2015 WAEC Physics Practical You have been provided with a resistance box, a voltmeter, a key, a battery, and...

WAEC 2015

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You have been provided with a resistance box, a voltmeter, a key, a battery, and other necessary materials.

Connect the circuit as shown in the diagram above.
With the key K closed, read and record the voltmeter reading V\(_{0}\).
Set the resistance R in the resistance box equal to I\(\Omega\).
Close the key, read and record the potential difference V on the voltmeter.
EvaluateR\(^{-1}\) and V\(^{-1}\) .
Repeat the procedure for five other values or R = 2\(\Omega\), 3\(\Omega\), 4\(\Omega\), 5\(\Omega\) and 6\(\Omega\).
Tabulate your readings.
Plot a graph with V\(^{-1}\) on the vertical axis and R on the horizontal axis.
Determine the slope, s, of the graph and the intercept, C, on the vertical axis.
Evaluate c\(^{-1}\)
State two precautions taken to obtain accurate results.

(b)i. Define the potential difference between two points in an electric circuit.

ii. Explain why the emf of a cell is greater than the p.d. across the call when it is supplying Current through an external resistance.

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Explanation

S/N	R/W	V/vat	R\(^{-1}\)/W\(^{-1}\)	V\(^{-1}\)/vat
1	1.0	1.80	1.00	0.56
2	2.0	2.10	0.50	0.48
3	3.0	2.30	0.33	0.43
4	4.0	2.40	0.25	0.42
5	5.0	2.50	0.20	0.40
6	6.0	2.60	0.17	0.30

Slope = \(\frac{0.65 - 0.40}{1.10 - 0.20} = \frac{0.25}{0.9}\)

= 0.3

X = c = 0.35 = C\(^{-1}\) = 2.86

Precautions; I

ensured that the key was open after each reading so that the battery will not run down quickly.
ensured there was no Zero error when taking the voltmeter reading.
ensured the electrical wire was tightly connected to the terminals in order to avoid current fluctuations.

(b) i. Potential difference is the amount of work done in moving a unit positive charge from a point to another, mostly from a point of lower potential to the point of higher potential. Its density p.d. measured in Volts.

ii. The e.m.f. of a cell is the p.d. across the terminals when it is in an open circuit i.e. hot supplying current to an external circuit. While terminal p.d. 5 is defined as the p.d, between the terminals of a cell when it is delivering current to an external circuit. Therefore, emf is greater because it has not yet been connected to an external circuit.

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