Define the ohm.

1. (a)Define the ohm.[1]
2. (b)A wire is made of metal of resistivity ρ. The length L of the wire is gradually increased. Assume that the volume V of the wire remains constant as its length is increased. Show that the resistance R of the extending wire is proportional to L².[2]
3. (c)(i)A battery of electromotive force (e.m.f.) E and internal resistance r is connected to a variable resistor of resistance R, as shown in Fig. 5.1. E r I A R V Fig. 5.1 An ammeter measures the current I in the circuit. A voltmeter measures the potential difference V across the variable resistor. The resistance R is now varied to change the values of I and V. The variation with I of V is shown in Fig. 5.2. 3 V/V 2 1 0 0 2 4 6 I/A Fig. 5.2 Use Fig. 5.2 to state the e.m.f. E of the battery.[1]
4. (c)(ii)Use Fig. 5.2 to determine the power dissipated in the variable resistor when there is a current of 5.0A.[3]
5. (c)(iii)State what is represented by the value of the gradient of the graph.[1]

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