As a result of the close interactions between magnetic fields and electric fields it is possible to produce a flow of current in a conductor by the movement of the conductor through a magnetic field. This process of creating a current with a magnetic field is called Electromagnetic induction.
As the wire ‘cuts’ the lines of the magnetic field the electric field around the electrons interacts with this magnetic field to produce a force. This force makes the electrons move creating an electrical current in the wire. The direction of the current depends on the direction of the magnetic field and the direction of motion of the wire, as illustrated in the animation below.
The magnitude of the current depends on the strength of the magnetic field (magnetic flux density), the speed of movement of the wire and the length of wire within the magnetic field.
Moving the wire up and then down in the magnetic field produces an alternating current in the wire. The change in the direction of movement creates a change in the direction of the current. This can also be achieved by holding the wire still and moving the magnet or changing the north and south poles over. As long as the wire and the magnetic field are moving relative to each other a current will be induced in the wire.
On a small scale this can demonstrated using a horseshoe magnet with a reasonable magnetic field strength and a length of wire connected to a sensitive ammeter or galvanometers. As the wire is moved inside the magnet field the ammeter will register a small alternating current in the wire.
Although this experiment only produces a small current the same principle is used in commercial electricity generation, but using stronger electromagnetic, long lengths of wire in a coil and by spinning the magnet inside the coil of wire to gain the movement, this is done at very high revolutions to maximise the current induced in the wire coil.
An alternator is a device that converts mechanical energy, in the form of kinetic energy into electrical energy via electromagnetic induction. An alternator generates an alternating current and is the type of generator is used for mains electricity generation from power stations (nuclear, fossil fuels and hydroelectric) and wind turbines.
To generate an electric current a conducting wire or coil must move relative to a magnetic field. The simplest way to achieve this is to spin the magnet inside the wire coil. This is both cheaper and technically less complicated to build and maintain.
In a large scale alternator used in a power station the spindle connected to the magnet is connected at the other end to the turbines that are driven by the steam or water to make the turbine spin.
As the magnet is spinning relative to the stationary wire the magnetic field is changing direction relative to the wire, so an alternating current is produced.
A dynamo is a device that converts kinetic energy to electrical energy via electromagnetic induction. A dynamo generates direct current. The kinetic energy, in the form of a rotating spindle moves a coil of wire within a magnetic field. To allow the output to be DC rather than
AC, the output wires connect to the external circuits via a commutator. This allows the flow outwards to the external wires to only ever be connected to one side of the coil within the dynamo. The structure is similar to that of an electric motor, in a some older cars the starter motor was used as a dynamo to recharge the DC battery once the engine was running.
The spindle in the centre of the coil of wire must be connected to a machine that provides some kinetic energy, such as a petrol engine.
In a bicycle dynamo the spindle is connected to a gear that rests against the wheel rim to gain kinetic energy.
- What causes a current to flow when a conductor is moved relative to a magnetic field?
- electrical field
Explanation: As the magnetic field passes through the conductor it interacts with the electrical field around the electrons, this causes a force that results in the movement of the electrons in the conductor. This forms a current in the conductor.
- State 3 ways an alternator can be made to produce a higher current.
- Your answer should include: strong magnet / stronger magnet / coils / wire / spun / spin
Explanation: A higher current will be generated from an alternative if a stronger magnet is used, if the alternator has more coils of wire within the magnetic field or if the magnet is spun at a higher speed.
- Explain how an alternator is different from a dynamo.
- alternating current
Explanation: An alternator produces alternating current, whereas a dynamo generates direct current. Generally in an alternator the kinetic energy is used to rotate a magnet inside a coil of wire, in a dynamo the kinetic energy is used to rotate a coil of wire within a magnet. A dynamo has a commutator connecting its output wires to the external circuit, an alternator does not have a commutator.