You are provided with cells, a potentiometer, an ammeter, a voltmeter, a bulb, a key, a jockey, and other necessary materials. i. Measure and record the e.m...
You are provided with cells, a potentiometer, an ammeter, a voltmeter, a bulb, a key, a jockey, and other necessary materials.
i. Measure and record the e.m.f E of the battery.
ii. Set up a circuit as shown in the diagram above.
iii. Close the key K and use the jockey to make a firm.
iii. Contact at J on the potentiometer wire such that PJ = x= 10cm.
iv. Take and record the voltmeter reading V and the () Corresponding ammeter reading.
v. Evalute log V and log I.
vi. Repeat the procedure for five other values of x = 20 cm, 30 cm, 40 cm, 50 cm, and 60 cm.
vii. Tabulate your readings.
viii. Plot a graph with log I on the vertical axis and log V on the horizontal axis.
ix. Determine the slope s, of the graph.
x. Determine the intercept, c, on the vertical axis.
xi. State two precautions taken to ensure accurate results.
(b)i. How is the brightness of the bulb affected as x increases?
ii. List two electrical devices whose actions do not obey Ohm's law.
Test of Practical Knowledge - Potentiometer, Bulb, Ammeter and Voltmeter
(i) E.m.f of the battery: On open circuit the voltmeter connected directly across the battery terminals reads \( E = 3.0\,\text{V} \).
(ii) Circuit: The circuit is connected as shown below. The battery, key K and the potentiometer wire PQ form the driver loop. The section PJ of length \( x \) is tapped by the jockey J and supplies the bulb through the ammeter A, with the voltmeter V connected across the bulb.
Circuit diagram: battery (e.m.f E) with key K drives the potentiometer wire PQ; the jockey J taps the section PJ = x, which supplies the bulb through the ammeter A, with the voltmeter V across the bulb.
(iii)-(vii) Readings: The key K is closed and the jockey pressed firmly at J so that \( PJ = x \). For each length \( x \) the voltmeter reading \( V \) and the corresponding ammeter reading \( I \) are taken, and \( \log V \) and \( \log I \) are evaluated. The results are tabulated below.
S/N
x / cm
V / V
I / A
log V
log I
1
10
0.30
0.15
-0.523
-0.824
2
20
0.45
0.19
-0.347
-0.721
3
30
0.60
0.23
-0.222
-0.638
4
40
0.75
0.26
-0.125
-0.585
5
50
0.90
0.29
-0.046
-0.538
6
60
1.05
0.32
0.021
-0.495
(viii) Graph: A graph of \( \log I \) (vertical axis) against \( \log V \) (horizontal axis) is plotted below. The points lie on a straight line, confirming the relation \( I = kV^{s} \), i.e. \( \log I = s\,\log V + c \).
Straight-line graph of log I (vertical) against log V (horizontal); the line of best fit has slope s = 0.60 and vertical-axis intercept c = -0.51.
(ix) Slope of the graph: Two well-separated points on the line of best fit are taken, \( (\log V_1, \log I_1) = (-0.523,\,-0.824) \) and \( (\log V_2, \log I_2) = (0.021,\,-0.495) \):
(x) Intercept on the vertical axis: Extending the line of best fit to \( \log V = 0 \), it cuts the vertical axis at
\[ c = \log I - s\,\log V = -0.824 - (0.60)(-0.523) = -0.824 + 0.314 \]\[ \boxed{c \approx -0.51} \]
(xi) Two precautions:
Parallax error was avoided by viewing the ammeter and voltmeter pointers directly (line of sight perpendicular to the scale).
The key K was opened whenever readings were not being taken, so that the cells did not run down and the wire did not heat up. (The jockey was also pressed firmly and not dragged along the potentiometer wire.)
(b)(i) Effect on brightness: As \( x \) increases, the potential difference tapped from the potentiometer wire increases, so both the voltage \( V \) across the bulb and the current \( I \) through it increase. The bulb therefore becomes brighter as \( x \) increases.
(b)(ii) Two electrical devices whose action does not obey Ohm's law:
A semiconductor diode.
A thermistor. (A filament lamp and a transistor are also acceptable - the slope \( s \approx 0.60 \ne 1 \) obtained above is itself evidence that the filament bulb used here is non-ohmic.)
Test of Practical Knowledge - Potentiometer, Bulb, Ammeter and Voltmeter
(i) E.m.f of the battery: On open circuit the voltmeter connected directly across the battery terminals reads \( E = 3.0\,\text{V} \).
(ii) Circuit: The circuit is connected as shown below. The battery, key K and the potentiometer wire PQ form the driver loop. The section PJ of length \( x \) is tapped by the jockey J and supplies the bulb through the ammeter A, with the voltmeter V connected across the bulb.
Circuit diagram: battery (e.m.f E) with key K drives the potentiometer wire PQ; the jockey J taps the section PJ = x, which supplies the bulb through the ammeter A, with the voltmeter V across the bulb.
(iii)-(vii) Readings: The key K is closed and the jockey pressed firmly at J so that \( PJ = x \). For each length \( x \) the voltmeter reading \( V \) and the corresponding ammeter reading \( I \) are taken, and \( \log V \) and \( \log I \) are evaluated. The results are tabulated below.
S/N
x / cm
V / V
I / A
log V
log I
1
10
0.30
0.15
-0.523
-0.824
2
20
0.45
0.19
-0.347
-0.721
3
30
0.60
0.23
-0.222
-0.638
4
40
0.75
0.26
-0.125
-0.585
5
50
0.90
0.29
-0.046
-0.538
6
60
1.05
0.32
0.021
-0.495
(viii) Graph: A graph of \( \log I \) (vertical axis) against \( \log V \) (horizontal axis) is plotted below. The points lie on a straight line, confirming the relation \( I = kV^{s} \), i.e. \( \log I = s\,\log V + c \).
Straight-line graph of log I (vertical) against log V (horizontal); the line of best fit has slope s = 0.60 and vertical-axis intercept c = -0.51.
(ix) Slope of the graph: Two well-separated points on the line of best fit are taken, \( (\log V_1, \log I_1) = (-0.523,\,-0.824) \) and \( (\log V_2, \log I_2) = (0.021,\,-0.495) \):
(x) Intercept on the vertical axis: Extending the line of best fit to \( \log V = 0 \), it cuts the vertical axis at
\[ c = \log I - s\,\log V = -0.824 - (0.60)(-0.523) = -0.824 + 0.314 \]\[ \boxed{c \approx -0.51} \]
(xi) Two precautions:
Parallax error was avoided by viewing the ammeter and voltmeter pointers directly (line of sight perpendicular to the scale).
The key K was opened whenever readings were not being taken, so that the cells did not run down and the wire did not heat up. (The jockey was also pressed firmly and not dragged along the potentiometer wire.)
(b)(i) Effect on brightness: As \( x \) increases, the potential difference tapped from the potentiometer wire increases, so both the voltage \( V \) across the bulb and the current \( I \) through it increase. The bulb therefore becomes brighter as \( x \) increases.
(b)(ii) Two electrical devices whose action does not obey Ohm's law:
A semiconductor diode.
A thermistor. (A filament lamp and a transistor are also acceptable - the slope \( s \approx 0.60 \ne 1 \) obtained above is itself evidence that the filament bulb used here is non-ohmic.)
