Series and parallel circuits: resistance, R, in ohms, Ω

The idea

Resistance measures how strongly a component opposes the flow of electric charge. It is defined by R = V ÷ I, where V is the potential difference across the component in volts and I is the current through it in amperes. The unit of resistance is the ohm, symbol Ω; one ohm means one volt is needed to drive one ampere through the component. In a series circuit the same current passes through each component, so a larger resistance produces a larger share of the supply potential difference. For example, a resistor with 6 V across it and 2 A through it has R = 6 V ÷ 2 A = 3 Ω. Resistance arises because charge carriers collide with ions in the material, transferring energy and heating the component.

What earns the marks

• State that resistance is measured in ohms, symbol Ω, and that one ohm corresponds to one volt per ampere.
• Recall and apply R = V ÷ I, rearranging correctly to V = I × R or I = V ÷ R when required.
• Substitute values with correct units, for example 12 V and 3 A giving R = 4 Ω.
• Explain that a larger resistance gives a smaller current for the same potential difference, since current and resistance are inversely related at constant V.
• Link resistance to energy transfer by heating as charge carriers collide with the ions of the material.

Common mistakes and how to put them right

• Writing the unit as 'ohms' without the symbol Ω or confusing it with the volt.

  Put it right by stating that resistance is measured in ohms, symbol Ω.

• Dividing current by potential difference instead of potential difference by current.

  Put it right by using R = V ÷ I and checking that the answer has units of ohms.

• Treating resistance as a fixed property of every component.

  Put it right by noting that resistance can change, for example with temperature in a filament lamp.

Exam tips

• Write the equation, substitute the values with units, then give the answer with the unit Ω.
• Check rearrangements by substituting simple numbers back into R = V ÷ I.
• If a question asks for the unit, give the symbol Ω as well as the word ohm.