P3.3 Keeping things moving

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Electromagnets

An electromagnet is made of insulated wire wrapped round an iron bar, this is the core. When a current is passed through the wire, a magnetic field is created around it. As a result, the magnetic field of the wire magnetises the iron bar strongly. When the current is switched off, the iron bar loses its magnetism. Iron easily loses magnetism when the current is switched off.

Electromagnets are used in scrapyard cranes to lift up vehicles. They are also used as circuit breakers in a switch series, the switch is held in place with an iron catch, when too much current flows the switch is pulled open by the electromagnet and opens. An electric bell is connected to a battery, an iron armature is pulled on to the electromagnet this opens a make and break switch and the electromagnet is switched off.

A relay is used to switch on an electrical machine. When a current passes through the electromagnet the armature is pulled onto the electromagnet turning the pivot and closing the switch gap, this means a small current is used in the switch and it is therefore safer to use.

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The motor effect

The motor effect is when a force is experienced by a wire that is carrying a current in a magnetic field.

The size of this force can be increased by increasing the current or using a stronger magnet. The size of the force depends of the angle between the wire and the magnetic field lines. the force is greatest when the wire is perpendicular to the magnetic field. The force is zero when the wire is parallel to the magnetic field lines. The direction of the force is always at right angles to the wire and the field lines. Also, the direction of the force is reversed if the direction of the current/ magnetic filed is reversed.

Fleming's left hand rule can be used to find out the field, current or movement. It is when three fingers are held at right angles to each other. The thumb is the movement of the wire, first finger is the field and second finger is the current.

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Uses of the motor effect

An electric motor uses the motor effect, this consists of a rectangular coil of insulated wire connected to the battery by two 'brushes'. The brushes press onto a metal 'split-ring commutor'. When a current passes through the coil it spins, this is due to the force acting on each side of the coil due to the motor effect. In addition to this the force on one side is in the opposite direction to the force on the other side. The split ring commutator reverses the current round the coil every half turn.

A loudspeaker can also use the motor effect, it is designed to make the diaphram attatched to a coil vibrate when an alternating current passes through. When a current passes through the coil a force due to the motor effect makes the coil move

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Electromagnet induction

Electromagnetic induction is the process of creating a voltage using a magnetic field. The takes place when a conductor e.g. a wire cuts across the magnetic field lines. If the wire is part of a complete circuit, an electric current is passed through the circuit.

When a bar magnet is pushed into an insulated coil of wire a current flows throught the circuit. This is because the movement of the bar magnet through the coil causes a potential difference to form, this induced potential difference then causes a current. If the magnet was held at rest inside the coil of wire, then no potential difference would be formed as no movement occured. However if the magnet was inserted throught the other end of the coil the current would flow in an opposite direction. To increase the size of the current formed more coils could be added and the magnet could be stronger.

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Transformers

There are two main types of transformers; step up and step down They both contain two coils, the primary and the secondary, joined with an iron core. Step-up transformers step the voltage up. They have more turns on the secondary coil than the primary coil. Step down transformers step the voltage down. They have more turns on the primary coil than the secondary.

An alternating current passing through the primay coil produces an alternating magnetic field within the iron core. This means it passes through the secondary coil.Because there is A.C. in the primary coil, the field in the iron core is constantly changing direction therefore a changing voltage is induced in the secondary coil. The relative number of turns on the two coils determinds whether the potential difference induced in the secondary coil is geate or less than the p.d. in the primary.

If a D.C  was supplied there would be no voltage in the secondary coil. This is because it is the changing magnetic field that induces the changing potential difference. Without this only a magnetic field would be made in the iron core.

The iron core carries the magnetic field in the transformer and has no electricity flowing through it.

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Switch mode transformers

Switch mode transformers are also a type of transformer. These are lighter and smaller than a traditional transformer and uses very little power when there is no device connected across output terminals. This makes them suitable for use in mobile phone chargers. These types of transformers have ferrite cores. These are lighter than iron cores and can work at higher frequency. In addition to this they are more efficient.

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