TEST OF PRACTICAL KNOWLEDGE QUESTION You are provided with a constantan wire, a2 \(\Omega\)standard resistor, an accumulator E, an ammeter A, a key K, and o...

Answer Details

Test of practical knowledge (current versus length of resistance wire)

Note: the ammeter readings, the table and the graph must come from your own measurements; the procedure and the fully determined theory parts are given below.

Method (summary): measure the e.m.f. \(E\) of the accumulator, connect the circuit with the constantan wire, standard \(2\,\Omega\) resistor, ammeter and key. Read \(I_0\) with the clip off the wire, then for each length \(d = 90, 80, 70, 60, 50\,\text{cm}\) record the current \(I\) and evaluate \(d^{-1}\). Tabulate, plot \(I\) (vertical) against \(d^{-1}\) (horizontal), and read the slope \(S\) and vertical intercept \(c\); then \(k = \dfrac{c}{S}\). The graph is used to read \(I\) at \(d = 55\,\text{cm}\).

Two precautions:

• Ensure clean, tight (low-resistance) connections and a firm crocodile-clip contact on the wire.
• Open the key immediately after each reading to avoid heating the wire (which would change its resistance) and to save the cell.

(b)(i) Potential difference between two points

The potential difference between two points in a circuit is the work done (energy converted from electrical to other forms) in moving one coulomb of positive charge from one point to the other. It is measured in volts \(\left(1\,\text{V} = 1\,\text{J C}^{-1}\right)\).

(b)(ii) Two factors on which the resistance of a wire depends

• The length of the wire (resistance increases with length).
• The cross-sectional area of the wire (resistance decreases as area increases). (Also its material/resistivity and temperature.)