Ampere force

Right-hand thumb rule

Fleming's left-hand rule

Fleming's left-hand rule shows the direction of the thrust on a conductor carrying a current in a magnetic field, Wikimedia Commons CC-BY-SA-3.0

Ampere's Force Experiment

Ampere first discovered the phenomenon of a force acting between two wires. He noticed that a compass needle would deflect perpendicularly when brought close to a current-carrying conductor. His next experiments consisted of studying the force acting on two current-carrying wires by varying:

• The current passing through them

• The direction of the currents

• The distance between the wires and

• Finally, the length of the wires

He found that two parallel wires carrying current in the same direction will attract each other and repel if the directions of current passing through them are opposite. And if the two wires are placed perpendicular to each other, then the force acting between them will be zero.

Ampere's Force Equation

There is a complicated mathematical derivation for the force between two wires which you do not need to know for your GCSE exam!

We know that the Ampere's force is proportional to the length of the wire and the current passing through them. The Amperes force between two parallel wires can be obtained as follows:

Ampere's force between two parallel wires, Wikimedia Commons

If we place two wires carrying a current of $1\mathrm{A}$ parallel to each other separated by a distance of $1\mathrm{m}$. Then the force between them will be equal to$2\times {10}^{-7}\mathrm{N}$. This can also be used to define the value of $1\mathrm{A}$. We know that ampere is the standard unit of current.$1\mathrm{A}$can also be defined as the current flowing through parallel wires at a distance of$1\mathrm{m}$from each other which produces a force of$2x{10}^{-7}\mathrm{N}$.

There are a few interesting properties of the Ampere's force.

• The force is attractive in nature when the current in both the wires flows in the opposite direction.

• The force is repulsive in nature when the current flows in the same direction.

• The force is zero when the two wires are perpendicular to each other.

• The force increases as the magnitude of current in the wires increases.

• It is also inversely proportional to the distance between the wires.

Ampere's Longitudinal Force

Ampere later discovered an additional force that would act along the axis of the current-carrying wire. This force is called Ampere's longitudinal force. This tensile force tended to stretch wires carrying large currents. This has also been referred to as Ampere's tension. The wire when exposed to its magnetic field experiences a longitudinal force that stretches it.