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AQA A2 Physics A Unit 4 Magnetic Fields

Magnetic Fields

A magnetic field is a force field surrounding a magnet or current-carrying wire which acts

on magnetic materials or magnetically susceptible materials.

Magnetic field lines go from NORTH to SOUTH (a line in the direction a 'free' north pole

would move in the field). The closer together the field lines, the stronger the field.

Magnetic Fields Around a Wire

A current flowing in a wire induces a magnetic field around the wire, the field lines are

concentric circles around the wire. The direction of the magnetic field is worked out using

the right-hand rule.

Force on a Current-Carrying Wire

A current-carrying wire will experience a force if it is placed at a non-zero angle to the field

lines of another magnetic field. The force is perpendicular to the wire and to the field lines,

it is known as the MOTOR EFFECT. The field lines from the wire and the external magnetic

field interact, this causes the field lines from the external magnetic field to contract so the

field lines are closer together, causing a force on the wire.

 The magnitude of the force experienced by the wire depends on the:

• Current

• Strength of the magnetic field

• Length of the wire

• Angle between the lines of force of the field and the direction of the current.

The force is largest when the wire is perpendicular to the magnetic field. The force

is zero when the wire is parallel to the magnetic field.

Fleming's left-hand rule can be used to relate the directions of the force, current

and field. If the current or magnetic field direction is reversed, the direction of the

force will also reverse. Passing an alternating current through a wire in a magnetic

field causes the wire to vibrate as the direction of the current is reversed, causing

the direction of the force on the wire to reverse.

• First finger - Field• SeCond finger - Current

• ThuMb - Motion

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Magnetic Flux Density

Magnetic flux density, B, is the force per unit length per unit current on a current-

carrying conductor at right angles to the magnetic field lines. It is a vector quantity

and is measures in teslas, T.

The force on a wire F = BIl

Charged Particles in a Magnetic Field

Charged particles moving in a magnetic field experience a force.

The force experienced by a particle of charge Q travelling at speed v perpendicular

to the magnetic field is shown by F = BQv

Charged particles in a magnetic field follow a circular path with the force always

acting towards the centre of curvature; this is because the force on the charge fromthe magnetic field is always at right angles to the direction of motion of the particle.

This means that no work is done on the particle by the field; the speed and kinetic

energy of the particle are unchanged. The path of the particle will be a complete

circle if the magnetic field is uniform and the particle remains within the field. The

directions of the force, current or field can be worked out using Fleming's left-hand

rule (REMEMBER - current direction is always opposite to the direction of electron

movement).

The radius of the circular orbit depends on the magnetic flux density and the speed

of the particles. r = mv/BQ

The path is more curved if particles with a larger specific charge are used, if

magnetic flux density is increased or if the speed of the particles is decreased.

The frequency of rotation of a particle in circular motion f = BQ/2#m

Particle Accelerators

A cyclotron is a particle accelerator used in medicine to produce high-energy

radiation beams for radiotherapy treatments. They are made up from two hollow

semicircular electrodes in a vacuum chamber with a uniform magnetic field applied

at right-angles to the electrodes and an alternating pd applied between the

electrodes.

The charged particles are produced and directed at one of the electrodes. The

magnetic field forces them to follow a circular path and then leave the electrode. The

applied voltage accelerates the particles across the gap between the electrodes.

The particle will be travelling at a higher speed so will travel in a circular path with

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a larger radius. The applied pd will have reversed direction so the particle will be

accelerated across the gap again to the next electrode. The process repeats and

the particle continues to gain speed until it leaves the cyclotron.