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 multiple resistors in series is their sum, many think 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)
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)
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)
This is an interactive teacher demonstration with a difference. It involves working not with apparatus, but with numbers in circuit calculations. The idea here is to explain to students the steps, or underlying strategy, involved in making calculations. This systematic guidance is often missed, and students struggle to make sense of relatively simple questions.
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References
 Millar, R. and Beh, K. L. () Students’ understanding of voltage in simple parallel electric circuits. International Journal of Science Education, 15 (4), 351361.