Motor effect and electromagnetic induction

​​In a nutshell

A current-carrying conductor placed in a magnetic field will experience a force. This is the motor effect. Electromagnetic induction of the production of electricity when a wire moves relative to a magnetic field or experiences a changing magnetic field. This is the generator effect.

Equations

Variable definitions

The motor effect

The motor effect occurs when a current-carrying conductor (e.g. a wire) is placed in a magnetic field. A current produces a magnetic field, and this interacts with the magnetic field that the conductor is placed in. The magnet and the conductor exert a force on each other.

1. Direction of current 2. Magnetic field lines 3. Direction of force

Fleming's left hand rule

Fleming's left hand rule can be used to determine the direction of the force on the current-carrying conductor.

When the current is at right angles to the magnetic field lines the force exerted on the conductor will be at a maximum. No force is exerted on a conductor that is placed parallel to the magnetic field lines. 

The thumb, index finger and middle finger of the left hand represent the directions of each component.

1. Middle Finger Direction of the current. 2. Index Finger Direction of the magnetic field lines. 3. Thumb Direction of force exerted on the wire.

​

Note: Fleming's rule uses conventional current. This goes from the positive (+) to negative (-) terminal of a cell. This is opposite to the direction of the flow of electrons in a circuit.

Magnetic force equation

The force exerted on a conductor due to a magnetic field can be found using this equation.

​$$force = magnetic \space field \space strength \times current \times length \space of \space wire \\ \ \\ F = B \times I \times l$$​

This means that the size of the force depends on the size of the magnetic field, the size of the current, and the length of the wire that's in the magnetic field.

Electric motors

Electric motors work by using the motor effect. This is a basic dc motor:

1. Current-carrying loop of wire (clockwise) 2. Split ring commutator 3. Upwards force produced on left side of loop 4. Downwards force produced on right side of loop 5. Clockwise motion of the loop of wire

A current-carrying coil of wire can be placed in a magnetic field between two opposite magnetic poles. A force, due to the motor effect, is exerted on the sides of the coil that are perpendicular to the magnetic field lines. No force is exerted on the sides that are parallel.

The forces act in opposite directions on each side of the coil. These forces act as either both clockwise or both anticlockwise moments, and produce a turning effect on the coil. The coil is attached to a spindle so is able to turn.

Once the coil completes one half turn, the current will appear to flow the other way. This is where the split ring commutator comes in. It switches the current, by swapping the polarity of the dc supply, so that it appears to flow in the original direction. This keeps the coil turning in the same direction.

The direction that the motor will spin can be found using Fleming's left hand rule.

Example

1. Direction the loop turns

The field lines are directed towards the left, from north to south pole. The current goes from positive to negative so on the right side of the coil, it flows upwards. Therefore, using Fleming's left hand rule, the force goes into the page. This means the current turns anticlockwise.

Tip: Only one side of the coil needs to be analysed using the left hand rule. This is because, although the force on the other side will act in the opposite direction, it produces a turning effect in the same direction!

The generator effect (electromagnetic induction)

The generator effect is caused by electromagnetic induction. Electromagnetic induction is the production of a potential difference when a conductor experiences a change in magnetic field. This can be used to create a current if the wire is part of a complete circuit.

There are two ways the generator effect can be shown. The first is by moving a magnet inside a coil of wire. The second is by moving a conductor in a magnetic field. Both methods produce a current in the wire.

	1. Coil of wire 2. Magnet 3. Movement of magnet inside coil
	1. Conductor (long, straight wire) 2. Movement of conductor

Moving the magnet back and forth creates an alternating current, as the potential difference produced keeps swapping directions. Rotating the magnet end-to-end also produces an alternating current. This is because the magnetic field in the coil changes. From this a potential difference is produced. The direction of the magnetic field reverses after the magnet completes a half turn.

The current produced is known as the induced current. 

To increase the size of the induced current:

• The conductor or magnet can be moved faster.
  
• Stronger magnets can be used.
• More loops of wire can be used

Generators

A generator uses a turning force on a coil in a magnetic field to produce an alternating current.

1. Slip rings 2. Brushes 3. Direction the loop of wire spins 4. Direction of the current produced in the wire 5. Alternating potential difference produced

A generator is similar to a motor but doesn't use a split-ring commutator. Instead it uses slip rings to allow the coil to spin, and brushes to create a completed circuit. As the direction of the force is not switched each half turn, an alternating current is produced.