Calculate a voltage or resistance

Calculating the voltage

If we know the resistance R of a component and the current I flowing through it, we can calculate the voltage across it using:

U = R × I

Example: a heating wire with a resistance R = 15 ohms has a current I = 4 A flowing through it. The voltage across it is U = 15 × 4 = 60 V.

Calculating resistance

If we measure U and I using a voltmeter and an ammeter, we can calculate the unknown resistance of a component:

R = U / I

Example: across an unknown resistor, we measure U = 8 V and I = 0.2 A. We calculate R = 8 / 0.2 = 40 ohms.

Using a graph of U versus I

For an ohmic conductor, if we plot the voltage U against the current I, we obtain a straight line passing through the origin (the point (0,0)). The slope of this line is precisely the value of the resistance R.

I (A)	U (V)
0	0
0.1	2
0.2	4
0.3	6

Here, R = U / I = 2 / 0.1 = 20 ohms for each row of the table: the straight line does indeed have a constant slope.

Pitfall to avoid

If the line obtained experimentally does not pass through the origin or is not straight, the component being tested is not an ohmic conductor (for example, an incandescent light bulb, whose resistance varies with temperature).

Key points

The three forms of Ohm’s law: U = R × I, I = U / R, R = U / I. Choose the appropriate form depending on the quantity you are seeking.