Two questions, and only one of them can be measured

Put a wire carrying a current between two magnets and it gets pushed. That is the whole of the motor effect, and it is the reason your washing machine turns. The topic feels arbitrary to most people because two completely different questions get answered in the same breath. How big is the push? That one is measured. You cannot see a magnetic field, so every single thing anybody knows about this came from somebody putting a wire on a balance and changing one quantity at a time. The formula is not three separate facts to learn: it is the summary of three experiments. Which way does the push go? That one cannot be measured by size at all. No reading on any balance tells you a direction, and the answer turns out to be strange enough that people need a hand to remember it. So carry one question through the module: am I being asked how big, or which way? Getting that straight also explains the one bit of a motor that looks like an arbitrary extra part. How a field is plotted in the first place, and how electromagnets work, belong to the other module on this topic.

Five terms, used exactly

The first two are about size and the last three are about direction, which is the division the whole module is built on.

Run the measurement properly

A class is measuring the force on a current-carrying wire. Put the method into the order it has to be done.

Set a length of wire lying across the field between two magnets
Stand the magnets on a balance and note the reading with no current flowing
Switch on a known current and note the new reading on the balance
Take the difference between the two readings, which gives the size of the force
Change the current, leaving the magnets and the length of wire exactly as they were
Repeat for several currents, then plot force against current

Testing the second variable

A student has shown that doubling the current doubles the force. They now want to find out whether the force also depends on how much wire is in the field. What must they do?

Change only the length of wire in the field, keeping the current and the magnets exactly as they were, because otherwise they could not tell which change caused any difference in the force
Double the current again, since that is what produced a change last time
Change the length and the current together, so the effect is large enough to measure reliably
Nothing, because the force already depends on current and so cannot also depend on length

Three variables, one at a time

These are the three experiments the formula summarises. Each one changes a single quantity and holds the other two still, and each one turns out to give a straight line through the origin.

Five results from the same bench

A wire lying along the field direction feels no force at all, however large the current
Reversing the current AND swapping the two magnets over leaves the force pointing exactly where it was
Only the part of the wire actually between the magnets seems to contribute to the push
A motor coil built without a split ring turns half a turn and then sticks
Two students measure forces of very different size but agree exactly about the direction

The push acts at right angles to both the current and the field, and when those two are already in line there is no such direction left for it to act in
Each of those reversals on its own flips the direction, so doing both flips it twice and lands it back where it started
The force is exerted where the field is, so what counts is the length sitting inside the field and not the length of the whole circuit
Once the coil is past the upright position those same forces would turn it back again, so the current has to be swapped over at exactly that moment
Size depends on the field, the current and the length, which they set up differently, while direction depends only on which way those two things point

Work backwards to the field

A straight wire has 0.4 metres of its length inside a magnetic field. It carries a current of 6 amps and experiences a force of 1.2 newtons. The force is equal to the magnetic flux density multiplied by the current multiplied by that length. What is the magnetic flux density?

Bigger, smaller, or no change

A wire lies across a magnetic field and feels a force. Decide what each change does to the SIZE of that force. Three lives.

The half you cannot measure

No reading on any balance will ever tell you which way the push goes, so the direction has to be worked out rather than measured. And the answer is genuinely odd: the force acts at right angles to both the current and the field at once. Not along either of them. That is why people need three fingers held apart to keep track of it. Hold the left hand with thumb, first finger and second finger all at right angles. First finger points along the Field. seCond finger points along the Current. The thuMb then points along the Motion, which is the direction of the force. The capital letters are the memory trick and they are worth writing in a margin before you start. And now the part that looks like an arbitrary extra component. In a motor the two sides of a coil carry current in opposite directions, so they get pushed opposite ways, and the coil turns. Half a turn later those same two sides have swapped places, and the same forces would now turn it straight back. So the current has to be swapped over at exactly that moment, and the split-ring commutator is the thing that does it. It is not a separate fact. It is this module question, which way, asked about a coil instead of a straight wire.

Two claims to strike out

Five statements from a set of revision notes. Tap the TWO that are wrong.

Doubling the current doubles the size of the force
Only the length of wire actually inside the field counts
The force acts along the direction of the magnetic field
Reversing both the current and the field reverses the direction of the force
A stronger magnetic field gives a bigger force for the same current

Inside a simple d.c. motor

Here is a simple d.c. motor with every part named. A coil of wire sits between the N and S poles of a magnet, where the magnetic field runs from N to S, and a d.c. supply drives a current round the circuit. The current gets into the turning coil through two brushes, small blocks that press on the split-ring commutator and stay in contact while it spins. Follow the current arrows. The current runs away from you along the left side of the coil and towards you along the right side, so the two sides are pushed in opposite directions: down on the left, up on the right. Check the left side with Fleming's left-hand rule: first finger along the field, second finger along the current, and your thumb points down. That pair of opposite forces is what turns the coil.

The revolving door

A revolving door has two quite separate things you might want to know about a push, and mixing them up is what makes people stand there looking foolish. How hard you push decides how fast it goes round, and anybody can feel the difference between a gentle shove and a hard one. Which way it goes has nothing to do with how hard. It depends entirely on which panel you put your hand on and which face of it you push. Shove as hard as you like on the wrong face and the door goes backwards, briskly. And here is the bit worth carrying across. Keep your hand in the same place as the door turns, and after a quarter of a turn your panel has swung round past you. Push in the same direction now and you are pushing it back the way it came. To keep the door going one way you have to let go at exactly the right moment and take hold of the next panel. Nobody thinks of that as a separate skill. It is the same question about direction, asked again once things have moved.

Put the proper words back

The strength of a magnetic field is called its magnetic _____ density and is measured in _____. The size of the force on a current-carrying wire is _____ to the current, to the length of wire inside the field, and to the strength of the field. The direction of that force is at right angles, or _____, to both the current and the field, which is what the left-hand rule is for. Reversing either the current or the field _____ the direction of the force, and in an electric motor a split-ring _____ swaps the current over every half turn so that the coil keeps going the same way.

flux tesla proportional perpendicular reverses commutator gauss newtons parallel armature

Label the d.c. motor

Tap a label, then tap the pin for the part of the motor it names.

Make the push bigger

A straight wire sits across a magnetic field and feels a force. Select every change that would increase the SIZE of that force.

Replacing the magnets with stronger ones
Increasing the current through the wire
Arranging for a greater length of the wire to sit inside the field
Reversing the direction of the current
Turning the wire so that it lies along the field direction

Four decisions at the bench

A student works through the motor effect over one afternoon. Each stage sets out where things stand and hands you the decision.

The student has a wire between two magnets on a balance, and wants to find out how the force depends on the current. Decide what to change and what to leave alone.
Later the student lays the wire so that it points along the field direction rather than across it, and records no force at all, however high the current goes. Decide what that result shows.
The student then reverses the direction of the current AND swaps the two magnets over, both at the same time. Decide what happens to the direction of the force.
Finally the student builds a motor coil with plain slip rings instead of a split ring. It turns half a turn and then sticks, rocking back and forth. Decide what is happening and what would fix it.

Build the two-part answer

Assemble the sentence that answers both halves of a motor effect question at once.

Explain both halves of the push

A wire carrying a current is placed between two magnets and experiences a force. Explain what decides how big that force is, explain what decides which way it acts, and explain why an electric motor needs a split-ring commutator.

Name the three quantities the size of the force depends on, and say how you would investigate one of them
Explain why only the length of wire inside the field counts
Explain which way the force acts relative to the current and the field, and how the direction is worked out
Explain why a wire lying along the field direction feels no force at all
Finish with the motor: say what would happen after half a turn without a commutator, and what the commutator does about it

The Motor Effect Lab

Get the method right under pressure

What you'll cover