Electric Motors and Electromagnetic Induction

Question 1

How can the force of the motor effect be made stronger?

Answer 1

If the magnetic field is made stronger or if the current is increased.

Question 2

What does Flemings left hand rule tell you and how does it work?

Answer 2

thuMb - the direction of the MOTION of the force.
First finger - the direction of the FIELD from North to South.
seCond finger gives the direction of the current in the wire.

Question 3

How does the moving-coil loudspeaker use the motor effect?

Answer 3

Signals (currents) from a source such as an amplifier are fed into a coil of the speaker that is wrapped around a magnet. These currents that are continually changing size and direction the overlapping fields of the coil and the magnet therefore create rapidly varying forces on the wire of the coil, which cause the speaker cone to vibrate. These create the sound waves that we can hear.

Question 4

How does an electric motor use the motor effect?

Answer 4

As current passes around the loop of wire one side will experience a force pushing it upwards, the other side downwards, so the loop will rotate. Due to the split ring commutator when the loop is vertical (p. 189 for picture) the connections to the supply through the carbon brushes swap over (i.e. positive becomes negative) and the current flowing through each side of the loop changes direction. The wire at the bottom is now pushed upwards and at the top downwards - the loop carries on turning. Every half turn the direction of current changes so the rotation can be continuous.

Question 5

How can we increase the rate at which the motor turns?

Answer 5

1) increase the number of turns or loops of wire.
2) increase the strength of the magnetic field.
3) increase the current flowing through the loop of wire.

Question 6

How do practical motors differ from the simple motor shown on p. 189?

Answer 6

The permanent magnets are replaced with curved electromagnets for very strong magnetic fields.
The single lip is replaced with several could of wire wrapped on the same axis.
The coils are wrapped on a laminated soft iron core.

Question 7

What is electromagnetic induction?

Answer 7

When a wire is MOVED across a magnetic field at right angles a voltage is induced or generated in the wire. If the wire is part of a complete circuit the current flows.

Question 8

How can the size of the induced voltage in electromagnetic induction be increased?

Answer 8

1) moving the wire more quickly
2) using a stronger magnet
3) wrapping the wire into a coil so that ore pieces of wire move through the magnetic field.

Question 9

Other than moving a wire through a magnetic field, how else can we generate a voltage and current?

Answer 9

Pushing a magnet into a cou and moving it.

Question 10

How can you increase the size of the induced voltage of a moving magnet and a coil of wire?

Answer 10

1. Moving the magnet more quickly
2. Using a stronger magnet.
3. Increasing the number of turns on the coil
4. Using a coil with a larger cross sectional area.

Question 11

What is Faradays Law of Electromagnetic Induction?

Answer 11

The size of the induced voltage across the ends of a wire depends on the rate at which the change that causes it takes place

Question 12

What happens when you place a current carrying wire at right angles to the magnetic field? What is this called?

Answer 12

When current flows in a wire there is a cylindrical magnetic field around it. If the wire is PLACED between the poles of a magnet, the fields will overlap. In some places the fields are in the same direction, producing a strong magnetic field. In some places they are in opposite directions, producing a weaker field. The wire experiences a force pushing it from the stronger part to the weaker part. This is called the motor effect.

Question 13

What determines the direction of the induced current?

Answer 13

The direction of the movement. If it is reversed, so too is the direction of the induced current.

Question 14

What can a dynamo (or simple generator) be used for and how?

Answer 14

As a cyclist pedals his wheels rotate and a small magnet with a simple generator wrapped around cutting through the magnetic field of the surrounding coil inducing a current in it. This can be used to work the cyclists lights.

Question 15

How is alternating current produced?

Answer 15

By an alternator

Question 16

How does a transformer work?

Answer 16

When an a.c. voltage (Vp) is applied to the primary coil of (Np) turns, it causes an alternating current to flow in this coil
This current produces an alternating magnetic field in the iron core.
This alternating field induces an a.c. voltage (Vs) in the second coil of (Ns) turns.

Question 17

Equation linking voltage and number of turns on a coil?

Answer 17

primary voltage / secondary voltage =

number of turns on primary coil / number of turns on secondary coil

Question 18

What is the equation if the transformer is 100% efficient?

Answer 18

input power = output power
primary voltage x primary current = secondary voltage x secondary current
Ip x Vp = Is x Vs

Question 19

Why are transformers used in the national grid?

Answer 19

As current passes through a wire it loses energy as heat. If the current is kept to a minimum then heat losses are reduced. Transformers are used in the national grid so that electricity is transmitted as low currents at high voltages.

Question 20

How are transformers used in the national grid?

Answer 20

Electric currents from alternators are passed through step-up transformers immediately after generation. Here to voltage is increased to 400 kV and the size of the current is decreased (because power = current x voltage). Close to where the energy is needed the supply is passed through a step-down transformer that decreases the voltage to about 230 V whilst at the same time increasing the current.

Question 21

What is a transformer? Describe its structure

Answer 21

A device for making a.c. voltages bigger or smaller. It consists of two coils - primary and secondary - wound around a magnetically soft iron core

Question 22

Why do transformers only work with a.c.?

Answer 22

Because they rely on the phenomenon of electromagnetic induction, and for that to work something has to be changing.

Question 23

Why can’t the electricity from power station generators be transmitted at their original voltage?

Answer 23

Power station generators produce electricity at high voltages of about 30,000V, but if the electricity was transmitted at this voltage, the heating losses in the power lines would be too great, and insufficient power would reach the destination.

Question 24

How are step-up and step-down transformers used in the large scale generation and transmission of electrical energy?

Answer 24

The electricity produced at power station generators is transformed by step-up transformers from about 30,000V to voltages of a few hundred thousand volts (between 132kV and 400kV) and then transmitted
When the voltage goes up, the current goes down (because of power = current x voltage) and so the heating of the power lines is reduced - because it is the current that causes the heating
At the other end, the voltage is transformed again by a step down transformer (to 230V for domestic use) to make it safe
Even so, about 2% of the energy generated in power stations is lost overall.

Question 25

What is electromagnetic induction?

Answer 25

If an electrical conductor is moved through magnetic field lines so that , a voltage is induced across the ends of the conductor
If the wire is part of a complete circuit, a current is induced in the wire