Page 266 - Physics Coursebook 2015 (A level)

Cambridge International AS Level Physics
Problem
Improvement
‘Taking readings for just two lengths was not enough.’
‘Difficult to measure the length of the wire as the clips have width and I don’t know where inside they grip the wire.’
‘The scale is not sensitive enough and can only measure to 0.05 V.’
‘The values of voltage are small, particularly at 0.30 m.’
‘The voltmeter reading fluctuates because of contact resistance.’
‘Other factors may have changed the resistance; for example, the temperature may have increased because of the current.’
Table P1.11 Suggestions for improving Experiment 5.
‘Use more than two lengths and plot a graph of the voltmeter reading against the length.’
‘Calculate more than just two values of k.’
‘Use narrower clips.’
‘Solder the contacts onto the wire.’
‘Use a voltmeter that reads to 0.01 V.’ ‘Use a digital voltmeter.’
‘Use a larger current so that when l = 0.50 m the voltmeter reading is at the top of the scale.’
‘Clean the wires with wire wool first.’
‘Wait a long time until the wire has reached a constant temperature.’
‘Use smaller currents, but with a more sensitive voltmeter.’
End-of-chapter questions
1 What is the uncertainty in the following sets of readings? All of them are written down to the smallest division on the instrument used in their measurement.
a 24.6, 24.9, 30.2, 23.6 cm [1]
b 2.66, 2.73, 3.02 s [1]
c 24.0, 24.0, 24.0 g [1]
2 Electrical experiments usually involve the reading of meters such as the voltmeters shown in Figure P1.19.
a What is the reading shown by each voltmeter, and the uncertainty in each reading? [3]
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0.5 1.5 0.5 1.5
VV 0202
Figure P1.19 Two voltmeter scales – for End-of-chapter Question 2.
b The voltmeters show the readings obtained when they were connected across two wires which were
identical apart from their different lengths. The current in each wire was 0.500 A and the length l of
the wire was 30.0 cm in the right diagram and 50.0 cm in the left diagram.
Use the scale readings to test the hypothesis that the resistance R of the wire is proportional to length l.
Consider the effect of the uncertainties on your conclusion. [4]
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