You are provided with cells, a potentiometer, an ammeter, a voltmeter, a bulb, a key, a jockey, and other necessary materials. Measure and record the emf E ...
Question 1 Report
You are provided with cells, a potentiometer, an ammeter, a voltmeter, a bulb, a key, a jockey, and other necessary materials.
- Measure and record the emf E of the battery.
- Set up a circuit as shown in the diagram above.
- Close the key K and use the jockey to make a firm contact at J on the potentiometer wire such that PJ = x = 10cm.
- Take the record of the voltmeter reading V and the corresponding ammeter reading I.
- Evaluate log V and log I.
- Repeat the procedure for other values of x =20, 30. 40, 50, and 60cm.
- Tabulate your readings.
- Plot a graph with the log I on the vertical axis and log V on the horizontal axis.
- Determine the slope, s, of the graph.
- Determine the intercept, c, on the vertical axis.
- State two precautions taken to ensure accurate results.
(b)i. How is the brightness of the bulb affected as x increases? Give a reason for your answer.
ii. List two electrical devices whose actions do not obey ohm's law
Answer Details
(a) Potentiometer measurement of V and I
The emf of the battery is first measured directly with the voltmeter across the battery terminals on open circuit, giving \(E = 3.0\ \text{V}\).
The circuit is connected as shown below. The key K is closed and the jockey is pressed firmly at J so that \(PJ = x\). For each length x the voltmeter reading V (across the bulb) and the ammeter reading I are recorded, and \(\log V\) and \(\log I\) are evaluated.
Observation / table of readings
| x (cm) | V (V) | I (A) | log V | log I |
|---|---|---|---|---|
| 10 | 0.30 | 0.06 | -0.523 | -1.222 |
| 20 | 0.40 | 0.09 | -0.398 | -1.046 |
| 30 | 0.50 | 0.13 | -0.301 | -0.886 |
| 40 | 0.60 | 0.18 | -0.222 | -0.745 |
| 50 | 0.70 | 0.24 | -0.155 | -0.620 |
| 60 | 0.79 | 0.30 | -0.102 | -0.523 |
Graph of log I against log V
A straight line of best fit is drawn through the plotted points.
Slope, s
Taking two widely separated points on the line of best fit, \((\log V_1, \log I_1) = (-0.523,\ -1.222)\) and \((\log V_2, \log I_2) = (-0.102,\ -0.523)\):
\[ s = \frac{\log I_2 - \log I_1}{\log V_2 - \log V_1} = \frac{-0.523 - (-1.222)}{-0.102 - (-0.523)} = \frac{0.699}{0.421} \]\[ s = 1.66 \]Intercept, c (on the vertical axis)
Using the line equation \(\log I = s\,\log V + c\) with the point \((-0.102,\ -0.523)\):
\[ c = \log I - s\,\log V = -0.523 - (1.66)(-0.102) = -0.523 + 0.169 \]\[ c = -0.35 \]Two precautions
- The jockey was pressed on the potentiometer wire only momentarily (and the key opened between readings) to prevent heating of the wire and the cells.
- The eye was placed directly in line with the pointer when reading the ammeter and voltmeter to avoid error of parallax.
(b)(i) Effect on the brightness of the bulb as x increases
The brightness of the bulb increases as x increases. This is because a longer length PJ of the potentiometer wire delivers a larger potential difference to the bulb, so both the voltage across the bulb and the current through it increase, raising the electrical power \(P = VI\) dissipated in the filament.
(b)(ii) Two electrical devices that do not obey Ohm's law
- Semiconductor diode (p-n junction).
- Thermistor.
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