A magnetic field is an area around a magnet in which a force will act on a magnetic object. Magnetic fields get stronger the closer an object is to a magnet. Their direction can be plotted using a compass.
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What is a permanent magnet?
A permanent magnet produces its own magnetic field. It always has a north and south pole.
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What is an induced magnet?
An induced magnet becomes magnetic when it is placed in a magnetic field.
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True or false? An electric current produces a magnetic field around a wire.
True
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How does the right-hand grip rule?
You point your thumb in the direction of the current. The way your fingers point when you curl them shows the direction of the magnetic field.
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What do a dot and cross symbolise with regards to current?
A dot shows current flowing out of the paper. A cross shows current flowing into the paper.
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What is a solenoid?
A solenoid is a long coil of wire that emitts a magnetic field.
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State four ways of increasing the strength of the magnetic field of a solenoid.
1. Increase the current. 2. Increase the number of coils of wire. 3. Decrease the space between the coils of wire. 4. Give the solenoid an iron core.
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Explain how a car relay work?
The ignition key is turned sending a small current down a thin wire that magnetises the solenoid. The solenoid is attracted to the heavy duty iron current contacts. They connect completing the circuit so a much higher current can flow to the motor.
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What is flux density?
The number of lines of magnetic flux in a given area measured in teslas.
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How do you calculate the force on a wire perpendicular to the field lines?
Force (Newtons, N) = magnetic flux density (teslas, T) X current (amps, A) X length (metres, m)
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What is the motor effect?
The motor effect causes the movement of a wire perpendicular to a magnetic field. It does this because of an interaction between the two magnetic fields.
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What factors effect the size of the force?
1. The magnetic flux density between the two poles. 2. The size of the current. 3. The length of wire fed through the poles.
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How do you work out the direction of movement.
Use the left-hand rule. Point your first finger in the direction of the magnetic field lines. Point your seconf finger in the direction of the current. Your thumb shows the direction of the force.
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How can we use the motor effect in an electric motor?
A coil on an axis is placed inside a permanent magnet. When a current flows through the coil the sides move in alternate directions due to the magnetic force. A split ring commutator maintains the direction of current so the wire continues to move.
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What is induced potential?
When a wire is moved through a magnetic field it induces a potential difference. A potential difference is also induced if the magnetic field changes around a stationary wire. If the wire is in a complete circuit it will induce a wire.
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Describe the four stages in an alternator.
1. The coil is vertical. The sides are parallel to the magnetic field and no p.d. is induced. 2. The coils is horizontal, the sides cut through the field at the greatest rate and the maximum p.d. is induced. 3. The coils is vertical again.
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Continued
4. The coil is horizontal again but this time the sides are opposite to wear they were. The maximum p.d. is induced in the other direction.
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State four ways the p.d. size can be increased.
1. Rotate the coil faster. 2. Use stronger magnets. 3. Use more turns of wire. 4. Wrap the wire round an iron core.
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