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What types of resistor are used in circuits, and how are their values read and chosen?

Fixed and variable resistors: preferred (E-series) values, the resistor colour code, tolerance, and variable resistors (potentiometers and rheostats).

An Eduqas GCSE Electronics answer on fixed and variable resistors: reading the four-band colour code, preferred E-series values and tolerance, and how variable resistors are used as potentiometers and rheostats.

Generated by Claude Opus 4.812 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
  4. Try this

What this dot point is asking

Eduqas wants you to identify and read resistors: decode the four-band colour code, understand preferred (E-series) values and tolerance, and explain how a variable resistor is used either as a potentiometer (variable voltage) or a rheostat (variable resistance). These are the most common components in any circuit you build or analyse.

The answer

The colour code

Tolerance

Preferred values

Variable resistors

Examples in context

Resistors are in every circuit you meet. The colour code lets you identify them on a breadboard; preferred values mean your calculated resistor is rounded to one you can actually buy; tolerance tells you whether a value is precise enough for a timing or gain circuit. A potentiometer sets the reference voltage for a comparator or the volume of an audio stage, and a rheostat dims a lamp. The non-exam assessment expects you to specify real, preferred resistor values.

Try this

Q1. State what the third band of a four-band resistor represents. [1 mark]

  • Cue. The multiplier (the number of zeros / power of ten to multiply the first two digits by).

Q2. A resistor is brown, black, red, gold. State its value and tolerance. [2 marks]

  • Cue. 1,0,×100=1000 Ω=1 kΩ1, 0, \times 100 = 1000\ \Omega = 1\ \text{k}\Omega, ±5%\pm 5\%.

Q3. How many terminals are connected when a variable resistor is used as a rheostat? [1 mark]

  • Cue. Two (one end of the track and the wiper).

Exam-style practice questions

Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Eduqas 20193 marksA resistor has the colour bands red, violet, orange, gold. State its resistance and its tolerance, and give the range of values it could actually have.
Show worked answer →

Read the first two bands as digits and the third as the multiplier (number of zeros): red =2= 2, violet =7= 7, orange =×1000= \times 1000. So the value is 27×1000=27 000 Ω=27 kΩ27 \times 1000 = 27\,000\ \Omega = 27\ \text{k}\Omega.

The gold band is the tolerance: ±5%\pm 5\%.

Tolerance range: 5%5\% of 27 kΩ27\ \text{k}\Omega is 1.35 kΩ1.35\ \text{k}\Omega, so the value lies between 25.65 kΩ25.65\ \text{k}\Omega and 28.35 kΩ28.35\ \text{k}\Omega.

Markers reward 27 kΩ27\ \text{k}\Omega, ±5%\pm 5\%, and a correct range. The usual error is treating the third band as a digit rather than the multiplier.

Eduqas 20212 marksExplain the difference between using a variable resistor as a potentiometer and using it as a rheostat.
Show worked answer →

Potentiometer (1 mark): all three terminals are used. The two ends are connected across a voltage and the wiper taps off a variable fraction of that voltage, so it acts as an adjustable potential divider giving a variable output voltage.

Rheostat (1 mark): only two terminals are used (one end and the wiper). It acts as a variable resistance in series with the rest of the circuit, so it controls the current.

Markers reward "potentiometer = three terminals / variable voltage" and "rheostat = two terminals / variable resistance or current".

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