Many students predict incorrectly that two resistors in parallel will have a larger resistance than each resistor alone
Misconception
While most students recognise that the effective resistance of two (or more) resistors in series is their sum, many predict incorrectly that two resistors in parallel will have a larger resistance than each resistor alone.
Diagnostic Resources
The following worksheets may help to identify whether students hold this particular misconception.
For more information, see the University of York EPSE website.
The following questions probe understanding of the total resistance of combinations of resistors in parallel.
EPSE Voltage, Resistance and Current Q34The next group of questions look at the effect on the circuit of adding a second resistor in parallel.
EPSE Voltage, Resistance and Current Q40Q46 extends Q36 a little, by asking about the relative sizes of the current when a second resistor is added in series or in parallel.
EPSE Voltage, Resistance and Current Q46Resources to Address This

Episode 114: Components in Series and Parallel (1619)
Source  TAP/ Electricity / Series and parallel circuits / ...
This teaching sequence describes demonstrations, classroom activities, and discussions aimed at helping older students understand the result of combining components in series and parallel circuits.
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Adding a Resistor in Parallel... Shifting Energy (1416)
Source  SPT/ Ee02TL07
When the resistor is added in parallel, an extra current loop is provided and energy is shifted from the cell at a greater rate.
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Talking and thinking circuit calculations (1416)
Source  SPT/ Ee02TA06
This is an interactive teacher demonstration with a difference. It involves working not with apparatus, but with numbers in carrying out circuit calculations. The idea here is to make explicit to students the steps, or underlying strategy, involved in making calculations. All too often this systematic guidance is missed out and students struggle to make sense of relatively simple questions.
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References
The following studies have documented this misconception:
 Millar, R. and Beh, K. L. () Students’ understanding of voltage in simple parallel electric circuits. International Journal of Science Education, 15 (4),
351361.