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Which parts of a magnet are the places where the magnetic field is strongest?
poles
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What do unlike poles of a magnet do?
attract
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What do like poles of a magnet do?
repel
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Is the magnetic force an example of a contact or non-contact force?
non-contact
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What do we call something that becomes a magnet only when placed within a magnetic field?
induced magnet
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What effect does induced magnetism have?
A) Always attracts
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What is a magnetic field?
An area where a magnetic material experiences a force
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What is the direction of all magnetic fields?
North to south
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What can be used to plot the magnetic field around a magnet?
A plotting compass
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Why does a compass needle point North?
A compass needle is small bar magnet which aligns itself with the earth's magnetic field
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How does the magnetic field around a bar magnet change as you get further away from the magnet?
It gets weaker (shown by field lines becoming further apart)
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What does the strength of the magnetic field around a current-carrying wire depend upon?
- The current flowing through the wire
- The distance away from the wire
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Use two words to describe the field within a solenoid
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Draw the magnetic field around a bar magnet
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How can the magnetic field strength around a solenoid be increased?
- More turns
- Increased current
- Add an iron core
- Put the coils closer together
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Draw the magnetic field around a wire carrying a current
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What rule is used to determine the direction of the field around a current-carrying wire?
- Right-hand grip
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Draw the magnetic field around a solenoid
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Higher Only
Name the effect whereby a current-carrying wire experiences a force when placed inside a magnetic field.
motor effect
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Higher Only
Name the effect whereby moving a wire within a magnetic field induces a p.d. across the ends of the wire
Generator effect
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What rule do we use to determine the direction of the motor effect.
- Flemings left-hand rule
- (remember, we motor on the left in the UK)
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Name the three things that affect the size of the force on a current-carrying wire placed within a magnetic field
- Magnetic field strength
- Length of wire (in field)
- Current
(F = BIL)
-
What is the unit of magnetic flux density (B)?
Tesla (T)
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Higher only
Name a device, other than a motor, that relies on the motor effect.
- (moving coil) loudspeaker
- headphones
-
Higher only
Name a device, other than a generator, that uses the generator effect
microphone
-
Higher only
When a current is induced using the generator effect, the current causes its own magnetic field. What is the direction of this field?
- The direction of the induced field is such as it opposes the change that created it.
- eg in the example below, a N pole will be created nearest the magnet to repel the N pole being pushed in.
-
-
Higher only
If an alternator coil is rotated at 30 rotations per second, what is the frequency of the induced a.c. p.d.?
30 Hz - the frequency of the induced p.d. is the same as the frequency of rotation
-
Higher only
Generator effect - how can the direction of the induced current be reversed?
- Any one of :
- reverse magnetic field
- reverse direction of movement/rotation
-
Higher only
Generator effect - how can the size of the induced current be increased?
- Any of:
- stronger magnetic field
- more turns
- rotate faster
-
Higher only
Which type of generator produces d.c. ?
dynamo
-
Higher only
Which type of generator produces a.c. ?
alternator
-
Higher only
Which type of generator uses a split-ring commutator?
dynamo (for d.c.)
-
Higher only
Name the device that converts
pressure variations in sound waves
into
variations in current in electrical circuits
microphone
-
Higher only
Name the key parts of a transformer
- primary coil
- secondary coil
- iron core
-
Higher only
Why is iron used as the iron core in a transformer?
It is easily magnetised (and demagnetised)
-
Higher only
If a transformer has 10 times as many turns on the secondary coil as the primary, which relationship will be true:
A) Vs = 10 x Vp and Is= Ip /10
-
Higher only
Describe how transformers work
1. The alternating current in the primary coil produces an alternating magnetic field
2. The alternating magnetic field is linked (guided) by the iron core into the secondary coil
3. The alternating magnetic field induces an alternating p.d. across the ends of the secondary coil
-
Higher only
Why don't transformers work with d.c.?
To induce a p.d. across the secondary coil the magnetic field needs to be changing.
If the primary coil had d.c. passing through it, a constant magnetic field would be produced.
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