The Motor Effect Lab

The Motor Effect Lab

Electricity and magnetism are two sides of one thing. Whenever a current flows through a wire, a magnetic field appears around that wire, whether you wanted one or not. Around a long straight wire that field is a set of concentric circles, centred on the wire and getting weaker the further out you go. Reverse the current and the circles run the other way. Turn the idea round and you get the motor effect: the push that spins every electric motor, from a phone on vibrate to an electric car. This module builds from one to the other.

The compass test

A compass is placed just above a straight wire. When the current is switched on, the needle swings clockwise away from north. The current is then reversed. What happens to the needle?

• It swings the other way, because reversing the current reverses the direction of the field
• It swings the same way, because the size of the current has not changed
• It settles back to north, because the two currents cancel out
• Nothing happens, because a straight wire has no magnetic field

Coils and cores

Wind the wire into a coil and the fields from every loop add together. Four words cover everything this specification asks about that:

Too weak to lift

A scrapyard electromagnet is not strong enough to lift the load. Select ALL THREE changes that would make it stronger.

• Winding more turns of wire onto the same coil
• Increasing the current through the coil
• Replacing the plastic former inside the coil with an iron bar
• Spreading the same number of turns over a longer coil
• Adding a thicker layer of insulation to the wire
• Painting the coil a darker colour

Say it back

A coil of wire carrying a current is called a _____, and its field looks like that of a bar magnet: strong and roughly _____ inside the coil, with a pole at each end. Put an iron _____ inside it and the field becomes far stronger, giving an electromagnet, which can be switched off because it is only magnetic while the _____ flows.

The motor effect

Now the payoff. Put a current-carrying wire inside a magnetic field and the two fields interact, pushing the wire sideways with a force. That push is the motor effect. Its direction comes from Fleming's left-hand rule. Hold the thumb and first two fingers of your left hand at right angles to each other: • thuMb = Motion (the force on the wire) • First finger = Field (north to south) • seCond finger = Current (positive to negative) Two things go wrong with it in exams: people use the right hand, and people forget that the field arrow runs from north to south. Get either wrong and the force comes out backwards.

Point your fingers

A wire carries a current directly away from you, into the page. The magnetic field runs from left to right across the page. Using Fleming's left-hand rule, which way is the force on the wire?

• Downwards, in the plane of the page
• Upwards, in the plane of the page
• Out of the page, towards you
• Along the wire, in the direction of the current

What happens if you reverse it?

A favourite exam move is to reverse something and ask what the force does. Fleming's rule answers it in a second, and the pattern is worth knowing on its own:

Both at once

A wire in a magnetic field is pushed upwards. An engineer reverses the current AND swaps the magnets round so the field also reverses. Which way is the wire pushed now?

• Upwards still, because the two reversals cancel each other out
• Downwards, because two changes must mean a bigger change
• There is no force at all now
• Sideways, at right angles to the original push

Work out the force

A wire of length 2 m carries a current of 3 A at right angles to a field of flux density 0.5 T. Using F = B I l, what is the force on the wire, in newtons (N)?

Find the current

A 0.4 m length of wire sits at right angles to a field of flux density 1.5 T and feels a force of 6 N. What current is flowing, in amps (A)?

Force against current

A 4 m length of wire sits at right angles to a field of flux density 0.5 T, and the current through it is varied. Work out the force at 1 A, 2 A and 3 A, then plot all three points. The x-axis is current in amps and the y-axis is force in newtons.

The problem a motor has to solve

A motor is a coil of wire in a magnetic field. Current flows, and because the two sides of the coil carry current in opposite directions, they feel forces in opposite directions. The coil turns. Then it reaches vertical, and the trouble starts. Carry on with the current flowing the same way and the forces now act to pull the coil back, so instead of spinning it would rock to a halt like a pendulum. Every simple motor faces this, and the fix has to arrive twice per revolution. The split-ring commutator is that fix. It is a ring cut into two halves, so that as the coil turns past vertical the contacts swap over and the current through the coil runs the other way.

How a motor spins

Put the stages of a simple d.c. motor into order, starting from the current switching on.

• Current flows through the coil, which is sitting in the magnetic field
• The two sides of the coil feel forces in opposite directions
• That turning effect rotates the coil
• As the coil passes vertical, the commutator contacts swap over
• The current in the coil reverses, so the forces keep pushing the same way round

Lab summary

A current in a wire makes a magnetic field. Wound into a solenoid with an iron _____ it becomes a switchable electromagnet. A current-carrying wire in a field feels a force, the motor effect, whose direction comes from Fleming's _____-hand rule and whose size is F = B I _____. Reversing the current or the field alone flips that force, while reversing both leaves it _____. In a motor the split-ring commutator reverses the current every half turn so the coil keeps spinning.