A –Level Physics: Magnetic Fields Electric Motors

Objectives:

FLASHBACK Semiconductors: increase the free electron density with an increase in heat or light (thermistor/LDR). This reduces resistance, allowing current to increase. Filament wires increase resistance with increasing energy (voltage) thus gradually stopping an increase in current. FLASHBACK: Explain how semiconductors can be used in circuits to control the current and how they differ from a filament bulb in this respect (6 marks)

Starter Activity Recall and explain the difference between magnetic flux and magnetic flux density. Include all relevant equations

What effect does this have? Flux Linkage Often a magnetic field (flux) is utilised by interacting it with a coil of wire. What effect does this have? The amount of magnetic flux interacting with a coil of wire is known as flux linkage and is simply the: “product of the quantity of flux in a particular region (ɸ) and the number of turns on the coil (N) that the region interacts with” I.e. Flux Linkage = N ɸ Or as ɸ = BA, then Flux Linkage= BAN 5.1 x 10-10 N

Motor Effect When a wire with a circulating current is placed within a magnetic field, the wire will experience a force upon it. This is known as the motor effect. Draw the structure below carefully and with detail/labelling. We will refer back to it a lot!

Fleming’s Left Hand Rule We can use this rule to figure out the direction that the force will be produced upon the wire Orientate your hand in to this form. To do so, make a fake ‘gun’ shape and then lift your second finger up

Fleming’s Left Hand Rule This is the wire, notice that the direction shown for the current flowing through the coil is going away from you. Point your second finger away from you B This is the magnetic field, it is always shown going from north to south so in this instance is going to the right. Point your first finger to the right Now, your thumb shows the direction of the motion (force) ThuMb=Motion First= Field SeCond= Current

Which direction is the wire forced (motion)? Practice Which direction is the wire forced (motion)? a) Left b) Left c) Towards you (out of the screen)

Motor Effect Now, let’s imagine we have a coil of wire instead of just a length. What will happen to the coil of wire in this case and why?

Motor Effect As the direction of the current going through one branch of the ring is different to the other, the force produced on each branch are opposites. This creates a torque (turning) force. So the wire spins! You’d create a current that changes direction known as an alternating current! What would be produced if you physically turned the coil within the field?

Write an A4 page on the micromotor and nanomotor Practice and I/S Write an A4 page on the micromotor and nanomotor Pg 65 of blue text book