Which of the following instruments can be used to compare the magnitudes of charges on two given bodies?

Analyze the question: We need an instrument that can compare how large the charges on two bodies are, not one that just makes or transfers charge.

Concept: Each option does a different job in electrostatics.

• Electrophorus: It produces charge by induction, so it is a charge generator, not a measuring device.
• Ebonite rod: This is just an insulating material. When rubbed with fur it becomes negatively charged, so it is used to charge other bodies.
• Gold-leaf electroscope: It detects charge and also gives a rough measure of its size. The more charge placed on the cap, the greater the divergence of the leaf. You can charge the electroscope with one body and note the leaf divergence, then repeat with the second body. The body that causes greater divergence carries the larger charge.
• Proof plane: It is a small conducting disc on an insulating handle. It is used to pick up charge from a surface to test it, but it does not display or measure the charge by itself. It needs an electroscope to show a result.

Solution:

1. Touch the first charged body to the electroscope cap and note the leaf divergence.
2. Discharge the electroscope.
3. Repeat with the second body.
4. Compare the two divergences. The larger divergence means the larger charge.

Answer: The gold-leaf electroscope, option C.

Exam tip: Remember the roles this way:

• Electrophorus produces charge.
• Proof plane transfers charge.
• Electroscope detects and compares charge.

If a question says "detect," "test," or "compare" charge, think electroscope.