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12.3.1 The motor effect

The force on a current-carrying conductor

Definition

Motor effect

The motor effect is the force experienced by a current-carrying conductor when it is placed in a magnetic field that is not parallel to the current.

  1. Place a wire between the poles of a magnet and nothing happens while the circuit is open. Close the switch so that a current flows and the wire jumps.
  2. That jump is the motor effect, and it needs three things present at once: a magnetic field, a current in the conductor, and the conductor lying inside the field.
  3. Remove any one of the three and the force disappears. No current means no force, and a wire held outside the field feels nothing either.
  4. There is one more condition. The conductor must not lie parallel to the magnetic field. A wire lying along the field lines feels no force at all, and the force is greatest when the wire is at right angles to the field.
  5. The force acts on the conductor at right angles to both the field and the current, so a horizontal wire in a horizontal field is pushed straight up or straight down.
  6. Reversing the current turns the force round, and reversing the magnetic field turns the force round as well. Reverse both together and the force ends up back where it started.
  7. The wire does not have to touch the magnet. The force appears wherever the two fields overlap, which is what makes this a non-contact effect.

Why the force appears

  1. A magnetic force is always the result of an interaction between magnetic fields. Two fields have to be present before any force can act.
  2. In the motor effect the two fields are:
    1. the field of the permanent magnet, running from its north pole across to its south pole
    2. the circular field produced by the current in the conductor, wrapping round the wire
  3. Where the two fields share the same space they combine. On one side of the wire the circular field runs the same way as the magnet's field, so the two reinforce and the combined field there is strong.
  4. On the opposite side of the wire the circular field runs against the magnet's field, so the two partly cancel and the combined field there is weak.
  5. The result is a lopsided pattern with the field lines bunched up on one side of the wire and thinned out on the other. This is often called a catapult field.
  6. The wire is pushed out of the crowded region and into the region where the field has been weakened, in the same way that a stone is flung out of a catapult.
  7. Reversing the current reverses the circular field, so the crowded side and the thinned side swap over and the wire is pushed the other way.
  8. Saying only that the magnet pushes the wire is not a full explanation, because it leaves out the field that the current itself produces.
Key Idea

The force in the motor effect comes from the interaction of the magnet's field with the field produced by the current, not from the current touching or being attracted by the magnet.

The force on the magnet

  1. The conductor is not the only object that feels a force. At the same moment, an equal and opposite force acts on the magnet.
  2. The two forces have three features in common and one difference:
    1. they are equal in size
    2. they are opposite in direction
    3. they are the same type of force, both magnetic
    4. they act on different objects, one on the conductor and one on the magnet
  3. So if the wire is pushed upwards, the magnet is pushed downwards with a force of exactly the same size.
  4. The two forces do not cancel each other out. Forces only cancel when they act on the same object, and here each force acts on a different one.
  5. In a school demonstration the magnet usually stays put while the wire flies up. That is because the magnet is heavy and clamped down, so other forces from its support balance the magnetic force on it.
  6. Rest the magnet on a top-pan balance instead and the reading changes the instant the current is switched on, which is direct evidence that the magnet really is being pushed.
Example

Reasoning through a wire that lifts

  • A stiff copper wire hangs horizontally between the poles of a clamped horseshoe magnet. When the switch is closed the wire swings sharply upwards.
  • The current in the wire produces its own circular magnetic field around the wire.
  • The magnet produces a second field across the gap, running from its north pole to its south pole.
  • The two fields overlap in the gap. Below the wire they act in the same direction and reinforce, while above the wire they act in opposite directions and partly cancel.
  • The combined field is therefore stronger below the wire than above it, so the wire is pushed upwards, out of the strong region and into the weak one.
  • At the same time an equal downward force acts on the magnet, and the clamp holding the magnet supplies the extra force that keeps it still.
  • Swapping the two supply leads reverses the current, the crowded and thinned regions swap over, and the wire now kicks downwards instead.
Practical

Demonstrating the motor effect with a kicking wire

  • Aim: to show that a current-carrying conductor in a magnetic field experiences a force, to show that the force reverses when the current or the field is reversed, and to show that the magnet experiences a force too.
  • Apparatus: a length of bare stiff copper wire, two horizontal brass or copper rails on insulating supports, a strong horseshoe magnet or a pair of slab magnets on a steel yoke, a low-voltage direct current supply, a rheostat, an ammeter, a switch, a top-pan balance, a clamp stand and connecting leads.
  • Variables: the direction of the current and the direction of the field are the independent variables, the direction in which the wire moves is the dependent variable, and the magnet, the wire, the current and the position of the wire in the gap are all kept the same.
  • Method, setting up:
    • Clamp the magnet so that its two poles face each other across a horizontal gap, with the field running straight across that gap.
    • Lay the two rails through the gap and rest the bare copper wire loosely across them at right angles to the field, so it can roll or swing freely.
    • Connect the rails in series with the switch, ammeter, rheostat and supply, and set the rheostat to its highest resistance before switching on.
    • Mark the poles of the magnet with N and S, and note on paper which way the conventional current will travel along the wire.
  • Method, observing the force:
    • Close the switch for no more than a second and record which way the wire moves.
    • Open the switch, swap the two leads at the supply, close it again and record the new direction of movement.
    • Restore the original connections, turn the magnet round so that N and S are swapped, and record the direction of movement again.
    • Finally reverse the current and the magnet together, and record that the wire moves the same way as it did at the start.
    • Turn the wire so that it lies along the field instead of across it, switch on and note that it does not move.
  • Method, showing the force on the magnet:
    • Stand the magnet on the top-pan balance and zero the reading, then clamp the wire firmly so that this time it is the wire that cannot move.
    • Close the switch briefly and watch the balance reading change, then open it and watch the reading return to zero.
    • Reverse the current and note that the reading now changes the other way, showing that the force on the magnet has also reversed.
  • Expected pattern: the wire kicks out of the gap while the current flows and stops the moment it is switched off. Reversing either the current or the field reverses the kick, reversing both restores the original direction, and a wire lying along the field does not move at all. The balance reading confirms that the magnet is pushed the opposite way.
  • Watch out: a wire that sticks to the rails because of dirt or oxide, a wire too heavy to move, a magnet that is not clamped so that both objects shift at once, and confusing the direction of conventional current with the direction the electrons drift.
  • Safety: the bare wire carries a large current and heats up quickly, so switch on only in short bursts and never leave the circuit closed. Keep the rheostat in circuit at all times, do not short the supply, and keep faces clear in case the wire flies out of the gap.
Exam technique

Explaining the motor effect

  • Build the answer as a chain: the current produces a magnetic field, the magnet produces a magnetic field, the two fields interact, and a force therefore acts on the conductor.
  • The word interact is worth writing explicitly, because it is what turns a description into an explanation.
  • Write current-carrying conductor rather than wire, since that is the wording the question will use.
  • The mark students most often drop is the one for the magnet. Add the sentence that an equal and opposite force acts on the magnet whenever the question mentions both objects.
  • If asked why the magnet does not visibly move, say that it is held in place and that other forces from its support balance the magnetic force, rather than saying no force acts.
Common Mistake
  • Do not say that only the conductor feels a force. Both the conductor and the magnet experience forces.
  • Do not say the equal and opposite forces cancel out. They act on different objects, so the conductor can still move.
  • Do not explain the force by saying that the magnet attracts the wire. Copper is not magnetic, and without a current there is no force at all.
  • Do not forget the orientation condition. A conductor lying parallel to the magnetic field experiences no force however large the current is.
Self review
  • State the three conditions needed for a conductor to experience a force in the motor effect.
  • Name the two magnetic fields that interact when a current-carrying wire is placed near a magnet.
  • Explain how the combining of those two fields produces a force on the conductor.
  • Describe the force that acts on the magnet and state how it compares with the force on the conductor.
  • Explain why these two forces do not cancel one another out.
  • State what happens to the force when the current is reversed, and when both the current and the field are reversed.
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12.3.1 The motor effect Revision Guide

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Revision notes for Edexcel GCSE Physics 12.3.1 The motor effect: explanations and worked examples.

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