Q3 · 2022 · Paper 2Complex familiar8 marks
QUESTION 3 (8 marks)
Negatively charged oil drops were placed in a uniform electric field generated by two parallel plates. By altering the applied voltage between the plates, the oil drops were suspended in the air between the plates.
The graph shows the electric field strength required to suspend negatively charged oil drops of varying weight.
a)[4 marks]
Determine the average charge on the oil drops. Show your working. Give your answer to two significant figures.
b)[4 marks]
Another oil drop was suspended between the plates with an electric field strength of . Determine the work done to move this oil drop a distance of towards the negatively charged plate. Show your working. Give your answer to two significant figures.
WORKED SOLUTION
8 marksQCAA guide · typeset solution
ANSWER
See worked solution below.
Worked solution
Since
F
E
q
=
, gradient of graph is equivalent to 1
q
7
1
1
9
6
2
1
2
1
2.8 10
0
1
6.43 10
19
1.0
4
18
y
y
gradient
x
x
−
=
−
−
=
−
=
×
×
×
17
6.43
18
6
1
1
10
1.
0
q =
×
−
=
×
Charge of electron = 1.6 × 10−18 C (to two significant figures)
From the graph:
33
Fg
N
=
33
0.005
1
1.7 10
U
W
Fs
∆
=
=
=
×
−
=
×
Work done = 0.17 J (to two significant figures)
• recognises the scenario relates to electric field
strength and electron charge [1 mark]
• provides appropriate mathematical reasoning
[1 mark]
• determines the gradient [1 mark]
• determines the average charge on the oil drops
[1 mark]
• determines the weight of the oil drop [1 mark]
• recognises the scenario relates to the work done
in an electric field [1 mark]
• demonstrates correct substitution [1 mark]
• determines the work done [1 mark]
One QCAA sample method typeset for web; criterion wording is adapted from the official marking guide.
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Compare your working with the guide above.