The Motor Effect

If a current passing through a wire in a magnetic field exerts a force on the wire this is called the motor effect. This is always the case unless the magnetic fields and wire are running in parallel. The force can increase in two ways:

• – if a stronger magnet is used
• – if the current is increased

The size of the force depends on the angle at which the magnetic field lines and the wire are:

• – if the wire and magnetic field lines are perpendicular then the force is at its largest
• – if the wire and magnetic field lines are parallel there’s no force

An electric motor uses the motor effect. Its speed can be altered by changing the current. If the current is reversed the speed is reversed also.

• The armature coil is connected to a battery by two brushes made of graphite or metal.
• The brushes are connected to a split-ring commutator.
• The split-ring commutator is fixed to the rectangular coil.

The armature coil is forced to rotate when a current passes through it. It acts like this because:

• – due to the motor effect the force is acting on either side of it
• – the forces acting on either side are forcing the coil in opposite directions

Each half-turn the direction in which the coil is travelling is reversed by the split ring commutator. As the sides are swapping each time the coil moves in the same direction continuously.

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The National Grid

The higher the potential difference is the larger the efficiency is to transfer electrical power from the grid. This is the reason why:

• – the potential difference is increased when transferred from the power station to the National Grid by step-up transformers
• – the potential difference is decreased when transferred from the National Grid to the mains by step-down transformers

The secondary potential difference (p.d.) is dependant on the primary potential difference as well as the number of turns on both of the coils.

V_P/V_S = N_P/N_S

• – V_P is the p.d of the primary
• – V_S is the p.d of the secondary
• – N_P is the number of turns on the primary
• – N_S is the number of turns on the secondary

• In a step-up transformer: V_S is more then V_P and N_S is more than N_P.
• In a step-down transformer: V_S is less than V_P and N_S is less than N_P.

Transformer are nearly 100% efficient:

• – power supplied to a transformer = primary current x primary p.d.
• – power delivered by a transformer = secondary current x secondary p.d.

If a transformer is 100% efficient:

• – primary current x primary p.d. = secondary current x secondary p.d.
• I_PV_P = I_SV_S