Chapter 7: Magnetism and electromagnetism

Question 1

What are north- and south-seeking poles?

Answer 1

Ends of the magnet that always point towards geographical north/south

Question 2

What is a magnetic field?

Answer 2

An area where a force acts on a magnetic object or other magnet

Question 3

What is the different between a permanent and an induced magnet?

Answer 3

An induced magnet becomes magnetic when placed in a magnetic field, a permanent magnet is always magnetic. Induced magnets are always magnetised in the direction of the permanent magnet’s field, so are attracted to them

Question 4

What are magnetic field lines?

Answer 4

Lines used to represent a magnetic field. They always start at the north pole and end at the south pole. The closer the lines are to each other, the stronger the field in that location

Question 5

How do you know if a magnet is permanent?

Answer 5

By placing it near another permanent magnet and seeing if they can repel each other

Question 6

What happens when an electric current runs through a conducting wire?

Answer 6

A circular magnetic field is produced around the wire, which gets stronger as the current increases and changes direction if the current is reversed

Question 7

What is the right-hand grip rule?

Answer 7

When you put the thumb of your right hand along the direction of the current, your fingers point in the direction of the magnetic field

Question 8

What is a solenoid?

Answer 8

A long coil of wire, whose magnetic field looks like a long bar magnet’s

Question 9

How can the strength of a solenoid’s magnetic field be increased?

Answer 9

Using a larger current, using more turns of wire, putting an iron cord in the middle of the solenoid, putting the turns closer together

Question 10

What is a relay?

Answer 10

A device that uses a small current to control a much larger current in a different circuit

Question 11

What equation links force, magnetic flux density, current and length of wire

Answer 11

F = BIL
F = Force (N)
B = Magnetic flux density (Tesla, T)
I = Current (A)
L = Length (m)

Question 12

What is magnetic flux density?

Answer 12

The number of lines of magnetic flux in a given area. The stronger a magnetic field is, the more lines there are

Question 13

What is the motor effect?

Answer 13

A current-carrying wire or coil can exert a force on a permanent magnet. This is called the motor effect. The wire could also exert a force on another nearby current-carrying wire or coil. If the current-carrying wire is placed in a magnetic field (whose lines of force are at right angles to the wire) then it will experience a force at right angles to both the current direction and the magnetic field lines.

Question 14

What happens when the magnetic field around a wire combines with the magnetic field between 2 magnets?

Answer 14

A force is exerted on the wire perpendicular to the direction of the magnetic field between the 2 magnets

Question 15

How would you use the left-hand rule?

Answer 15

Place your first finger in the direction of the magnetic field, second finger in the direction of the current (positive to negative), and your thumb will point in the direction of the force. This works when the field and the current are perpendicular

Question 16

How can a motor be designed so that direct current can keep a coil rotating all the time?

Answer 16

A split-ring commutator rotates with the coil between the carbon brush contacts. When the coil passes the vertical position, the two halves of the commutator change contact from one carbon brush to the other. This causes the direction of the current to reverse, so the forces act along the coil in the same direction

Question 17

How do moving-coil loudspeakers work?

Answer 17

They are designed so that when a current flows, the paper experiences a force to the left, and when it’s reversed, the paper experiences a force to the right

Question 18

What is induced potential?

Answer 18

When a piece of conducting wire is moved through a magnetic field, a potential difference is produced across the ends of the wire. The same happens when there is a change in the magnetic field around a piece of wire

Question 19

What is induced current?

Answer 19

If a wire is part of a complete circuit, the induced potential will cause a current in the circuit

Question 20

What happens when a current is induced?

Answer 20

There is a force which opposes the movement of the magnetic field or wire, meaning you do extra work

Question 21

How does an electromagnetic flow meter work?

Answer 21

Liquid flows past a turbine with magnets attached to it. The magnets move past a solenoid, inducing a current which is detected by an oscilloscope

Question 22

What is an alternator?

Answer 22

A device that converts kinetic energy into alternating current. The graph of pD and time is a sine-wave. When the axle is vertical, the pD generated is 0

Question 23

How can the pD generated by an alternator be increased?

Answer 23

Using stronger magnets, rotating the coil faster, using more turns of wire, wrapping the wire around a soft iron core

Question 24

What is a dynamo?

Answer 24

Like an alternator but uses a split-ring commutator so the current always flows in the same direction

Question 25

What is a transformer?

Answer 25

A device that changes the current/voltage of a circuit

Question 26

How do transformers work?

Answer 26

When an alternating current is supplied to the primary coil of a transformer, a changing magnetic field is produced. The iron core becomes magnetised and carries the changing magnetic field to the secondary coil, inducing a changing pD. Transformers only work with an AC power supply.

Question 27

What is the difference between a step-up and a step-down transformer?

Answer 27

Step-up transformers have more turns on the secondary coil, increasing the voltage. Step-down transformers have more turns on the primary coil, decreasing the voltage.

Question 28

How would you calculate the voltages in a transformer?

Answer 28

Vp/Vs = Np/Ns
V = voltage
N = number of turns
p = primary coil
s = secondary coil

Question 29

What equation links voltage and current for 100% efficient transformers?

Answer 29

Vp * Ip = Vs * Is
V = voltage
I = current
p = primary coil
s = secondary coil

Question 30

How is electricity transmitted across the country in a way that reduces energy loss?

Answer 30

A step-up transformer is used to increase the voltage to 250 kV and decrease the current before it enters power cables. This is because P (power loss) = R * i^2, so by reducing the current, less energy is lost as heat. A step-down transformer reduces the voltage and increases the current before it enters homes