You are provided with a potentiometer, an ammeter, a voltmeter, a standard resistor, and other necessary apparatus. Using the circuit diagram above as a gui...

Answer Details

(a) The potentiometer experiment

Close K with the jockey J off the wire and record the open-circuit ammeter reading \(I_o\) and voltmeter reading \(V_o\). Make contact at P for each length OP (10, 20, 30, 40, 50, 60 cm). As OP increases, more of the potentiometer wire is placed in parallel with (or in circuit with) the load, changing the current I and terminal voltage V. Record I and V for each length.

Specimen table

Graph and slope. A plot of V (vertical) against I (horizontal) gives a straight line of negative gradient. The slope \(s=\dfrac{\Delta V}{\Delta I}\) has the unit of resistance (ohms) and represents the internal resistance of the source (magnitude). The intercept on the V-axis (value of V when I = 0) is the e.m.f. E of the cell, since \(V=E-Ir\).

Two precautions:

• Ensure all connections and the jockey make firm, clean contact to avoid contact resistance.
• Open the key immediately after each reading to prevent heating of the wire and running down the cell.

(b)(i) Advantages of a lead-acid accumulator over a dry Leclanche cell:

• It can be recharged and reused, whereas the Leclanche cell cannot.
• It has a much lower internal resistance and can deliver a large current.

(b)(ii) E = 2 V, r = 1 \(\Omega\), R = 9 \(\Omega\).

Current \(I=\dfrac{E}{R+r}=\dfrac{2}{9+1}=0.2\,\text{A}\).

Potential drop across the cell (terminal p.d.) \(V=E-Ir=2-(0.2)(1)=1.8\,\text{V}\).