Every time you switch on an electric fan or a power drill, you are relying on invisible magnetic fields to create physical movement. This phenomenon is known as the motor effect, which occurs when a current-carrying conductor is placed within an external magnetic field. The interaction between the magnetic field produced by the wire's current and the external magnet's field creates a mechanical force that pushes the wire. In the laboratory, this is often demonstrated using the "jumping wire" experiment. When the power is switched on, the wire visibly jumps upwards or downwards due to the sudden force. The size of this force can be increased by using a stronger magnet, increasing the current, or placing a longer stretch of wire inside the field.
To calculate exactly how much force acts on a wire, we need to know the strength of the magnet being used. In physics, we measure this strength as the magnetic flux density, which is a vector quantity represented in diagrams by how closely packed the magnetic field lines are. The standard unit for magnetic flux density is the Tesla (T). For the motor effect to work at its maximum potential, the wire must be exactly perpendicular () to the magnetic field lines. If the wire is placed completely parallel to the magnetic field, the two fields do not interact in the right way, and the resulting force is exactly 0 N. To predict the direction of the force, scientists use . By pointing your First finger in the direction of the magnetic Field (North to South) and your seCond finger in the direction of the Current (positive to negative), your Thumb will point in the direction of the resulting Force (or motion).
Because the force is directly proportional to the magnetic strength, current, and length, we can link them together in a single equation. This calculation is strictly for Higher Tier students and is typically provided on the OCR Physics Equations Sheet.
Where:
A copper wire is placed at exactly to a magnetic field with a magnetic flux density of 0.15 T. The section of wire inside the magnetic field is 40 cm long, and a current of 3.0 A flows through it. Calculate the force experienced by the wire.
Step 1: Identify and convert the values.
Step 2: State the formula.
Step 3: Substitute the values into the equation.
Step 4: Calculate the final answer with units.
A straight wire of length 0.50 m carries a current of 2.5 A perpendicular to a uniform magnetic field. It experiences a magnetic force of 0.75 N. Calculate the magnetic flux density of the field.
Step 1: Identify the values.
Step 2: Rearrange the formula for .
Step 3: Substitute the values.
Step 4: Calculate the final answer with units.
Students often forget to convert the length of the wire from centimetres (cm) or millimetres (mm) into metres (m) before calculating, which leads to incorrect answers.
In exam questions, if a wire is placed completely parallel to the magnetic field, do not attempt a calculation — the force experienced is always exactly 0 N.
You do not need to memorise the equation as it is provided on the OCR Physics Equations Sheet, but you must be confident rearranging it to solve for , , or .
When using , check the number of significant figures in the question; your final answer should typically match the lowest number of significant figures provided.
Motor effect
The force experienced by a current-carrying conductor placed in a magnetic field due to the interaction of the two magnetic fields.
Current-carrying conductor
A material, such as a metal wire, that currently has an electric current flowing through it.
Magnetic flux density
A measure of the strength of a magnetic field, defined as the force exerted per unit current per unit length on a conductor placed perpendicular to the field.
Tesla
The standard SI unit of magnetic flux density, equivalent to one Newton per Ampere-metre.
Fleming's Left-Hand Rule
A visual rule used to determine the relative direction of the force, magnetic field, and current in the motor effect.
Put your knowledge into practice — try past paper questions for Physics A
Motor effect
The force experienced by a current-carrying conductor placed in a magnetic field due to the interaction of the two magnetic fields.
Current-carrying conductor
A material, such as a metal wire, that currently has an electric current flowing through it.
Magnetic flux density
A measure of the strength of a magnetic field, defined as the force exerted per unit current per unit length on a conductor placed perpendicular to the field.
Tesla
The standard SI unit of magnetic flux density, equivalent to one Newton per Ampere-metre.
Fleming's Left-Hand Rule
A visual rule used to determine the relative direction of the force, magnetic field, and current in the motor effect.