Magnetism and Current-Carrying Wires
An electric motor: current + magnet = spin
- Inside every electric motor, a current-carrying wire sits in a magnetic field — and gets pushed.
- Because a current is just moving charges, the field pushes the whole wire sideways.
- Reverse the current every half-turn and that push becomes continuous rotation.
- This "motor effect", and its reverse, power almost all our machines.
The motor effect
- A wire carrying current $I$ and length $L$ in a field $B$ feels a force $F = BIL$.
- Like the force on a single charge, it is perpendicular to both the current and the field.
- Reverse the current or the field and the force flips direction.
- Bigger current, stronger field, or longer wire all give a bigger force.
Force on a current
Change the field, current and length and watch the motor-effect force respond.
A $0.5\ \text{m}$ wire carries $4\ \text{A}$ across a $2\ \text{T}$ field. What force does it feel, in $\text{N}$?
$F = BIL = 2 \times 4 \times 0.5 = 4\ \text{N}$.
The force on a current-carrying wire in a magnetic field is:
$F = BIL$ — the motor-effect force on a current.
Reversing the current through the wire reverses the direction of the force.
Reversing the current (or the field) flips the force direction.
Select all changes that increase the motor-effect force on a wire.
$F = BIL$ grows with field, current and length. Parallel current gives zero force.
Fleming's left-hand rule
- Point your left hand: first finger = field, second finger = current, thumb = force.
- Hold them at right angles and they give the three perpendicular directions.
- It is the quick way to find which way the wire is pushed.
- (Use the left hand for the force on a current; the right hand was for a positive charge's velocity.)
Which hand's rule gives the force on a current-carrying wire?
Fleming's left-hand rule gives the force on a current: field, current, force.
A current makes its own field too
- The reverse is also true: a current-carrying wire creates a magnetic field around itself.
- Coil the wire and the fields add to make a strong electromagnet.
- Switch the current off and the field vanishes — a controllable magnet.
- Electromagnets lift scrap cars, ring doorbells and drive loudspeakers.
A coil of current-carrying wire acts as an ____.
A coil's magnetic fields add to make a strong, switchable electromagnet.
Use the left hand (Fleming's left-hand rule) for the force on a current-carrying wire — it is easy to grab the wrong hand. And the force needs the current to run across the field: a wire carrying current parallel to the field feels no force.
A wire of length $0.5\ \text{m}$ carries $4\ \text{A}$ across a $2\ \text{T}$ field.
- $F = BIL = 2 \times 4 \times 0.5 = 4\ \text{N}$.
Double the current to $8\ \text{A}$ and the force doubles to $8\ \text{N}$.
A current-carrying wire in a magnetic field feels the motor-effect force $F = BIL$, perpendicular to the current and field (Fleming's left-hand rule). Reversing the current or field flips it. A current also creates a field — the basis of the electromagnet.