Required Practical: Investigating Magnetic Fields in Wires (AQA A Level Physics): Revision Note

Required practical: investigating magnetic fields in wires

Aim 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

  • This is just one example of how this required practical might be carried out

Variables

• Independent variable = Current, $I$

• Dependent variable = mass, $m$

• Control variables:

  • Length of wire, $L$

  • Magnetic Flux density, $B$

  • E.m.f. of the power supply

Equipment list

• Resolution of measuring equipment:

  • Ammeter = 0.01 A

  • Variable resistor = 0.01 Ω

  • Top-pan balance = 0.01 g

  • Ruler = 1 mm

Method

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 eight to ten readings for the current (not exceeding 6 A).

8. Repeat the experiment at least three times, and calculate the mean of the mass readings.

• An example table might look like this:

Analysing the results

• The magnetic force on the wire is:

$F=BIL$

• 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 mass $m$:

$m=\frac{BIL}{g}$

• Comparing this to the straight-line equation: $y=mx+c$

  • $y=m$ (mass)

  • $x=I$ (current)

  • Gradient, $m=\frac{BL}{g}$

  • y-intercept, 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:

$B=\frac{g\times gradient}{L}$

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 (up to 6 A) pass through the copper wire, otherwise, the wire’s resistance will increase and affect the experiment

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