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GCSE Level Questions: Magnets and Electromagnets

Q4. The diagram shows a solenoid.

(a) Draw the magnetic field of the solenoid onto the diagram.

The mark was given for lines going in and out of the solenoid - if they were not shown through the cardboard you still got the mark - they were after the direction of the field lines for the second mark.

They were not bothered about you drawing the full magnetic field line pattern - that mimicks a bar magnet shaped field.

[2 marks]

(b) This diagram shows two iron rods placed inside a solenoid:

 

Explain why the iron rods move apart when the switch is closed.

Magnetism is induced in the rods and they become magnets with the same polarity at each end so they repel each other.

Be careful not to say the rods 'become charged' - they are not charged - a magnetic field is induced around them. Talk of 'charge' loses you the mark!

[2 marks]

(c) Ross investigated the strength of an electromagnet. He was instructed to investigate how the strength depended on:

the current in the wire

the number of turns of wire around the iron core.

Here is the equipment he used

Ross measured the strength of the electromagnet by finding the maximum weight the electromagnet could hold.

The table below shows his results.

Current in amps
Number of turns of wire
Maximum weight held/ N
1.0
30
6.5
1.5
20
6.4
2.0
10
3.7

 

Explain why the method used by the student is not valid for this investigation.

The results show that Ross changed two independent variables at the same time, so it is not possible to know the effect of one independent variable or the other. He did not control all but one of the independent variables.

He needed to keep current or number of turns constant and should only change one variable at a time. This makes it a 'fair test' but examiners do not give you a mark for using that phrase without explaining what makes a fair test in this situation because in the past students just wrote 'fair test' as an answer without understanding what it meant.

[2 marks]

(d) Chandler used the same equipment to perform his investigation.

Here are Chandler's results displayed on a graph:

(i) How does increasing the current in the wire affect the strength of the electromagnet, when the electromagnet has 30 turns of wire?

Increasing the current increases the strength of the electromagnet until the current reaches 2.75A after which the strength has reached a maximum value of 11.4 N.

[1 mark]

(ii) How does increasing the number of turns of wire from 10 to 20 affect the strength of the electromagnet, compared to increasing the number of turns of wire from 20 to 30?

Increasing the number of turns from 10 to 20 increases the strength more than increasing from the turns from 20 to 30.

[1 mark]

(TOTAL 14 marks)