S-Cool Revision Summary

S-Cool Revision Summary

Force on Parallel Wires

In two parallel wires carrying current:

-like currents attract

-unlike currents repel.

Learn the shapes of the fields.

Current flowing in opposite directions:

 Forces in Magnetic Fields

Currents flowing in the same direction:

 Forces in Magnetic Fields

The Motor Effect

When two magnets are close together, they affect each other and produce a force. The same happens when any two magnetic fields are close together.

If a wire carrying a current is placed in a magnetic field a force is produced. This is called the motor effect.

The direction of the force will depend on the direction of the magnetic field and the direction of the current in the field.

To make the force bigger:

  1. Increase the size of the current.
  2. Increase the strength of the permanent magnet.

Fleming's Left Hand Rule

Use Fleming's Left Hand Rule to get the direction of the force.

Second finger - conventional current

First finger - field direction

Thumb - thrust or force direction.

This can also be used to find the direction of force on a single charge travelling in a field.

Forces on Charged Particles

When a wire carrying a current through a field feels a force it is because the magnetic field pushes the electrons inside the wire to one edge of the wire. These electrons actually then apply force to the wire.

The same effect occurs if the electrons are not inside a piece of wire - for example, if they are in a beam crossing a vacuum.

We can calculate the force on a charged particle in a magnetic field using the equation:

F = Bq v sin θ


F = force (N)

B = magnetic field strength (T)

θ = charge on the particle (C)

v = velocity of the particle (m/s)

Note: θ is the angle between the direction of the beam and the magnetic field direction.

Use Fleming's left hand rule to work out the direction of the force. Align your second finger with the beam of particles remembering that it points the way positive particles flow, the opposite way to electron flow.

Finding the Charge-to-Mass Ratio of a Particle

When a charged particle enters a magnetic field we now know it will be forced to change direction. If it stays in the field it will continue to change direction and will move in a circle. The force produced will provide the centripetal force on the moving particle.


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Copyright S-cool

This idea is used in velocity-selectors, where particles of different mass-to-charge ratio will rotate in circles with a different radius.

The Hall Effect

The force acting on charged particles moving through a conductor in a magnetic field can become balanced by an equal but opposite force due to the build up of charge on the edges of the conductor.

The Hall Effect

This results in a p.d. across the width of the conductor which is proportional to the strength of the magnetic field around the conductor. This is the Hall Effect. It can be used to measure the strength of a magnetic field because the size of the pd set up is directly proportional to the magnetic field strength.


  Magnetic Field Strength   Force on a Beam of Charged Particles
F= force, N F= force, N
B= magnetic field strength or magnetic flux density, T B= magnetic field strength or magnetic flux density, T
I= current, A θ= the angle between the magnetic field and the current.
l= length, m Q= the charge on the particle, C
θ= the angle between the magnetic field and the current. v= the speed of the charged particle, ms-1