# Magnetic fieldsMagnetic fields

Magnetic fields are regions where a force is exerted on magnetic materials. They are created by moving charges and permanent magnets. Magnetic fields can be mapped by magnetic field lines, which pass from north to south, The closer the

lines, the stronger the field. Field lines are equally spaced an in the same direction if the field is uniform. Check the textbook/internet for the diagrams of magnetic fields created by a wire, a loop of wire and a coil.

Fleming’s Left Hand Rule: allows you to predict the direction of current, motion or a magnetic field when current in a conductor is permanent to the field lines in a uniform magnetic field. First=field, second=current, thumb=motion.

The force on a current-carrying conductor is proportional to the magnetic flux density, B. It also only depends on the component of the current perpendicular to the wire. Hence, it is given by:

Magnetic flux density is a measure of the strength of a magnetic field. It is defined as the force on one metre of wire when a current of one amp flows perpendicular to the magnetic field. It’s a vector quantity whose unit is the tesla, T. It’s like the number of field lines per unit area.

You can determine uniform magnetic flux density using a digital balance. A square hoop of wire is positioned such that the top of the hoop, length l, passes through the magnetic field, perpendicular to it. When current flows, the wire experiences a downwards force. If the balance is zeroed when no current flows, the balance will indicate the force exerted on the scale. A variable resistor can be used to vary the current. If a graph of F against I is plotted, the gradient will be equal to Bl.