The Motor EffectAQA GCSE Physics: Revision notes

The motor effect is the force that acts on a current-carrying conductor when it is placed in a magnetic field. This force causes the conductor to move, and it is the principle behind electric motors.

When an electric current flows through a wire that is positioned perpendicular to a magnetic field, the magnetic field exerts a force on the moving charges in the conductor. This force is always perpendicular to both the direction of the current and the direction of the magnetic field.

Key points:

• The force only occurs if the current-carrying conductor is not parallel to the magnetic field lines
• If the conductor is parallel to the field lines, no force acts on it
• The stronger the magnetic field and the larger the current, the greater the force

Fleming's left-hand rule is a simple method to find the direction of the force on a current-carrying conductor in a magnetic field.

To use Fleming's left-hand rule:

1. Hold your left hand with thumb, first finger, and second finger all at right angles to each other
2. Point your first finger in the direction of the magnetic field (from North to South pole)
3. Point your second finger in the direction of the current (from positive to negative terminal)
4. Your thumb then points in the direction of the force (or motion)

Memory aid: Thumb = mThmotion, First finger = Field, seCond finger = Current

This rule only applies to the left hand—using the right hand gives the wrong answer. The force direction is always perpendicular to both the field and the current.

The force on a current-carrying conductor in a magnetic field is given by the equation:

F = BIl

Where:

• F = force (in newtons, N)
• B = magnetic flux density (in tesla, T)
• I = current (in amperes, A)
• l = length of the conductor in the magnetic field (in metres, m)

This equation shows that the force is directly proportional to:

• The magnetic flux density (B)
• The current flowing through the conductor (I)
• The length of the conductor in the field (l)

The equation assumes that the conductor is perpendicular to the magnetic field. If the conductor is at an angle to the field, the effective length would be reduced.

A simple d.c. (direct current) motor uses the motor effect to convert electrical energy into mechanical energy (motion).

Key components of a simple d.c. motor:

• Magnetic field (from permanent magnets or electromagnets)
• Coil of wire (called the rotor or armature) carrying current
• Commutator (a split ring that reverses the current direction)
• Brushes (carbon contacts that supply current to the commutator)
• Axle on which the coil rotates

How it works:

1. Current flows into the coil through the brushes and commutator
2. The magnetic field exerts a force on the current-carrying sides of the coil using the motor effect
3. The forces on opposite sides of the coil are in opposite directions, creating a turning effect (moment)
4. The coil rotates
5. As the coil passes the vertical position, the commutator reverses the current direction
6. This reversal keeps the forces in the same rotational direction, so the coil continues to rotate
7. The rotation continues as long as current flows

Without the commutator, the coil would rotate only halfway before stopping.

The turning effect (or moment) of a motor is the rotational force that makes the coil rotate. It can be increased in several ways:

Methods to increase the turning effect:

The most practical methods used in real motors are:

• Using electromagnets instead of permanent magnets to increase B
• Increasing the number of turns in the coil
• Using a stronger power supply to increase the current
• Increasing the radius of the coil