Testing Materials in a Simple Circuit
Students built a circuit using one 1.5 V D-cell, one bulb, and three wires. They clipped each test material between two alligator clips so the material became part of the circuit, then recorded what the bulb did. Each material was tested once.
| Material | What the bulb did | | copper wire | bright glow | | aluminum foil | bright glow | | coin | bright glow | | pencil lead (graphite) | dim orange glow | | paper soaked in salt water | dim orange glow | | dry paper | no glow | | rubber band | no glow | | glass rod | no glow |
One student says the results prove that only metals can carry electric current.
1. Which materials carried current in this circuit? Explain why the 'dim' materials still count, and why 'no glow' is weaker evidence than it looks.
Read the worked answer
Current flowed through the copper wire, aluminum foil, coin, pencil lead, and salt-water paper. The first three gave a bright glow because they are good conductors with low resistance. The pencil lead and salt water gave only a dim glow, but a glow of any kind means the circuit was complete and charge was moving. A dim bulb means less current, not zero current, so those materials are poorer conductors but still conductors.
'No glow' is weaker evidence, because the bulb only lights when the current is above some minimum. Dry paper, rubber, and glass may have allowed a small current that this cell and bulb were simply unable to show. So the honest statement is that they are not good conductors in this circuit, not that they can never carry current.
2. Design a fair test for the question: does adding more salt to water make it a better conductor? List what you would change, what you would keep the same, and how you would measure the result more precisely than 'dim'.
Read the worked answer
Change only the amount of salt: four cups of water, each with the same volume, holding 0, 1, 2, and 3 teaspoons of dissolved salt. Keep everything else identical - same battery, same bulb, same size and type of paper strip, same 2 cm gap between the clips, same dipping time, and rinse and dry the clips between trials so salt does not build up.
The bulb's brightness is too coarse to compare, so turn it into a scale: off, dim, medium, bright, very bright, scored 0 to 4. Repeat each cup three times and use the most common score. If equipment allows, use two cells or a small motor, since a motor's speed changes more smoothly with current than a bulb's brightness.
3. The glass rod gave no glow. A student concludes that glass can never conduct electricity. Give two reasons this conclusion goes beyond the evidence.
Read the worked answer
First, the test was weak. One 1.5 V cell pushes only a small current, and the bulb ignores anything below a certain threshold, so a material with very high resistance would look identical to one with no current at all. A stronger source or a sensitive meter could give a different answer.
Second, the surface condition matters and was not controlled. A glass rod handled by many students may carry a film of moisture or salts, and that film, not the glass, can change what happens. The result also rests on a single trial with one sample, so a poor connection at the clips could have produced the same 'no glow'.
A fair conclusion is: glass did not conduct well enough to light this bulb under these conditions.