, a own school or a factory, the voltage is stepped d___. © Boardworks Ltd 2003 Step-down More/less turns on the primary coil than the secondary coil. Primary Secondary © Boardworks Ltd 2003 Step-up More/less turns on the primary coil than the secondary coil. Primary Secondary © Boardworks Ltd 2003 Transformer calculations The size of the output voltage depends upon the ratio of the turns on the primary and secondary coils. Guess what? There’s a formula (it is Physics after all!) - V – voltage V1 = N1 V2 = N2 N - number of turns © Boardworks Ltd 2003 Transformer Question 1 A transformer has 200 turns on its primary coil and 50 turns on its secondary coil. The input voltage is 920V. a) Is is a step-up or step-down transformer? Step-down a) What is the output voltage? 230V © Boardworks Ltd 2003 Transformer Question 2 A transformer has 100 turns on its primary coil. It has an input voltage of 35V and an output voltage of 175V. a) Is is a step-up or step-down transformer? Step-up a) How many turns are on the secondary coil? 500 turns © Boardworks Ltd 2003 What happens if you reverse the direction of current in an electromagnet? A. The magnetic field is increased B. The voltage increases C. The current increases D. The direction of the magnetic field is reversed © Boardworks Ltd 2003 What happens if a wire carrying a current is placed in a magnetic field? A. The current is increased B. The magnetic field is reversed C. The magnetic field is increased D. The wire will experience a force © Boardworks Ltd 2003 Which below will not increase the strength of an electromagnet? A. More coils B. More current C. Less current D. Iron core © Boardworks Ltd 2003 Which use of electromagnets is similar in function to a fuse? A. Relays B. Transformer C. Circuit breaker D. Loud speaker © Boardworks Ltd 2003 Which of the following is not a use of electromagnets? A. Fridge magnets B. Loud speakers C. Relays D. Circuit breakers © Boardworks Ltd 2003 A transformer has 30 turns on its primary coil and 240 turns on its secondary coil. If the output voltage is 128V, what is the input voltage? A. 8V B. 16V C. 1024V D. 512V © Boardworks Ltd 2003