Siyapath
Sign up

Lesson: Chapter 19 — Current Electricity

19.4. Ohm's law 1 of 5

The nichrome experiment

Current and potential difference have each been defined. This activity asks the obvious next question: is there a relationship between them?

Materials

A nichrome wire coil, an ammeter, a voltmeter, a rheostat, four dry cells, connecting wires, a switch.

What each instrument is for

  • The voltmeter measures the potential difference across the conductor — that is, across the nichrome coil.
  • The ammeter measures the current flowing through the conductor.
  • The rheostat is adjusted to change both the potential difference across the coil and the current through it.

Figure to be added

Figure 19.15 — a rheostat: a coil of wire wound on a cylinder with a sliding contact along a bar above it. Its circuit symbol is a resistor with an arrow through it.

Figure to be added

Figure 19.16 — the circuit diagram for the experiment: the nichrome coil with the voltmeter across it, and in series with it a 6 V supply of four cells, the ammeter, the switch S and the rheostat.

Method

  • Set up the circuit shown in the diagram using the apparatus supplied.
  • Close the switch S, take the voltmeter reading and the ammeter reading as quickly as you can, and open the switch again.
  • After a short while, adjust the rheostat a little, close the switch again and take another pair of readings.
  • Obtain at least five pairs of readings in this way.

Why the switch is opened again at once

The readings are taken the moment the switch is closed because the temperature of the nichrome coil can rise while a current passes through it. This activity has to be done with the temperature kept constant, and the next pages explain why that condition is not a detail but part of the law itself.