To make a current flow through a resistance there must be a voltage across that resistance. Ohm's Law shows the relationship between the voltage (V), current (I) and resistance (R). It can be written in three ways:
|V = I × R||or||
|where:||V = voltage in volts (V)
I = current in amps (A)
R = resistance in ohms ()
|or:||V = voltage in volts (V)|
I = current in milliamps (mA)
R = resistance in kilohms (k)
The Ohm's Law equations work if you use V, A and , or if you use V, mA and k. You must not mix these sets of units in the equations so you may need to convert between mA and A or k and .
The VIR triangle
|V I R|
Write down V, I and R in a triangle like the one in the yellow box on the right.
- To calculate voltage, V: put your finger over V,
this leaves you with I R, so the equation is V = I × R
- To calculate current, I: put your finger over I,
this leaves you with V over R, so the equation is I = V/R
- To calculate resistance, R: put your finger over R,
this leaves you with V over I, so the equation is R = V/I
Ohm's Law CalculationsUse this method to guide you through calculations:
|V I R|
- Write down the Values, converting units if necessary.
- Select the Equation you need (use the VIR triangle).
- Put the Numbers into the equation and calculate the answer.
- 3 V is applied across a 6
resistor, what is the current?
- Values: V = 3 V, I = ?, R = 6
- Equation: I = V/R
- Numbers: Current, I = 3/6 = 0.5 A
- A lamp connected to a 6 V battery passes a current of 60 mA, what is the lamp's resistance?
- Values: V = 6 V, I = 60 mA, R = ?
- Equation: R = V/I
- Numbers: Resistance, R = 6/60 = 0.1
(using mA for current means the calculation gives the resistance in k)
- A 1.2 k
resistor passes a current of 0.2 A, what is the voltage across it?
- Values: V = ?, I = 0.2 A, R = 1.2 k
(1.2 k is converted to 1200 because A and k must not be used together)
- Equation: V = I × R
- Numbers: V = 0.2 × 1200 = 240 V
- Values: V = ?, I = 0.2 A, R = 1.2 k = 1200