How to play Material Tester
- Choose a test and read the sample's size on the card and its resistivity in the question.
- Use R = ρ·L ÷ A with L in m and A in m² (1 mm² = 10⁻⁶ m²), and I = V ÷ R.
- Type your prediction and press Start. The switch closes after half a second.
- Read the ammeter, and see where your answer lands on the resistivity ladder.
Conductors, insulators, and resistivity
Whether a material conducts depends on how freely its electrons can move. Resistivity ρ measures how hard it is to push a current through a 1 m cube of it. Metals have free electrons and tiny resistivities, about 10⁻⁸ Ω·m for copper; glass and rubber hold their electrons tightly, at 10¹² Ω·m or more. No other everyday property spans so many powers of ten, which is why conductors and insulators behave so differently. The resistance of a particular piece also depends on its shape: R = ρ·L ÷ A, so a long thin wire resists more than a short thick one. Nichrome, with a resistivity about 65 times copper's, is used for heater elements; copper and aluminium carry current with as little loss as possible; and graphite, in between, makes pencil lead and brushes in electric motors.
A focused learning model
The battery, wires, clips, switch, and ammeter are ideal, so the sample is the only resistance; it doesn't warm up, so its resistivity stays the same. The current is steady from the moment the switch closes at 0.5 s; the run lasts 3 s. Resistivities are typical values: glass ranges from 10¹⁰ to 10¹⁴ Ω·m and rubber is higher still, and graphite depends on the lead. The electron dots move at a display speed set by the logarithm of the current, not at their real drift speed. Answers within 2% are right.