Detection of Charges

Charging process (by induction with earthing)

Charging the polythene slab:

1. Rub the polythene slab with fur.
2. The slab becomes negatively charged by gaining electrons from the fur.

Placing the brass disc on the slab:

1. Carefully place the brass disc (part of the electroscope) on top of the charged slab.
2. Because the brass disc is a conductor, the negative charge on the slab repels electrons in the brass disc upward.
3. This creates positive charges on the side of the disc near the slab (due to electron repulsion) and negative charges on the upper surface.

Touching the brass disc (earthing):

1. Touch the top of the brass disc with a finger.
2. The repelled electrons on the top surface flow through your body to the ground (earthing).
3. Now, the disc is left with an overall positive charge because the electrons have been removed.

Removing your finger first: Always remove your finger before removing the charged slab, so that the induced charge remains on the electroscope.

Removing the polythene slab:

1. Now, take away the polythene slab.
2. The electroscope is now positively charged by induction, and the gold leaf diverges, showing that it has stored charge.

Discharging process

Bring a negatively charged object near:

1. Bring a negatively charged object (e.g., another polythene rod) close to the brass disc.
2. The electrons in the brass disc are repelled downward to the gold leaves.

Neutralizing the positive charge:

1. The negative charges cancel out the positive charge in the gold leaves.
2. The gold leaves collapse, showing that the electroscope has been discharged.

Removing the object: Once the charged object is removed, the leaves may diverge again slightly if there's leftover charge, or they may remain collapsed if fully discharged.

The electrophorus can now be used to charge a gold leaf electroscope.

It can be used to charge a gold leaf electroscope by:

1. Contact
2. Induction

Charging by contact is the process of transferring electric charge to an object by directly touching it with another charged object.

Explanation

i. When a charged object (like a positively charged electrophorus) is touched to a neutral conductor (like the brass cap of a gold-leaf electroscope), some of the charges are transferred from the charged object to the electroscope.

ii. As a result, the electroscope becomes charged with the same type of charge as the object that touched it. This is because direct contact allows free movement of electrons.

iii. The gold leaves diverge because like charges repel. The more charge transferred, the wider the leaves spread.

iv. To test the type of charge received, bring a known charged rod (like a positively charged glass rod) near the cap:

• If the leaves collapse, the rod has the same charge as the electroscope.
• If the leaves diverge more, the rod has the opposite charge.

Example

A positively charged electrophorus is touched to the brass cap of an uncharged gold-leaf electroscope.

• The electroscope acquires a positive charge through contact.
• The leaves diverge due to repulsion of like charges.
• When a positively charged glass rod is brought near, the leaves collapse, confirming the electroscope is also positively charged (like charges attract and neutralize).

Key point

Charging by contact always results in the electroscope (or object being charged) receiving the same type of charge as the charging object, and it only works with conductors, not insulators.

Induction is the process of transferring electric charge to an object without touching it, by using the electric field of a nearby charged object.

Explanation

i. When a charged object (e.g., a negatively charged polythene rod) is brought close to an uncharged conductor (like a metal rod), the electric field of the charged object causes redistribution of charges in the conductor.

ii. In the conductor, electrons move either toward or away from the charged object, depending on the type of charge:

• If the inducing object is negatively charged, electrons in the conductor are repelled and move to the far end, leaving the near end positively charged.
• If the inducing object is positively charged, electrons in the conductor are attracted and move toward the near end, making the far end negatively charged.

iii. While this redistribution occurs, if you touch the conductor, the excess like-charges (e.g., electrons) will flow into or out of your body to the ground. This process is called earthing.

iv. After removing your finger (to cut off the grounding path) and then the charged object, the conductor is left with a net charge opposite to the charge of the inducing object.

Example

Bring a negatively charged polythene rod near a metal ball suspended by an insulating thread.

• Electrons in the metal ball are repelled and move to the far side.
• You touch the metal ball briefly: excess electrons flow into your body.
• Remove your finger first, then remove the rod.
• The metal ball now has a net positive charge.

Key point about insulators

1. In insulators, electrons are tightly bound and cannot move freely.
2. Therefore, charging by induction is not effective with insulators.

Discharging an electroscope means removing the electric charge from it so that the gold leaves collapse back together.

i. Charging by Induction; First, bring a negatively charged rod near the brass cap of the electroscope without touching it. The negative charges (electrons) in the electroscope are repelled to the leaves, and the upper part becomes positively charged.

Example: Bringing a negatively charged polythene rod close to the brass cap.

ii. Touching the Cap; While the rod is still near the cap, you touch the brass cap with your finger. This provides a path for the repelled electrons to leave the electroscope and go into the Earth through your body.

Example: You gently touch the brass cap with your finger while the charged rod is nearby.

iii. Removing the Finger then the Rod; After that, you first remove your finger, then remove the charged rod. The electroscope is now left with a positive charge, and the gold leaves remain diverged.

iv. Discharging the Electroscope; Now, bring a negatively charged object near the cap again. This repels electrons back into the leaves, cancelling out the positive charge.

Example: Bringing the same negatively charged rod near the cap again causes electrons to return to the leaves.

v. Removing the Object; Once the object is removed, the excess electrons that returned to the leaves go back to the brass cap, and the leaves are left with a positive charge again, causing them to separate once more.

1. To detect electric charge; An electroscope helps us know whether an object is electrically charged. Example: If you rub a plastic pen with a dry cloth and bring it near the metal cap of an electroscope, the leaves will diverge. This shows the pen is charged.
2. To identify the type of charge; It helps determine whether a charge is positive or negative by comparing it with a known charge. Example: If a negatively charged electroscope shows increased leaf divergence when a rod is brought near, the rod is also negatively charged. If the leaves collapse, the rod has a positive charge.
3. To test insulating materials; An electroscope can be used to check whether a material is an insulator or a conductor. Example: If you touch the electroscope cap with plastic and the leaves do not react, the plastic is an insulator. But if you touch it with metal and the leaves collapse or move, the metal is a conductor.
4. To observe charging by friction or contact; It shows how charges can be transferred to the electroscope by rubbing or touching. Example: Rubbing a balloon on your hair and then touching the electroscope cap transfers charge and makes the leaves diverge.
5. To show discharge of charges (earthing); It can demonstrate how charges can be removed through grounding. Example: If you touch a charged electroscope with your finger, the leaves collapse, showing the charge has been earthed through your body.
6. To compare amounts of charge; The amount of leaf divergence indicates the strength of the charge. Example: Bringing two different charged rods to the electroscope, the one causing greater leaf movement has more charge.