Magnetism and magnetic forces

Current offcoil

Molecular magnets aligned randomly

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Current oncoil

Magnetic field lines

(Representing magnetic flux Φ)

Magnetic field lines

(Representing magnetic flux Φ)

Molecular magnets aligned North to South

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Flux density

Flux lines

Flux density (B) is the amount of flux (Φ)

(represented by flux lines) passing

perpendicularly through a given area

(A)

B = Φ ÷ A

Φ = B x A

Force on a conductor

• When a current flows through a conductor in a magnetic field a force acts on the conductorThe direction of the force depends on the

direction of the magnetic field and the direction of the current.

These directions can be found using Fleming’s Left Hand Rule

This is called the motor effect

Direction of current

Cross sections of conductor

(wire)

Current coming out of page(like an arrow coming towards you)

Current going into page(like an arrow

going away from you)

Force on a conductor

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Field Direction

Current coming out to page

direction of Force (movement) up

Force on a conductor

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Field Direction

Current going in to

page Force (movement) Down

Electric motor effect

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If the conductor is part of a coil with the current going into the coil on the right and out on the left, the coil will spin ( as per an

electric motor)

Force on a conductor

Force = Flux density (B) x current (I) x length of conductor in magnetic field (L)

F = B x I x L

EMF induced in a conductor

• If a conductor is moved through a magnetic field an EMF (electro-motive- force) is induced

in the conductor which causes a current to flow in the direction of the force.

• The directions can be found using Fleming’s right hand rule

• This is called the generator effect

EMF induced in a conductor

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Field Direction

Current (from induced EMF)

going into page

Movement up through the field

EMF induced in a conductor

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Field Direction

Current (from induced EMF)

coming out to pageMovement down though field

EMF induced in a conductor

EMF induced in a conductor

The magnitude of the EMF (hence current) induced depends on the rate at which the

conductor ‘cuts‘ through the flux lines or ‘the rate of change in flux:

E = -dФ/dtThe minus sign means that the induced emf opposes

the change in flux (Lenz’s Law)

EMF induced in a conductor

E = -dФ/dtdФ/dt = dBA/dt (Ф = BA)

= dBLxL/dt ( L x L = A)E = BLv (v (velocity) = dL/dt

B = flux density L = length of conductor in field v = velocity of conductor through field