Electrical principles - Terminology

Ohm's law

Voltage, current and resistance are present in all operational circuits and a relationship exists between these three things. The relationship is named after the person who discovered it, George Ohm.

Ohm's law states, 'Current flow in a circuit is directly proportional to the voltage across the circuit and inversely proportional to the resistance contained in the circuit, providing circuit conditions remain the same.

In other words:

• if you increase the voltage (or electrical pressure) in a circuit then the current (flow of electrons) will increase in direct proportion, eg if you double the voltage the current flow will double
• if you increase the resistance (the opposition to current flow) in a circuit then the current flow will decrease in direct proportion, eg if the resistance in a circuit doubles then current flow will halve.

The image below shows that increasing battery voltage from 6 volts to 9 volts will cause the current flow to increase in direct proportion.

Voltage and current relationship

 

The following image shows that If the resistance of the circuit is increased from 3Ω to 6Ω (doubled) then the current flow will decrease inversely, ie it will reduce by half.

 

Resistance and current relationship

 

Ohm's law can be stated as a mathematical formula.

V = IR

Voltage is equal to current multiplied by the resistance. If the current and resistance of a circuit is known, the voltage can be calculated.

Transposing the formula allows us to calculate current or resistance.

I = ^V/_R

R = ^V/_I

If any two values in a circuit are known, the third value can be calculated.

Use the following Ohm's law triangle animation to help you practise using the formula.

Text alternative

 

For extra practice, draw an Ohm's law triangle the same as the one shown above. Place your thumb (instead of the mouse) over the unit you wish to determine and the appropriate formula is shown.

Ohm's law examples

1. What voltage would be measured on the voltmeter (V) if the resistor (R) has a value of 50Ω and has a current flow reading on the ammeter (A) of 0.5A?

   V = IR

   V = 0.5 x 50

   V = 25 V

2. What would be the current flow (I) measured on the ammeter (A) in the circuit if 24V is applied to the resistance (R) of 120Ω?

   I = ^V/_R

   I = 24/120

   I = 0.2 A or 200 mA

3. What would be the resistance value (R) if a current flow of 1.5A is recorded on the ammeter (A) and voltage of 18V on the voltmeter (V)?

   R = ^V/_I

   R = 18/1.5

   R = 12 Ω

4. How much current will be recorded on the ammeter (A) if the resistance (R) is 3kΩ (3000 ohms) and the applied voltage (V) is 12V?

   I = ^V/_R

   R is a multiple (k = 1,000) and must be converted to a base

   R = 3kΩ = 3 x 1,000 = 3000Ω

   I = 12/3000

   I = 0.004 A or 4m