Last time, we saw that a current-carrying wire will experience a force in a magnetic field, as long as the current is not parallel to the magnetic field lines.

Now, we’ve seen previously that force is… 

We’ve seen previously that force is a vector.

So it has direction and magnitude.

But how do we work out the magnitude and direction of the force on a current-carrying wire in a magnetic field?

Well, first, let’s look at the direction.

For example, suppose we put a current-carrying wire in this magnetic field.

The wire would experience a force in this direction.

And if we reversed the current, it would experience a force in this direction.

And if we reversed the magnetic field, it would experience a force in this direction.

So, the force on a current-carrying wire in a magnetic field is always perpendicular to both the current and the field lines.

And if both the current and the field lines are horizontal, then we know that the direction of the force is going to be vertical.

But how do we know whether the force acts in this direction…or this direction?

Well, we could memorise the direction of the force for every possible combination of current and field lines… but that would be quite confusing!

Thankfully, there is a handy  rule, which tells us the direction of the force for any combination of current and field lines. 

Here’s how it works…

Take your left hand, and make it into this shape

Line up your first finger with the field lines.

And your second finger with the current.

Then your thumb gives the direction of the force.

So, if we had a current-carrying wire in a magnetic field, like this, the force would act in this direction.

But if we reversed the current, the force would act in this direction.

And this time, the force would act in this direction.

So now, in which direction would this wire experience a force?

This wire would experience a force in this direction.

And this handy rule is called Fleming’s left hand rule. It was created in the 19th century by the engineer John Ambrose Fleming to help work out which way an electric motor would spin.

It works for any combination of current, magnetic field and force, you just might need to rotate your hand in a weird way!

Now, in your exam, the questions won’t tell you which finger is which.

But thankfully there’s an easy way to remember!

The ‘f’ in ‘first finger’, tells us it represents field lines.

And the ‘c’ in ‘second finger’ tells us it represents current.

Then the ‘th’ in ‘thumb’ tells us it represents ‘thrust’, which is another word for force.

Technically, thrust is a particular type of force created by things like jet engines, but if it helps you remember which finger is which, then go for it!

So now, which of these diagrams show the correct direction of the force on the wire?

These diagrams show the correct direction of the force on the wire.

Finally, sometimes you might see the direction that electrons are moving in a wire…but remember, the current is in the opposite direction… because the direction of conventional current is defined for positive charges. 

So, to sum up…

The force on a current-carrying wire is always perpendicular to the current and the magnetic field lines.

And to find the direction of the force, we can use Fleming’s left hand rule, which shows…

In Fleming’s left hand rule…

Your first finger shows the direction of the field lines.

And your second finger shows the direction of the current.

Then your thumb shows the direction of the force.