A-LevelPhysicsMagnetic fields9702/33 · Feb/Mar 2020 · Q220 Marks

In this experiment you will investigate the magnetic field produced by an electrical current.

(a)Count and record the number N of turns of wire in the coil. N = ................................................................[1]
(b)The distance between the first and last turns of wire is L, as shown in Fig. 2.2. Measure and record L. L = ................................................................[1]
(c)Close the switch. The compass arrow will rotate through an angle θ. Measure and record θ. θ = ................................................................ ° Record the ammeter reading I. I = ................................................................[2]
(d)Estimate the percentage uncertainty in your value of θ. percentage uncertainty = ................................................................[1]
(e)Calculate the value of B using B = μ₀(N-1)I / L where μ₀ = 1.26 × 10⁻⁶ NA⁻². B = ................................................................ NA⁻¹m⁻¹[1]
(f)Repeat (a), (b), (c) and (e). N = ................................................................ L = ................................................................ θ = ................................................................ ° I = ................................................................ B = ................................................................ NA⁻¹m⁻¹[3]
(g)(i)Using your data, calculate two values of k. first value of k = ................................................................ second value of k = ................................................................[1]
(g)(ii)Justify the number of significant figures you have given for your values of k.[1]
(g)(iii)Explain whether your results in (g)(i) support the suggested relationship.[1]
(h)(i)Describe four sources of uncertainty or limitations of the procedure for this experiment. 1 ................................................................................................................................................................................................................... 2 ................................................................................................................................................................................................................... 3 ................................................................................................................................................................................................................... 4 ...................................................................................................................................................................................................................[4]
(h)(ii)Describe four improvements that could be made to this experiment. You may suggest the use of other apparatus or different procedures. 1 ................................................................................................................................................................................................................... 2 ................................................................................................................................................................................................................... 3 ................................................................................................................................................................................................................... 4 ...................................................................................................................................................................................................................[4]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2020). The mark or mark type is shown in brackets.

[1] Value of N, with no unit, in range 10 to 14.
[1] Value of L to nearest mm, with unit, in range 5.0 to 15.0 cm.
[1] Value(s) of raw θ to nearest degree and < 90°.
[1] Value of I to nearest mA or nearest 0.1 mA, with unit
[1] Absolute uncertainty in θ value of 2 to 5° and correct method of calculation to obtain percentage uncertainty. If several readings have been taken, then the absolute uncertainty can be half the range (but not zero if values are equal) if the working is clearly shown.
[1] Correct calculation of B
[1] Second values of N and L
[1] Second values of θ and I
[1] Quality: second θ > first θ
[1] Two values of k calculated correctly.
[1] Justification based on s.f. in θ, I and L.
[1] Sensible comment relating to the calculated values of k, testing against a criterion specified by the candidate.
[4] 4 max Two readings are not enough to draw a valid conclusion. Difficult to determine I as reading not steady. Parallax error when measuring θ. Large % uncertainty in θ. Difficult to re-wind and fix the wire / wires may touch / wire is kinked. Channel moves on bench.
[4] 4 max Take more readings and plot a graph / calculate more k values and compare. Method of cleaning crocodile clips / contacts. Method of overcoming parallax error, e.g. measure change of angle by turning channel on bench/measure θ on a photo/angle markings on compass / mark 'before' and 'after' positions on compass. Use larger current / voltage. Use two ready-wound channels / use new wire. Method of fixing channel, e.g. tape channel to bench.