Magnetism and magnetic forces -Section B - Maths questions

Don't forget to take a ruler and calculator into the exam.

Maths questions often start with the command words like 'calculate', 'determine', 'estimate' and 'measure'. They will then include blank space for you to show your working.

When an answer to a maths question is marked:

  • full marks are given for the right answer (but it's wise to show your working so you can check your answer)
  • marks are given for working, including substitution and rearrangement
  • errors carried forward are taken into account

Errors carried forward are related to what happens if a later answer depends on an earlier answer, and you get the earlier one wrong. You could still get full marks in the later answer if your working is correct but you use the incorrect earlier answer.

If your answer has many decimal places or figures, make sure you give it to an appropriate number of decimal places or significant figures. You may be asked to give units. This may earn you an additional mark, so don't forget to check whether you need to do this.

Some maths questions might ask you to plot or complete a graph or table. When you draw a graph, make sure you:

  1. plot each point accurately
  2. draw a best fit straight line or curve

You may be given a grid with axes and scales already given. Sometimes you may be given an empty grid for you to supply your own axes. When you do this:

  • put the independent variable on the x-axis and the dependent variable on the y-axis
  • make sensible scales so that the plotted points cover at least 50% of the area of the graph
  • label the axes with their quantity and unit, eg time (s)

Sample question 1 - Foundation

Question

Two students decide to investigate the magnetic effect of a current-carrying wire.

Look at the graph of their results.

A graph showing the results of magnetic field strength against the distance from a current-carrying wire.

What trend does the graph show? [2 marks]

OCR Gateway Science, GCE Physics, Paper J249, 2016.

As the distance from the wire increases, the strength of the magnetic field falls - (idea of) the non-linear nature of the relationship.

Sample question 2 - Foundation

Question

A magnetic field forms around a wire carrying an electric current. Describe what happens to the strength of this field as the current and distance from the wire increases. [3 marks]

This question has been written by a Bitesize consultant as a suggestion to the type of question that may appear in an exam paper.

Strength increases as the current increases.

Strength decreases as the distance increases.

Strength is directly proportional to the current/inversely proportional to the distance squared.

Sample question 3 - Higher

Question

A transformer has 4,000 turns on its primary coil and 150 turns on its secondary coil.

The input voltage is 10,000 V. Calculate the output voltage. [2 marks]

Use:

\frac{\text{potential difference across primary coil}}{\text{potential difference across secondary coil}} = \frac{\text{number of turns in primary coil}}{\text{number of turns in secondary coil}}

OCR Gateway Science, GCE Physics, Paper B752, June 2015 - Higher.

Numbers correctly substituted into the equation, eg 10,000 ÷ output = 4,000 ÷ 150

output = 375 V

Sample question 4 - Higher

Question

A student places four wires of different lengths perpendicular to different magnetic fields with different currents flowing.

Look at the table of their results:

WireMagnetic field strength (T)Current (A)Length (m)
A0.102.50.50
B0.152.00.75
C0.204.50.25
D0.255.01.00

Use the data to show that wire D experiences the highest force.

Show your working. [2 marks]

Use force (N) = magnetic field strength (T) × current (A) × length (m)

OCR Gateway Science, GCE Physics, Paper J249, 2016 - Higher.

Reference to field strength, current and length being the largest in the table.

Four correct calculations:

A = 0.125 N

B = 0.225 N

C = 0.225 N

D = 1.250 N