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Voltage, current and resistance

Resistance and Ohm's Law

You should know and understand the relationship between voltage, current and resistance [resistance: The degree to which a component impedes the passage of current. Shown by the letter R. The unit of resistance is the ohm. ].

Why do we get resistance?

An electric current flows when charged particles called electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ] move through a conductor. The moving electrons can collide with the atoms [atoms: An atom is the smallest part of an element that still has the properties of that element, comprising electrons surrounding a nucleus of protons and neutrons. ] of the conductor. This makes it more difficult for the current to flow, and causes resistance. Electrons collide with atoms more often in a long wire than they do in a short wire. A thin wire has fewer electrons to carry the current than a thick wire. This means that the resistance in a wire increases as:

  • the length of the wire increases
  • the thickness of the wire decreases

Ohm's Law

Resistance is measured in ohms. The symbol for an ohm looks like this:  Ω 

The greater the number of ohms, the greater the resistance.

The equation below shows the relationship between voltage, current and resistance:

potential difference (volt, V) = current (ampere, A) × resistance (ohm, Ω )

 graph showing current on the y axis and voltage on the x axis. A line is drawn through the centre of the graph at 45 degrees.

Relationship between current and voltage when a resistor follows Ohm's Law

The current flowing through a resistorresistor: An electrical component that restricts the flow of electrical current. Fixed-value resistors do not change their resistance, but with variable resistors it is possible to vary the resistance. at a constant temperature is directly proportional to the voltage across the resistor. So, if you double the voltage, the current also doubles. This is called Ohm's Law. The graph shows what happens to the current and voltage when a resistor follows Ohm's Law.


Bicycles with battery operated lights often have different size bulbs for the front and rear lights. The filament in the front lamp has a resistance of 3 ohms. It takes a current of 0.6A. What voltage does it work at?

  1. 0.2V
  2. 1.5V
  3. 1.8V
  4. 5V

toggle answer


The answer is 1.8V. If you didn't get the correct answer, have another look at the formula before you try again.

The activity below concerns the relationship between voltage, current and resistance - check how much you understand.

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