The ratio that does not change
Five pairs of readings, and the interesting thing is not in either column. It is in what you get when you divide one by the other.
The task
For every one of your five readings, work out the value of the potential difference (V) divided by the current (I).
| Reading | Potential difference (V) / V | Current (I) / A | V / I |
|---|---|---|---|
| 1 | 1.0 | 0.5 | 2.0 |
| 2 | 2.0 | 1.0 | 2.0 |
| 3 | 3.0 | 1.5 | 2.0 |
| 4 | 4.0 | 2.0 | 2.0 |
| 5 | 5.0 | 2.5 | 2.0 |
A set of readings of the kind this activity produces. Your own numbers will differ; the last column is what should not.
Observation
The value of V ÷ I comes out the same for every reading. The potential difference changed and the current changed with it, but the ratio of the two did not move.
The constant ratio depends on one condition
A constant value is obtained only if the temperature of the conductor used — the nichrome coil — was kept unchanged. Let the coil heat up and the last column drifts, which is why the method insisted on opening the switch at once.
A constant is a property of the conductor
If a number stays the same while the two things that produced it both change, that number is telling you about the conductor rather than about the reading. The next two pages give it a name and a unit.