A-LevelPhysicsPhysical quantities and units9702/33 · Feb/Mar 2018 · Q220 Marks

In this experiment, you will investigate the size of a drop of liquid between two glass or perspex plates.

(a)You are provided with two pieces of wire, each labelled A, and two pieces of wire, each labelled B. Measure and record the diameter d_A of one the pieces of wire labelled A, and the diameter d_B of one of the pieces of wire labelled B. d_A = d_B =[2]
(b)Place the larger glass or Perspex plate on top of the piece of graph paper, as shown in Fig. 2.1. Fill the liquid dropper with water from the beaker. Use the dropper to put a single drop of water on the plate, as shown in Fig. 2.1. Place the four pieces of wire on the larger plate in the approximate positions shown in Fig. 2.2. The wires must not be touching each other. Place the smaller glass or Perspex plate on top of the wires as shown in Fig. 2.3. Gently press the smaller plate down onto the wires labelled B. Use the 2 mm graph grid to determine the average width W_B of your water drop. W_B =[1]
(c)Estimate the percentage uncertainty in your value of W_B. percentage uncertainty =[1]
(d)Holding the smaller plate in position, carefully pull out the two wires labelled B (leaving the wires labelled A) as shown in Fig. 2.4. Gently press the smaller plate down onto the wires labelled A. Use the 2mm graph grid to determine the average width W_A of your water drop. W_A =[2]
(e)Remove the smaller plate and the wires. Use a paper towel to remove all water from both plates. Repeat (b) and (d) using a larger drop of water made using two drops from the liquid dropper. W_B = W_A =[3]
(f)(i)It is suggested that the relationship between d_A, d_B, W_B and W_A is d_A/d_B = k (W_B/W_A)² where k is a constant. Using your data, calculate two values of k. first value of k = second value of k =[1]
(f)(ii)Justify the number of significant figures you have given for your values of k.[1]
(f)(iii)Explain whether your results in (f)(i) support the suggested relationship.[1]
(g)(i)Describe four sources of uncertainty or limitations of the procedure for this experiment. 1. 2. 3. 4.[4]
(g)(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 2018). The mark or mark type is shown in brackets.

[1] 2(a) Values for dA and dB, with unit, both <1 mm
[1] Raw values for dA and dB recorded to nearest 0.01 mm.
[1] 2(b) Value for WB, with unit, >2 mm
[1] 2(c) Absolute uncertainty in WB value of 0.2 cm to 0.5 cm 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).
[1] 2(d) Value for WA.
[1] Evidence of several raw measurements used to find the average, either here or in 2(b).
[1] 2(e) Value for second WB.
[1] Quality: Second WB greater than first WB.
[1] Value for second WA.
[1] 2(f)(i) Two values of k calculated correctly.
[1] 2(f)(ii) Justification based on s.f. in dA, dB, WB and WA.
[1] 2(f)(iii) Sensible comment relating to the calculated values of k, testing against a criterion specified by the candidate.
[4 max] 2(g)(i) Two k values are not enough to draw a valid conclusion
Difficult to: place glass plate / remove (B) wires / press down, without moving A wires
(Water drop) irregular shape so difficult to find average width
Parallax when measuring water drop
Graph grid is 2 mm (or just 'grid is large') / large % uncertainty in w
Outline of drop not clear because colourless / drop is colourless so difficult to see edge / drop is colourless so difficult to see width
Force not constant
[4 max] 2(g)(ii) Take more readings and plot a graph / calculate more k values and compare
Tape / clamp / weight / stick the wires
Measure with ruler / calipers
Grid marked on glass / place drop on waterproof graph paper
Use smaller (or 1 mm) grid / use large drops to reduce uncertainty
Use coloured water
Use masses / small load / other defined workable method of producing consistent force