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Conductors and Insulators

Primary schoolPhysicsOhmic material tester with temperature-dependent resistivity, finite contacts and a fixed-resistance lamp

Click a material in the tray and close the orange tester contact. Drag the voltage handle and compare contact quality, sample length and cross-sectional area.

3D

Drag the apparatus to adjust it. Drag the background to rotate. Scroll to zoom.

Measurement notebook

Change the setup, test a prediction and save the result. Up to 12 snapshots are kept.

About this experiment

Insert a sample between tester clips, close the circuit and compare its measured resistance with the lamp response. Control sample length, cross-sectional area, temperature and contact quality to separate a material property from the rest of the circuit.

Learning goals

  • Relate sample resistance to resistivity, length and area.
  • Explain why contact resistance can hide a good conductor.
  • Distinguish a dark lamp, a very small current and exactly zero current.

How to use it

  1. 1Choose copper, close the tester and record current and lamp power.
  2. 2Replace it with graphite, nichrome and dry glass without changing the geometry.
  3. 3For one sample, double length or cross-sectional area and compare the resistance ratio.
  4. 4Enable poor contacts and compare the current with a good-contact trial at the same voltage.

The model

Sample resistance depends on both material and geometry.
The tester current responds to the entire series circuit, not only the sample.

Assumptions

  • The battery has 0.5 Ω internal resistance, the lamp has fixed 20 Ω resistance and contacts contribute either 0.2 Ω or 50 Ω.
  • Material resistivity uses illustrative reference values and linear room-temperature coefficients. Heating does not feed back into temperature automatically.
  • Dry glass and rubber have large finite resistivities. The 1 µA detection threshold is a chosen display criterion, not a universal conductor-insulator boundary.
  • The model omits moisture, dielectric breakdown, semiconductor junctions and nonlinear incandescent filament resistance. Sample shape and lamp brightness are schematic.

Try this in class

  1. Q1Can a dim lamp tell you whether the sample or the contacts have high resistance?
  2. Q2Why is doubling area different from doubling length?
  3. Q3Does an insulating sample have precisely zero current in this model?

Frequently asked questions

Why does glass show a tiny current?

The model uses a very large finite resistivity. Its current is far below the selected display threshold, which is different from assuming an infinite resistance.

Does the sample temperature rise when current flows?

No. Temperature is an independently controlled test condition. The model does not integrate thermal energy or temperature changes.