In this experiment, you will measure the current through and the potential difference across a resistor to determine its resistance. You connects the resist...
Question 1 Report
In this experiment, you will measure the current through and the potential difference across a resistor to determine its resistance. You connects the resistor in a circuit with a battery, a switch, an ammeter and a voltmeter. The ammeter is connected in series with the resistor and the voltmeter is connected in parallel across it. You closes the switch and reads both meters. The ammeter and voltmeter scales are shown in Fig. 15.1 and Fig. 15.2. The ammeter has a range of 0 to 1.0 A with ten divisions between each numbered mark. The voltmeter has a range of 0 to 10 V with four divisions between each numbered mark.
(a) Measure and record the ammeter reading shown in Fig. 15.1. [1]
(b) Measure and record the voltmeter reading shown in Fig. 15.2. [1]
(c) Calculate the resistance of the resistor using these readings. Show your working. [2]
(d) State the resolution of the ammeter. [1]
(e) State the resolution of the voltmeter. [1]
(f) You wants to find out whether the resistance changes with voltage. Describe the measurements you should take and the calculations you should do. [3]
(g) State one precaution you should take when reading an analogue meter to reduce parallax error. [1]
Answer Details
(a) The ammeter reading is 0.63 A. The pointer lies between the 0.6 A and 0.7 A division marks, approximately three-tenths of the way from 0.6 to 0.7. [1]
(b) The voltmeter reading is 6.5 V. The pointer lies between the 6 V and 7 V marks, approximately halfway between them. [1]
(c) Using \( R = \frac{V}{I} \): [1]
\[ R = \frac{6.5}{0.63} = 10.3 \; \Omega \]The resistance of the resistor is approximately 10 Ω. [1]
(d) The ammeter scale runs from 0 to 1.0 A. The smallest division on the ammeter represents 0.1 A, so the resolution is 0.1 A. [1]
(e) The voltmeter scale runs from 0 to 10 V with divisions every 1 V. The resolution is 1 V (accept 0.5 V if half-divisions are counted). [1]
(f) To investigate whether resistance changes with voltage:
- Change the voltage using a variable power supply or add a variable resistor to the circuit. [1]
- Record several pairs of V and I readings at different voltages. [1]
- Calculate \( R = \frac{V}{I} \) for each pair and compare the values. If R is constant, the resistor obeys Ohm's law. Alternatively, plot V against I: a straight line through the origin confirms ohmic behaviour. [1]
(g) Read the meter with your eye positioned directly in front of the pointer, perpendicular to the scale, to avoid parallax error. If the meter has a mirror strip behind the scale, align the pointer with its reflection to confirm you are viewing from directly in front. [1]
Why this matters: Reading analogue instruments correctly is a core practical skill. Parallax error occurs when your line of sight is at an angle to the scale, making the pointer appear to be at a different position than it truly is. The resolution of an instrument determines the smallest measurable change. A measurement can never be more precise than the resolution of the instrument used. When calculating resistance from V and I, the uncertainties in both readings combine to affect the reliability of the final value.
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