3.6 Resistance in Series & Parallel

Resistors In Series

• When two or more components are connected in series:
  • The combined resistance of the components is equal to the sum of individual resistances

Resistors connected in series

• The equation for combined resistors in series is derived using the electric current rule and the electrical voltages rule
• These rules describe that for a series circuit:
  
  • The current is the same through all resistors
  • The potential difference is  split between all the resistors

• The equation for the combined resistance of resistors in series is therefore: 

Resistors In Parallel

• In a parallel circuit, the combined resistance of the components requires the use of reciprocals
  • The reciprocal of the combined resistance of two or more resistors is the sum of the reciprocals of the individual resistances

Resistors connected in parallel

• The equation for combined resistors in parallel is derived using the electric current rule and the electrical voltages rule
• These rules describe that for a parallel circuit:
  • The current is the split at the junction (and therefore between resistors)
  • The potential difference is the same across all resistors

• The equation for the combined resistance of resistors in parallel is therefore:

• This means the combined resistance decreases
  • The combined resistance is  less than the resistance of any of the individual components
  • For example, If two resistors of equal resistance are connected in parallel, then the combined resistance will halve

Series Circuits

Parallel Circuits