Charging by induction
Two spheres and a charged rod
This topic, charging by induction, is beyond the current NCERT syllabus. It was in the older textbook, and it is still useful, for example for competitive exams.
Rubbing charges a body by moving electrons through contact. Induction charges a conductor without touching it at all. Let us see how.
Take two metal spheres, A and B, on insulating stands, touching each other. Both are neutral. Remember, in a metal, some electrons are free to move.
Bring a positively charged glass rod near sphere A, without touching it. The rod attracts the free electrons. They crowd onto the near side of A.
The far side of B is left short of electrons, so it becomes positive. The total charge of A and B together is still zero: the charges have only been separated.
Now, keeping the rod in place, pull the spheres apart. The negative charge is trapped on A, and the positive charge on B.
Finally, take the rod away. The charges on each sphere spread out evenly over its surface. A is negative and B is positive, by exactly equal amounts.
The rod has not touched anything, so it has lost none of its charge. And the near sphere always gets the charge opposite to the rod’s.
With a negatively charged rod, everything reverses. Electrons are pushed away from the near side, so A ends up positive and B negative.
Induction with the earth
You can also charge a single metal sphere by induction, using the earth. Start with one neutral sphere on an insulating stand.
Bring a positively charged rod near it. Free electrons gather on the near side, making it negative. The far side is left positive.
Now connect the sphere to the earth with a wire. The earth is a huge conductor, able to give or take any number of electrons. Electrons flow up from the earth and cancel the positive charge on the far side.
The negative charge on the near side cannot leave: the positive rod holds it in place.
Disconnect the earth wire first, while the rod is still near. Then take the rod away. The negative charge spreads evenly over the sphere.
The order matters. If you removed the rod while the sphere was still earthed, the extra electrons would just flow back to the earth, and the sphere would end up neutral.
Induction also explains why a charged rod attracts light, uncharged things. Bring a charged comb near a small bit of paper. The comb induces unlike charge on the near side and like charge on the far side.
The unlike charge is closer to the comb, so the attraction beats the repulsion. The net force is attractive, and the paper jumps up to the comb.
Try it: charging by induction
- Press Next step to go through the method with two touching spheres. Watch where the charges gather.
- At each step, check the readouts: A and B together always have zero charge, and the rod never changes.
- Switch to “Using earth” and step through again. Which way do the electrons flow through the earth wire?
- Tick “Use a negatively charged rod”. Every induced charge changes sign.
Which sphere is which?
Two touching metal spheres A and B stand on insulating stands. A negatively charged rod is brought near A. The spheres are separated, then the rod is removed. What are the charges now?
- A negative, B positive
- A positive, B negative
- Both negative
- Both neutral, because the rod never touched them
Answer
B. The negative rod repels free electrons from A towards B. A is left positive and B gets the extra electrons, so B is negative. Separating the spheres before removing the rod traps these charges. The near sphere always ends up opposite in sign to the rod.
Earthing in the right order
A positively charged rod is held near an earthed metal sphere. The rod is removed first, and only then is the earth connection broken. What charge does the sphere have?
- Negative
- Positive
- Zero
- Half the charge of the rod
Answer
C. While the sphere is earthed, the extra electrons on it are held only by the rod. Once the rod is removed, they flow back to the earth through the wire. Breaking the connection afterwards leaves the sphere neutral. To charge it, break the earth connection before removing the rod.
Summary
- Induction charges a conductor without touching it: a nearby charged body separates the free charges in the conductor.
- Two-sphere method: bring the rod near, separate the spheres, then remove the rod. The spheres get equal and opposite charges.
- Earthing method: bring the rod near, earth the sphere, break the earth connection, then remove the rod. The sphere gets a charge opposite to the rod’s.
- The inducing rod loses no charge. Induction also explains why a charged body attracts light, uncharged objects.