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
Explanation
Observation/table of values
| S/N | V(volts) | I(Amps) | Log V | Log I |
| 1 | 0.790 | 0.030 | -0.100 | -0.520 |
| 2 | 0.690 | 0.240 | -0.160 | -0.620 |
| 3 | 0.600 | 0.180 | -0.220 | -0.740 |
| 4 | 0.600 | 0.120 | -0.220 | -0.920 |
| 5 | 0.500 | 0.060 | -0.300 | -0.220 |
| 6 | 0.300 | 0.060 | -0.520 | -0.220 |
Slope (s) = \(\frac{\bigtriangleup \text{log I}}{\bigtriangleup \text{log V}} = \frac{y_{2}-y_{1}}{x_{2}-x_{1}}\)
(s) = \(\frac{-0.4-(-1.4)}{-0.04-(-0.62)} = \frac{-0.4+1.4}{-0.04+0.62}\)
= \(\frac{1}{0.58}\)
s = 1.72
Graph Log I against Log V
PRECAUTIONS; i
- Ensured clean/tight terminals
- Opened key when reading was not taken
- Avoided parallax error in ammeter/ voltmeter.
- Avoided sliding or jockey on the potentiometer wire M Zero error of voltmeter/ammeter were noted.
(b)i. The brightness of the bulb increases. Voltage/current through the bulb increases.
ii. The devices that do not obey ohm's law;
- Diode
- Transistor
- Thyristor
- Capacitor
- Thermistor