Magnetic Flux Density (OCR A Level Physics)

Revision Note

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Determining Magnetic Flux Density

Aims of the Experiment

  • The overall aim of this experiment is to calculate the magnetic flux density of a magnet
    • This is done by measuring the force on a current-carrying wire placed perpendicular to the field


  • Independent variable = Current, I
  • Dependent variable = mass, m
  • Control variables:
    • Length of wire, L
    • Magnetic Flux density, B
    • Potential difference of the power supply

Equipment List

Magnetic Force Equipment List Table, downloadable AS & A Level Physics revision notes

  • Resolution of measuring equipment:
    • Ammeter = 0.01 A
    • Variable resistor = 0.01 Ω
    • Top-pan balance = 0.01 g
    • Ruler = 1 mm


Magnetic Force Apparatus Setup 1, downloadable AS & A Level Physics revision notesMagnetic Force Apparatus Setup 2, downloadable AS & A Level Physics revision notes

  1. Set up the apparatus as shown above
    • Make sure the wire is completely perpendicular in between the magnets
  2. Measure the length of one of the magnets using the 30 cm ruler
    • This will be the length of the wire L in the magnetic field
  3. Once the magnet is placed on the top-pan balance, and whilst there is no current in the wire, reset the top-pan balance to 0 g
  4. Adjust the resistance of the variable resistor so that a current of 0.5 A flows through the wire as measured on the ammeter
  5. The wire will experience a force upwards.
    • Due to Newton’s third law, the force pushing downwards will be the mass on the balance.
    • This movement will be very small, so it may not be completely visible
  6. Record the mass on the top-pan balance from this current
  7. Repeat the procedure by increasing the current in intervals of 0.5 A between 8−10 readings for the current (not exceeding 6 A)
  8. Repeat the experiment at least 3 times, and calculate the mean of the mass readings

  • An example table might look like this:

Magnetic Force Example Table, downloadable AS & A Level Physics revision notes

Analysing the Results

  • The magnetic force on the wire is:


  • Where:
    • F = magnetic force (N)
    • B = magnetic flux density (T)
    • I = current (A)
    • L = length of the wire (m)


  • Since F = mg where m is the mass in kilograms, equating these gives:

mg = BIL

  • Rearranging for m:

  • Comparing this to the straight-line equation: y = mx + c
    • y = m (mass)
    • x = I
    • m = BL / g
    • c = 0
  • Plot a graph of m against I and draw a line of best fit
    • Calculate the gradient
  • The magnetic flux density B is:

Magnetic Force Example Graph, downloadable AS & A Level Physics revision notes

Evaluating the Experiment

Systematic Errors:

  • Make sure top-pan balance starts at 0 to avoid a zero error

Random Errors:

  • Repeat the experiment by turning the magnet in the metal cradle and the wire by 90º
  • Make sure no high currents pass through the copper wire,
    • High current will lead to heating, causing the wire’s resistance to increase

Safety Considerations

  • Keep water or any fluids away from the electrical equipment
  • Make sure no wires or connections are damaged and contain appropriate fuses to avoid a short circuit or a fire
  • High currents through the wire will cause it to heat up
    • Make sure not to touch the wire when current is flowing through it

Worked example

A student investigates the relationship between the current and the mass produced from the magnetic force on a current-carrying wire. They obtain the following results:Magnetic Force Worked Example Table 1, downloadable AS & A Level Physics revision notesThe mean length of the wire in the magnetic field was found to be 0.05 m. Calculate the magnetic flux density of the magnets from the table.

Step 1: Complete the table

    • Add an extra column ‘Average mass m / × 103 kg and calculate this for each mass

Magnetic Force Worked Example Table 2, downloadable AS & A Level Physics revision notes

Step 2: Plot the graph of average mass m against current I

Magnetic Force Practical Graph 1, downloadable AS & A Level Physics revision notes

    • Make sure the axes are properly labelled and the line of best fit is drawn with a ruler

Step 3: Calculate the gradient of the graph

Magnetic Force Practical Graph 2, downloadable AS & A Level Physics revision notes

    • The gradient is calculated by:

Step 4: Calculate the magnetic flux density, B

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