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Dirt Changes Coin Resistance
The vending machine identifies the value of the coin by measuring the resistance of the coin.
Coins that are dirty are not recognised by the vending machine.
Suggest one reason why.
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Marking points
- 1 Links dirt to a changed measured resistance, an expected-value mismatch, or an explicitly allowed interruption of current.
Full-mark answer
The dirt changes the resistance measured by the machine, so the reading no longer matches the expected resistance of the coin.
Why this answer loses marks
The machine cannot see the dirty coin.
The machine identifies the coin electrically, not by appearance; link the dirt to a changed resistance reading and an expected-value mismatch.