The field around a wire and a solenoid, how to strengthen an electromagnet, the motor effect with F = BIL, Flemings left hand rule, and how an electric motor works.
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1.What happens when a current flows through a wire?
A magnetic field is produced around the wire.
2.What shape is the field around a straight wire?
Concentric circles around the wire, at right angles to it.
3.What two things affect the strength of that field?
The size of the current and the distance from the wire.
4.What is a solenoid?
A coil of wire carrying a current.
5.Why is the field of a solenoid strong?
The fields from each turn of the coil add together inside it.
6.What is the field inside a solenoid like?
Strong and uniform, with the field lines parallel and evenly spaced.
7.What does the field outside a solenoid look like?
The same shape as the field around a bar magnet.
8.Give three ways to strengthen an electromagnet.
Increase the current, increase the number of turns on the coil, and add an iron core.
9.What is the advantage of an electromagnet over a permanent magnet?
It can be switched on and off, and its strength can be varied.
10.Give a use of an electromagnet.
Scrapyard cranes, electric bells, and relays.
11.What is the motor effect?
When a conductor carrying a current is placed in a magnetic field, the two fields interact and the conductor experiences a force.
12.When is the force in the motor effect greatest?
When the conductor is at 90 degrees to the magnetic field.
13.When is the force zero?
When the conductor is parallel to the magnetic field.
14.State the equation for the force on a conductor.
force = magnetic flux density x current x length
15.What is the unit of magnetic flux density?
The tesla, T.
16.A wire of length 0.5 m carries 3 A in a field of 0.2 T. Calculate the force.
0.2 x 3 x 0.5 = 0.3 N.
17.State Flemings left hand rule.
With the left hand, the first finger points along the field, the second finger along the current, and the thumb shows the direction of the force or motion.
18.Which way does the field finger point?
From north to south.
19.What happens to the force if the current is reversed?
The force acts in the opposite direction.
20.What happens to the force if both the current and the field are reversed?
The force acts in the original direction, because the two reversals cancel out.
21.How does an electric motor work?
A current carrying coil in a magnetic field experiences forces in opposite directions on its two sides, which makes it rotate.
22.What is the purpose of the split ring commutator?
It reverses the direction of the current every half turn, so the coil keeps turning the same way.
23.What would happen without a commutator?
The coil would turn half a turn and then stop or oscillate, because the forces would push it back.
24.Give three ways to make a motor turn faster.
Increase the current, use a stronger magnetic field, or increase the number of turns on the coil.
25.How does a loudspeaker work?
An alternating current in a coil in a magnetic field produces a varying force, which moves a cone back and forth and creates sound waves.
26.Why does a loudspeaker need an alternating current?
The force must change direction repeatedly to make the cone vibrate rather than simply move one way.
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