ExpertEasy1 markShort Answer
AQA GCSE · Question 05.3 · Electricity
Coins that are dirty are not recognised by the vending machine.
Suggest one reason why.
Coins that are dirty are not recognised by the vending machine.
Suggest one reason why.
How to approach this question
The machine identifies coins by their resistance. What effect would a layer of dirt have on the electrical properties of the coin's surface?
Full Answer
Dirt is an insulator / increases the resistance of the coin.
The vending machine works by measuring the specific resistance of the metal alloy of the coin. Different coins have different, precisely known resistances. Dirt, grease, and oxidation on the surface of a coin are typically electrical insulators. This layer of dirt adds extra resistance, so when the machine measures the resistance, it gets a value that is higher than the expected value for any valid coin. As the measured resistance does not match any of the pre-programmed values, the machine rejects the coin.
Common mistakes
✗ Just saying "the dirt blocks it" without mentioning resistance or insulation.
✗ Suggesting it changes the size or mass, as the question is about resistance measurement.
Practice the full AQA GCSE Physics Higher Tier Paper 1
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