Describe simple experiments to show the production of electrostatic charges by friction and to show the detection of electrostatic charges

4.2.1 Electric Charge

Learning Objective

Describe simple experiments to show the production of electrostatic charges by friction and to show the detection of electrostatic charges.

Key Concepts

• Electrostatic charge is a property of matter that causes it to experience a force when placed in an electric field.
• Charges are transferred by friction, conduction or induction.
• Like charges repel; unlike charges attract.
• The unit of charge is the coulomb (C); the elementary charge is \$e = 1.602 \times 10^{-19}\,\text{C}\$.

Experiment 1 – Producing Charge by Friction

This experiment demonstrates how rubbing two different materials together can transfer electrons and create static charge.

Apparatus

Procedure

1. Secure the plastic rod on the insulating stand.
2. Take the woollen cloth and rub it vigorously along the length of the rod for about 10 seconds.
3. Hold a small piece of paper near the rod without touching it.
4. Observe whether the paper is attracted to the rod.
5. Repeat the experiment using a glass rod and silk cloth; note any differences.

Explanation

Rubbing transfers electrons from one material to the other. In the first case, electrons move from the woollen cloth to the plastic rod, giving the rod a negative charge and the cloth a positive charge. The charged rod attracts neutral paper because the electric field induces a separation of charges within the paper.

Experiment 2 – Detecting Charge with a Simple Electroscope

A basic electroscope can show the presence and type of charge on an object.

Apparatus

Construction

1. Insert the metal rod through a small hole in the lid of the jar so that it hangs down inside.
2. Attach the two aluminium leaves to the lower end of the rod so they hang side by side.
3. Seal the jar; the interior is now isolated from the external environment.

Procedure

1. Observe the leaves; they should hang vertically and touch each other (neutral state).
2. Bring the negatively charged plastic rod close to the metal rod without touching it. Note the leaves' movement.
3. Touch the plastic rod briefly to the metal rod, then remove it. Observe the leaves again.
4. Repeat the steps using a positively charged glass rod (rubbed with silk).

Explanation

When a charged object is brought near the metal rod, the electric field induces a separation of charge in the rod and leaves, causing them to repel each other and diverge. Direct contact transfers charge to the leaves; like charges on each leaf cause them to stay apart. The direction of deflection indicates the sign of the charge transferred.

Experiment 3 – Detecting Charge with a Charged Pith Ball

A lightweight pith ball suspended by a thread can be used to test for the presence of charge.

Apparatus

Procedure

1. Hang the pith ball from the silk thread so it can swing freely.
2. Bring a negatively charged plastic rod close to the pith ball without touching it. Observe the motion.
3. Touch the rod to the pith ball, then move the rod away and note the behaviour.
4. Repeat with a positively charged glass rod.

Explanation

The pith ball is initially neutral. When a charged rod is brought near, the electric field induces a separation of charge, causing the ball to be attracted regardless of the sign of the rod. After direct contact, the ball acquires the same type of charge as the rod and will be repelled by a similarly charged rod and attracted by the opposite charge.

Safety and Precautions

• Do not touch the metal parts of the electroscope with wet hands; moisture provides a conductive path to ground.
• Avoid rubbing materials for too long; excessive static can cause small sparks.
• Keep flammable materials away from any sparks produced.
• Handle glass rods carefully to prevent breakage.

Summary

Friction between two different materials transfers electrons, producing electrostatic charge. The charge can be detected using simple devices such as an electroscope, a pith ball, or by observing the attraction of neutral objects. These experiments illustrate the fundamental properties of static electricity and lay the groundwork for further study of electric fields and forces.

Review Questions

1. Explain why rubbing a woollen cloth on a plastic rod makes the rod negatively charged.
2. What observation would indicate that an electroscope has been positively charged?
3. Why does a neutral pith ball get attracted to both positively and negatively charged rods before it is touched?
4. Describe how you could use the electroscope to determine whether an unknown object is positively or negatively charged.