Electroscope - Definition, Working Principle, Types & Uses - Vedantu

An electroscope is a simple device used to detect the presence and nature of electric charges. It visually demonstrates fundamental electrostatic effects, particularly the principle that like charges repel and unlike charges attract. The electroscope's operation builds a strong conceptual foundation for understanding key ideas in electricity and magnetism. To explore electrostatics in action, you can create a basic electroscope using accessible materials like tape, plastic straws, and a plastic comb. This hands-on approach makes invisible electrical forces clear and memorable.

Electroscope Construction: Hands-On Model

A simple electroscope can be built using:

• Two plastic film canisters or small cups—partially filled with modeling clay for stability
• Four plastic drinking straws (preferably with flexible ends)
• Common 3M Scotch® Magic™ Tape (3/4-inch wide for consistent results)
• A plastic comb and either hair or wool cloth (for charging)

Press the inflexible ends of two straws into each canister and bend the flexible ends outward to form a "T" shape. Ensure both arms are at the same height. This setup allows suspension of tape strips that act as charge sensors.

Electroscope Working Principle

The key operation rests on charge transfer through contact or friction. When you press tape onto a table and peel it off, a transfer of electrons occurs. Depending on surface materials, tape either gains or loses electrons.

If both pieces of tape are prepared the same way, they end up with identical charges. When suspended, these tapes repel each other—demonstrating that like charges repel.

Experiment Procedure: Detecting and Comparing Charges

• Tear off two strips of tape (about 10 cm long each). Stick each firmly to a tabletop, leaving one end sticking up.
• Quickly pull both pieces from the surface, and suspend each from the arms of separate straws.
• Bring them face to face and observe—repulsion confirms both tapes have the same charge.
• Now, stick one strip to another (so their sticky and smooth sides contact). Rip apart and hang each. These attract each other, as the charge distribution now differs: one is positive, the other negative.

Rubbing a comb through hair (or wool) gives the comb a negative charge. Bring this comb near each tape and observe if it repels (same charge) or attracts (opposite or uncharged).

Electroscope Observations and Physics Explanations

What you see with the tape strips demonstrates the main law of electrostatics:

• Like charges repel: Two identical tapes push away from each other when suspended.
• Unlike charges attract: When a tape "sandwich" is separated, the two strips pull together, as one is now positive and one negative.

Attraction between an initially uncharged object and a charged tape occurs by induction: electrons in the neutral object shift slightly, creating a temporary dipole that attracts the tape.

Step-by-Step Charge Testing with an Electroscope

1. Charge the tape using friction (peel from table or another tape).
2. Bring an object near the tape. If tape and object repel, both are similarly charged.
3. If tape and object attract, the object could be oppositely charged or neutral.
4. Use a charged comb to identify the polarity of your test tape, then use this tape as a reference for other tests.

Test Scenario	Observation	Physics Interpretation
Two tapes, same preparation	Repel each other	Both tapes have same type of charge
Two tapes from sandwich arrangement	Attract each other	Tapes have opposite types of charge
Charged comb near tape	Attracts or repels	If repelled: same charge; if attracted: opposite or uncharged

Key Formula: Electrostatic Force of Repulsion

Formula	Application	Physical Meaning
F = (1/(4πε0)) × (q1q2)/r2	Force between charges on suspended tapes	Measures repulsion when tapes have like charge

Practical Tips for Consistent Results

• Use only recommended tape at first; different brands may produce different results.
• Some table surfaces may not charge the tape—testing is essential before demonstration.
• If tapes lose their charge, re-prepare them to ensure accurate tests.

Physics in Action: Electroscope Applications

The electroscope not only detects if an object is charged, but also helps identify the sign of charge through systematic testing. It makes the concept of electric fields and electrostatic forces tangible. By wiggling a charged comb near the tape, you create oscillating electric fields—demonstrating that the force between charges can even produce electromagnetic waves, the foundation of much of modern physics.

Application	Observation	Principle
Testing unknown objects for charge	Repulsion/Attraction of tape	Identifies similar/opposite charge or neutrality
Modeling charge induction	Hand attracts charged tape	Induced dipole in neutral objects

Practice Questions

1. Describe how you can use a homemade electroscope to distinguish between positively and negatively charged objects.
2. Explain why an uncharged hand attracts both positively and negatively charged tape strips.
3. Given two charges of +1 × 10-7C each placed 5 cm apart, calculate the force using the formula above.

Continue exploring more advanced electroscope concepts and other electrostatic instruments through additional resources:

• Electroscope: Detailed Physics Explanation
• Gold Leaf Electroscope: Working and Uses

Strengthen your understanding with experiments, key formulas, and guided lessons to prepare for physics problem-solving in all boards and entrance exam patterns.