QCE Physics: Exam practice Q6 (2025, Paper 2)

Physics — Question 6

QCAA 2025, Paper 2 · 11 marks

Q6 · 2025 · Paper 2Complex unfamiliar11 marks

QUESTION 6 (11 marks)

A current through a wire is coming out of the page.

Two parallel wires above an electronic scale, with current shown in wire 2.

Describe the direction and shape of the magnetic field produced by the current-carrying wire.

[2 marks]

Identify the theoretical trend between distance from the wire and magnetic field strength.

[1 mark]

0.90\,\mathrm{m}

wire (wire 1) is placed on an electronic scale which then reads

8.0\times10^{-6}\,\mathrm{kg}

. A second wire (wire 2) of the same length is secured

0.015\,\mathrm{m}

above wire 1, and has a current of

25\,\mathrm{A}

running through it, as shown.

A current is introduced to wire 1 that reduces the reading on the scale to

3.0\times10^{-6}\,\mathrm{kg}

Determine the magnitude and direction of the current in wire 1. Show your working.

[8 marks]

Question link

WORKED SOLUTION

QCAA guide · typeset solution

11 marks

ANSWER

See worked solution below.

Worked solution

a) The magnetic field produced would be in an anticlockwise direction, in concentric circles around the wire.

b) As the distance from the wire increases, the magnetic field strength decreases.

c) Magnitude: The magnetic field strength at wire 1 produced by wire 2 is given by:

B_2=\frac{\mu_0I_2}{2\pi r}

=\frac{(4\pi\times10^{-7})\times25}{2\pi\times0.015}=3.33\times10^{-4}\,\text{T}

The B force required (

F_{1,2}

) is the same as the force due to gravity due to wire 1.

F_W=m_1g=(8.0\times10^{-6}-3.0\times10^{-6})\times9.8=4.9\times10^{-5}\,\text{N}

Current in wire 1 and

B_2

are perpendicular:

\sin90^\circ=1

F_{1,2}=B_2I_1L

I_1=\frac{F_{1,2}}{LB_2}=\frac{4.9\times10^{-5}}{0.90(3.33\times10^{-4})}=0.163

Magnitude of current =

0.16\,\text{A}

Direction: Using RH grip rule, the current must run in same direction to attract wires together. Direction of current = left

describes the magnetic field’s anticlockwise direction

[1 mark]

describes the magnetic field’s circular shape around the wire

[1 mark]

identifies trend between distance and magnetic field strength

[1 mark]

recognises that the scenario relates to the strength of a magnetic field around a wire

[1 mark]

calculates strength of magnetic field

[1 mark]

calculates apparent weight of wire

[1 mark]

identifies perpendicular angle between B and I

[1 mark]

recognises that the scenario relates to force acting on a wire in a magnetic field

[1 mark]

provides appropriate mathematical working

[1 mark]

determines magnitude of current in wire 1

[1 mark]

determines the direction of current in wire 1

[1 mark]

One QCAA sample method typeset for web; criterion wording is adapted from the official marking guide.

How many marks did you earn?

Compare your working with the guide above.

Physics — Question 6

QUESTION 6 (11 marks)

QCAA guide · typeset solution

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