Forces between magnets and currents
Definition
Motor effect
The force that acts on a current-carrying conductor when it is placed across the field lines of a magnetic field.
Definition
Current-carrying conductor
A wire or other piece of conducting material that has an electric current flowing through it.
Definition
Magnetic field
The region around a magnet in which another magnet or a magnetic material experiences a force.
Definition
Interaction pair
The two forces produced by one interaction, one acting on each of the two interacting objects.
- A current-carrying conductor is surrounded by its own magnetic field, made of circular field lines centred on the wire.
- This field exists only while charge is flowing, so a wire with no current in it has no magnetic field of its own.
- When the conductor is placed between the poles of a permanent magnet, the field of the wire and the field of the magnet overlap in the same region of space.
- The two magnetic fields interact, and this interaction produces a force on the conductor.
- This effect is called the motor effect.
- The force is a non-contact force, because the magnet and the wire act on each other through their fields without touching.
- The magnet feels a force as well, because a force always arises from an interaction between two objects.
- The force on the magnet is equal in size to the force on the conductor and acts in the opposite direction, so the two forces form an interaction pair, as Newton's third law describes.
- In most arrangements the magnet is held still and only the conductor is free to move, so the conductor is the object you see moving.
- If the conductor were clamped in place and the magnet were free to move, the magnet would move instead, in the opposite direction.
- Like every other force, the motor-effect force is measured in newtons (N\text{N}N).
The combined magnetic field
Definition
Magnetic field line
A line drawn to represent a magnetic field, whose arrow shows the direction of the force on a north pole placed at any point along it.
- Between the flat, opposite poles of two magnets, the magnetic field lines run straight across from the north pole to the south pole, evenly spaced and parallel.
- Around the wire, the field lines form concentric circles, and the direction of these circles depends on the direction of the current.
- On one side of the wire, the circular field lines point the same way as the magnet's field lines, so the two fields add together and the combined field is stronger.
- On the opposite side of the wire, the circular field lines point the opposite way to the magnet's field lines, so the two fields partly cancel and the combined field is weaker.
- The combined field lines are therefore crowded together on the strong side of the wire and spread out on the weak side.
- The conductor is pushed out of the region of stronger field towards the region of weaker field.
- The crowded field lines behave like the stretched elastic of a catapult that flings the wire towards the weaker side, which is why this pattern is often called a catapult field.
- The magnet is pushed the opposite way by the same interaction, which is the equal and opposite force on the magnet.
When the force acts
- There must be a current in the conductor, because without a current the wire has no magnetic field to interact with the field of the magnet.
- The conductor must lie across the magnetic field lines rather than along them.
- The force is largest when the conductor is at right angles to the magnetic field lines.
- The force becomes smaller as the conductor is turned towards the direction of the field lines, and it is zero when the conductor is parallel to them.
- Reversing the direction of the current reverses the direction of the force on the conductor.
- Reversing the direction of the magnetic field, by swapping over the poles of the magnet, also reverses the direction of the force.
- Reversing both the current and the field at the same time leaves the force acting in its original direction.
- Switching the current off removes the force straight away, so the conductor is no longer pushed.
Practical
Observing the force on a wire
- Aim: to show that a magnet and a current-carrying conductor exert forces on one another, and to find how the direction of the force depends on the current and the magnetic field.
- Apparatus: two parallel horizontal copper rails fixed about 5 cm5\ \text{cm}5 cm apart on a wooden base, a short bare copper rod that rests across the rails, two flat magnets on a U-shaped steel yoke, a low-voltage d.c. power supply, a switch, a variable resistor, an ammeter, connecting leads with crocodile clips, a top-pan balance and a clamp stand.
- Method, the rolling rod:
- Rub the rails and the rod with fine emery paper until they shine, because a dull, oxidised surface stops the current passing between them.
- Clip a lead to the end of each rail and connect the rails in series with the switch, the variable resistor, the ammeter and the power supply, so the only path for the current from one rail to the other is through the loose copper rod.
- Slide the magnet yoke over the rails so the rod sits between the poles, with the north pole above and the south pole below, so the magnetic field passes vertically through the rod.
- Set the supply to about 2 V2\ \text{V}2 V, close the switch for a moment, and adjust the variable resistor until the ammeter reads about 2 A2\ \text{A}2 A.
- Close the switch for only a second or two, watch which way the rod rolls along the rails, and then open the switch.
- Swap the two leads at the power supply so the current flows through the rod the other way, close the switch again, and note the direction the rod rolls.
- Turn the yoke over so the poles are swapped, return the leads to their original positions, and repeat.
- Finally swap both the leads and the poles together and repeat once more.
- Method, the force on the magnet:
- Stand the magnet yoke on the top-pan balance and press the tare button so the reading is zero.
- Clamp a stiff straight copper wire horizontally so it passes through the gap between the poles without touching the yoke or the balance.
- Connect the wire into the same circuit, close the switch and read the balance.
- Reverse the current and read the balance again.
- Results:
- The rod starts to roll the moment the current flows and is no longer pushed once the switch opens.
- Reversing the current, or reversing the magnetic field, makes the rod roll the opposite way.
- Reversing both the current and the field makes the rod roll in its original direction.
- The balance shows a positive reading for one direction of current and a negative reading of the same size for the other, even though nothing touches the magnet, which shows that the wire exerts a force on the magnet.
- Watch out: the rails and rod warm up quickly at large currents, so keep the switch closed only briefly, and make sure the rod lies straight across both rails so it does not jam.
- Safety: use only a low-voltage supply, let the rod and rails cool before touching them, and handle the magnets carefully because they can snap onto the steel yoke and trap fingers.
Common Mistake
- Do not say that the wire is attracted to one pole of the magnet, because the force acts across the field, at right angles to the line joining the poles.
- Do not say that the current pushes on the magnet directly, because the current produces a magnetic field and it is the interaction of the two fields that produces the force.
- Do not forget the force on the magnet, which is equal in size and opposite in direction to the force on the wire.
Exam technique
Explaining why a wire moves
- A two-mark explanation needs a cause and an effect: the current in the wire produces a magnetic field, and this field interacts with the field of the magnet to produce a force.
- Name both fields in your answer, because writing "the magnetic field" on its own does not tell the examiner which field you mean.
- When a question asks about the forces on both objects, state that the force on the magnet is equal and opposite to the force on the wire.
- When asked how to make the wire move the other way, give one change, such as reversing the current, because reversing the current and the field together leaves the force unchanged.
Self review
- What is the motor effect?
- Explain why a wire with no current in it feels no force between the poles of a magnet.
- How does the force on the magnet compare with the force on the current-carrying wire?
- At what angle to the field lines is the force on a wire largest, and when is it zero?
- What happens to the force when both the current and the magnetic field are reversed?
