Electromagnetic Effects (Cambridge O Level Physics)

Fig. 10.1 is a simplified top view of a flat coil. There is an alternating current (a.c.) in the coil.

Describe the magnetic effect of this alternating current.

Fig. 10.2 shows a pan placed above the coil. The base of the pan is made of steel.

State what quantity is induced in the base of the pan.

The pan contains water.

State and explain the effect of the quantity induced in part (b) on the temperature of the water in the pan.

Fig. 11.1 shows in each of the diagrams a current-carrying conductor and a magnetic field pattern.

State the diagram which correctly shows the magnetic field around a current-carrying conductor.

Fig. 11.2 shows three pieces of equipment.

Fig. 9.1 shows a simple direct current (d.c.) electric motor. The coil rotates about the axis when there is a current in the coil. The coil is connected to the rest of the circuit by the brushes.

State any difference each of the following changes makes to the rotation of the coil in Fig. 9.1:

(i)

changing the polarity of the power supply to that shown in Fig. 9.2

 

[1]

 

(ii)

changing the coil to the new coil shown in Fig. 9.3

[1]

 

A student makes an electromagnet. He places an iron rod inside a coil of wire and connects
the coil to a d.c. power supply, as shown in Fig. 9.2.

A student has a model electric railway. The model railway uses a step-down transformer.

 
The input voltage is 230 V. The transformer has 1710 turns on the input coil and 90 turns on the output coil.

 
Calculate the output voltage of the transformer.

A step-up transformer is used to increase voltage.

Step-up transformers and step-down transformers have different coil arrangements.

 
Describe the differences in the coil arrangement for the two types of transformer.

Explain the advantage of transmitting electricity at high voltages, rather than at low voltages.

A transformer consists of two coils of wire wound on a metal core. Fig. 10.1 represents the transformer.

State the name of the metal from which the core is made.

Extended

The primary coil of the transformer is connected to the output voltage of an a.c. generator which supplies an alternating current.

Transformers are used to increase the voltage when electrical energy is transmitted in cables across long distances.

 
Explain why power losses in the cables are lower when the voltage is high.

A student moves a metal bar upwards between the poles of a magnet, as shown in Fig. 6.1.

Fig. 6.1

The ammeter connected to the metal bar shows a small positive reading as the bar moves.

Explain why there is a reading on the ammeter.

Complete Table 6.1 by stating what the ammeter shows when the metal bar moves in the direction shown by the arrow in each diagram.

Table 6.1

The student finds that using a stronger magnet increases the reading on the ammeter.

State one other way in which the student can produce a larger reading on the ammeter, using the same rod.

Describe how Lenz’s law applies when the bar is moved upwards.

Fig. 7.1 shows a horizontal, current-carrying wire PQ in the gap between the poles of a permanent magnet.

Fig. 7.1

[2]

[1]

Fig. 7.2 shows part of a torch. The torch does not contain a battery.

Fig. 7.2

The torch is shaken and this causes the magnet to move backwards and forwards through the coil.
 

[2]

. [1]

 [2]

A teacher demonstrates electromagnetic induction using the apparatus shown in Fig.10.1.

The teacher pulls the magnet down until one end is inside the coil. The teacher then releases the magnet. The magnet moves up and down repeatedly. As it moves, one end of the magnet enters and leaves the coil.

 
Describe and explain the readings on the centre-zero millivoltmeter as the magnet enters and leaves the coil.

Fig.10.2 shows a system for transmitting electricity from a power station.

State the name of the device used in X and the name of the device used in Y.

A student is experimenting with electromagnetic effects.

Describe an experiment, using any standard laboratory equipment, to demonstrate electromagnetic induction. You may draw a diagram.

Fig. 11.1 shows a transformer connected to an input voltage of 12 V a.c.

[1]

[1]

Fig. 10.1 shows a straight wire AB placed in the magnetic field between the poles of a magnet.

 
The ends of AB are connected to a galvanometer.

When AB is moved vertically, the needle of the galvanometer shows a deflection.

 
State three factors that affect the size of the deflection.

Extended

Fig. 10.2 shows a transformer.

A student makes a transformer that uses an alternating current (a.c.) supply with an electromotive force (e.m.f.) of 12.0 V to induce an output potential difference (p.d.) of 2.0 V.

 
The student is provided with two lengths of insulated wire and the U-shaped piece of iron shown in Fig. 7.1.

Another transformer is used in a school laboratory to step down a mains supply with a p.d. of 110 V to 12 V. This transformer is mounted in a metal case.

 
State and explain an essential safety feature required for this arrangement.

Fig. 11.1 shows a vertical conductor passing through a horizontal piece of card.

A student wants to demagnetise a permanent bar magnet. She suggests these steps:

1. Place the magnet in a long coil.
2. Switch on a large alternating current in the coil.
3. Switch off the current.
4. Remove the bar from the coil.

State and explain whether the steps will always be able to demagnetise the magnet.

Explain why the voltage of the supply to the primary coil of a transformer must be alternating.

Fig. 10.1 shows a transformer.

There are 8000 turns in the primary coil of the transformer. The primary coil is connected to a 240 V mains supply. A 6.0 V lamp connected to the secondary coil operates at full brightness.

Fig. 9.1 shows a coil ABCD with two turns. The coil is in a magnetic field.

When there is a current in the coil, the coil experiences a turning effect.

Extended

Fig. 9.2 shows a magnet held just below a vertical coil connected to a galvanometer.

The magnet is released.