The Leaves Move Before Any Charge Arrives
Learn how a body can be charged by contact or without any contact at all, how a gold leaf electroscope and a pith ball detect charge, how to find the sign of an unknown charge, and how a lightning conductor protects a building.
How can an electroscope react to a charge that never touches it?
Bring a charged rod near the metal cap of an electroscope — close but not touching — and the thin leaves inside immediately spring apart.
No charge has crossed the gap. What happened instead is that the rod's charge rearranged the charges already inside the instrument, pushing like charges down into the leaves, which then repel each other.
Take the rod away without ever touching it and the leaves fall back together, because nothing was ever added or removed. This is induction, and it is one of two ways a body's charge can be affected. This page covers the fourth part of the ICSE Class 8 Physics chapter on electricity: charging without friction, the instruments that detect charge, and lightning.
No charge has crossed the gap. What happened instead is that the rod's charge rearranged the charges already inside the instrument, pushing like charges down into the leaves, which then repel each other.
Take the rod away without ever touching it and the leaves fall back together, because nothing was ever added or removed. This is induction, and it is one of two ways a body's charge can be affected. This page covers the fourth part of the ICSE Class 8 Physics chapter on electricity: charging without friction, the instruments that detect charge, and lightning.
How do charging by conduction and by induction differ?
Conduction needs contact and transfers charge. Induction needs no contact and only rearranges it.
Charging by conduction. Touch a charged body to an uncharged conductor and some charge flows across. Both bodies end up with the same sign of charge, and the total is shared between them.
Worked example. A rod carries and is touched to an identical uncharged metal sphere. Being identical, they share equally:
So each carries half, and both are the same sign as the original rod. Note the total is unchanged — charge has only been divided.
Charging by induction. Bring a charged body near a conductor without touching. The conductor's free electrons shift: the near face acquires the opposite charge and the far face the same charge. The conductor as a whole is still neutral, and removing the rod undoes everything.
To make induction permanent, the far charge must be let go:
- Bring the charged rod near the conductor and hold it there.
- Earth the far side by touching it briefly with a finger. The like charge escapes to ground.
- Remove the finger first, keeping the rod in place.
- Now remove the rod. The conductor is left charged opposite to the rod.
The comparison in short:
- Contact. Conduction: required. Induction: not required.
- Sign obtained. Conduction: same as the charging body. Induction: opposite.
- Charge on the charging body. Conduction: it loses some. Induction: it loses none.
- Permanence. Conduction: permanent. Induction: temporary unless earthing is used.
The step students get wrong. The finger must be lifted before the rod. Remove the rod first and the separated charges simply flow back together through your finger, leaving the conductor neutral again — the whole procedure wasted.
Charging by conduction. Touch a charged body to an uncharged conductor and some charge flows across. Both bodies end up with the same sign of charge, and the total is shared between them.
Worked example. A rod carries and is touched to an identical uncharged metal sphere. Being identical, they share equally:
So each carries half, and both are the same sign as the original rod. Note the total is unchanged — charge has only been divided.
Charging by induction. Bring a charged body near a conductor without touching. The conductor's free electrons shift: the near face acquires the opposite charge and the far face the same charge. The conductor as a whole is still neutral, and removing the rod undoes everything.
To make induction permanent, the far charge must be let go:
- Bring the charged rod near the conductor and hold it there.
- Earth the far side by touching it briefly with a finger. The like charge escapes to ground.
- Remove the finger first, keeping the rod in place.
- Now remove the rod. The conductor is left charged opposite to the rod.
The comparison in short:
- Contact. Conduction: required. Induction: not required.
- Sign obtained. Conduction: same as the charging body. Induction: opposite.
- Charge on the charging body. Conduction: it loses some. Induction: it loses none.
- Permanence. Conduction: permanent. Induction: temporary unless earthing is used.
The step students get wrong. The finger must be lifted before the rod. Remove the rod first and the separated charges simply flow back together through your finger, leaving the conductor neutral again — the whole procedure wasted.
How does a gold leaf electroscope work?
It turns an invisible charge into a visible movement of two thin leaves.
Construction. A metal disc or cap sits on top of a metal rod, which passes down through an insulating plug of ebonite or cork into a glass jar. At the lower end of the rod hang two very thin gold or aluminium leaves, touching each other when uncharged. The glass jar keeps out draughts and moisture, and the insulating plug stops charge leaking to the stand.
Working. Charge given to the cap travels down the metal rod to both leaves. Both leaves therefore receive the same sign of charge, they repel each other, and they diverge. The greater the charge, the wider they spread.
Why gold leaf. The leaves must be as light as possible so that even a tiny electrostatic force can move them. Gold and aluminium can be beaten into extremely thin, light sheets, and they conduct.
A pith ball electroscope is the simpler version. A light ball of pith — the soft dry core of a plant stem — is hung from an insulating stand by a silk thread. A charged body brought near attracts the neutral ball first; once contact is made, the ball takes the same charge and is then repelled, swinging away and staying away.
Which to use when. The pith ball is easy to make and shows attraction and repulsion clearly. The gold leaf version is far more sensitive and, as the next section shows, can tell you the sign of an unknown charge — something a single pith ball cannot do reliably.
And the link to the previous chapter. Both instruments rest on one fact: like charges repel. Everything the leaves do is that law made visible.
Construction. A metal disc or cap sits on top of a metal rod, which passes down through an insulating plug of ebonite or cork into a glass jar. At the lower end of the rod hang two very thin gold or aluminium leaves, touching each other when uncharged. The glass jar keeps out draughts and moisture, and the insulating plug stops charge leaking to the stand.
Working. Charge given to the cap travels down the metal rod to both leaves. Both leaves therefore receive the same sign of charge, they repel each other, and they diverge. The greater the charge, the wider they spread.
Why gold leaf. The leaves must be as light as possible so that even a tiny electrostatic force can move them. Gold and aluminium can be beaten into extremely thin, light sheets, and they conduct.
A pith ball electroscope is the simpler version. A light ball of pith — the soft dry core of a plant stem — is hung from an insulating stand by a silk thread. A charged body brought near attracts the neutral ball first; once contact is made, the ball takes the same charge and is then repelled, swinging away and staying away.
Which to use when. The pith ball is easy to make and shows attraction and repulsion clearly. The gold leaf version is far more sensitive and, as the next section shows, can tell you the sign of an unknown charge — something a single pith ball cannot do reliably.
And the link to the previous chapter. Both instruments rest on one fact: like charges repel. Everything the leaves do is that law made visible.
How do you find whether a charge is positive or negative?
Give the electroscope a known charge first, then watch which way the leaves move.
A neutral electroscope cannot tell you the sign, because both a positive and a negative body make its leaves diverge. To get the sign you need a reference.
The procedure.
- Charge the electroscope to a known sign — say negative, by touching its cap with an ebonite rod rubbed with fur. The leaves now stand apart.
- Bring the unknown body near the cap, without touching.
- If the leaves diverge further, the unknown charge is the same sign as the electroscope — negative. More like charge has been driven into the leaves.
- If the leaves collapse towards each other, the unknown charge is the opposite sign — positive. It has drawn some of the leaves' charge up towards the cap.
What each observation on a neutral electroscope means instead.
- Leaves diverge — the body is charged, but the sign is unknown.
- Leaves stay together — the body is uncharged.
So a neutral electroscope answers is it charged? and a pre-charged one answers which sign?
The same distinction as before. This is the practical form of the rule that repulsion is the sure test of charge: only the increase in divergence — a repulsion effect — identifies the sign with certainty.
A third use. Touching a charged body to the cap lets you compare charge sizes. A wider divergence means a larger charge, so an electroscope works as a rough measuring instrument as well as a detector.
Why the leaves eventually fall on their own. Charge slowly leaks away through the air, especially in humid weather, and through any dust on the insulating plug. A collapsing divergence after a few minutes is the instrument discharging, not a change in the body being tested.
A neutral electroscope cannot tell you the sign, because both a positive and a negative body make its leaves diverge. To get the sign you need a reference.
The procedure.
- Charge the electroscope to a known sign — say negative, by touching its cap with an ebonite rod rubbed with fur. The leaves now stand apart.
- Bring the unknown body near the cap, without touching.
- If the leaves diverge further, the unknown charge is the same sign as the electroscope — negative. More like charge has been driven into the leaves.
- If the leaves collapse towards each other, the unknown charge is the opposite sign — positive. It has drawn some of the leaves' charge up towards the cap.
What each observation on a neutral electroscope means instead.
- Leaves diverge — the body is charged, but the sign is unknown.
- Leaves stay together — the body is uncharged.
So a neutral electroscope answers is it charged? and a pre-charged one answers which sign?
The same distinction as before. This is the practical form of the rule that repulsion is the sure test of charge: only the increase in divergence — a repulsion effect — identifies the sign with certainty.
A third use. Touching a charged body to the cap lets you compare charge sizes. A wider divergence means a larger charge, so an electroscope works as a rough measuring instrument as well as a detector.
Why the leaves eventually fall on their own. Charge slowly leaks away through the air, especially in humid weather, and through any dust on the insulating plug. A collapsing divergence after a few minutes is the instrument discharging, not a change in the body being tested.
How is lightning produced, and how does a lightning conductor help?
Lightning is an enormous electrostatic discharge — the same spark as a charged comb touching a tap, on a vastly larger scale.
How the charge builds up. Inside a tall storm cloud, violent air currents drive water droplets and ice particles past one another. The friction between them transfers electrons, exactly as rubbing a comb on hair does. The charges separate within the cloud, so the lower part of the cloud typically becomes strongly negative and the upper part positive.
The negatively charged cloud base then induces a positive charge on the ground and on tall objects beneath it — that is induction again, over a distance of hundreds of metres.
The discharge. When the potential difference becomes great enough, the air between cloud and ground stops insulating and momentarily conducts. A huge charge rushes across as a flash of lightning. The path heats the air so violently that it expands explosively, and that expansion is what we hear as thunder.
Why the flash comes before the sound. Light travels at and sound in air at about , so the flash arrives almost instantly while the sound takes noticeable time. The delay gives the distance.
Worked example. Thunder is heard after the flash:
So the strike was about kilometres away. As a rough rule, every three seconds of delay means about one kilometre.
The lightning conductor. A thick copper strip runs from a set of pointed metal spikes fixed above the highest part of a building, down its outer wall, to a large copper plate buried deep in moist earth.
It protects the building two ways:
- The sharp points concentrate the induced charge so strongly that they quietly leak charge into the air, partly neutralising the cloud overhead and reducing the chance of a strike.
- If a strike does happen, the conductor offers a path of very low resistance straight to earth, so the charge passes down the copper instead of through the building's walls, wiring or occupants.
The misconception to clear. A lightning conductor does not repel lightning or make a building immune. It gives the discharge a safe route, which is the same principle as the earth wire of the previous part — a deliberately easy path to ground so the dangerous one is not taken.
Safety during a storm. Stay indoors; avoid open ground, isolated tall trees and water bodies; keep away from metal railings, poles and windows; and unplug sensitive appliances.
How the charge builds up. Inside a tall storm cloud, violent air currents drive water droplets and ice particles past one another. The friction between them transfers electrons, exactly as rubbing a comb on hair does. The charges separate within the cloud, so the lower part of the cloud typically becomes strongly negative and the upper part positive.
The negatively charged cloud base then induces a positive charge on the ground and on tall objects beneath it — that is induction again, over a distance of hundreds of metres.
The discharge. When the potential difference becomes great enough, the air between cloud and ground stops insulating and momentarily conducts. A huge charge rushes across as a flash of lightning. The path heats the air so violently that it expands explosively, and that expansion is what we hear as thunder.
Why the flash comes before the sound. Light travels at and sound in air at about , so the flash arrives almost instantly while the sound takes noticeable time. The delay gives the distance.
Worked example. Thunder is heard after the flash:
So the strike was about kilometres away. As a rough rule, every three seconds of delay means about one kilometre.
The lightning conductor. A thick copper strip runs from a set of pointed metal spikes fixed above the highest part of a building, down its outer wall, to a large copper plate buried deep in moist earth.
It protects the building two ways:
- The sharp points concentrate the induced charge so strongly that they quietly leak charge into the air, partly neutralising the cloud overhead and reducing the chance of a strike.
- If a strike does happen, the conductor offers a path of very low resistance straight to earth, so the charge passes down the copper instead of through the building's walls, wiring or occupants.
The misconception to clear. A lightning conductor does not repel lightning or make a building immune. It gives the discharge a safe route, which is the same principle as the earth wire of the previous part — a deliberately easy path to ground so the dangerous one is not taken.
Safety during a storm. Stay indoors; avoid open ground, isolated tall trees and water bodies; keep away from metal railings, poles and windows; and unplug sensitive appliances.
Exam tip
Exam tip: remove the finger before the rod
In any charging-by-induction sequence, the order is fixed: rod near, earth briefly, remove the earthing finger first, then remove the rod. Reversing the last two steps leaves the conductor neutral, and examiners mark on the order.
State the sign obtained: conduction gives the same sign as the charging body, induction the opposite sign.
Say that in induction the charging body loses no charge, unlike conduction where it shares its own.
For the electroscope, name the parts precisely — metal cap, metal rod, insulating plug, two gold or aluminium leaves, glass jar — and give the reason for each: light leaves for sensitivity, insulating plug to stop leakage, glass jar to exclude draughts and moisture.
Explain divergence as the two leaves receiving like charge and repelling. That is the whole mechanism.
For a sign-of-charge question, always begin by charging the electroscope with a known sign. Answering from a neutral electroscope cannot give a sign, and saying so earns the mark.
For lightning, use the words friction for the charge build-up, induction for the ground charge and discharge for the flash. And for the conductor, say it provides a low-resistance path to earth — never that it repels lightning.
When calculating a strike distance, use the speed of sound, not the speed of light.
State the sign obtained: conduction gives the same sign as the charging body, induction the opposite sign.
Say that in induction the charging body loses no charge, unlike conduction where it shares its own.
For the electroscope, name the parts precisely — metal cap, metal rod, insulating plug, two gold or aluminium leaves, glass jar — and give the reason for each: light leaves for sensitivity, insulating plug to stop leakage, glass jar to exclude draughts and moisture.
Explain divergence as the two leaves receiving like charge and repelling. That is the whole mechanism.
For a sign-of-charge question, always begin by charging the electroscope with a known sign. Answering from a neutral electroscope cannot give a sign, and saying so earns the mark.
For lightning, use the words friction for the charge build-up, induction for the ground charge and discharge for the flash. And for the conductor, say it provides a low-resistance path to earth — never that it repels lightning.
When calculating a strike distance, use the speed of sound, not the speed of light.
Did you know
Why are the tips of a lightning conductor made sharp?
Charge on a conductor does not spread itself evenly. It crowds onto the most sharply curved parts, so a point carries far more charge per unit area than a flat or rounded surface does.
That crowding makes the electric effect at a sharp tip intense enough to pull charge out of the tip and into the surrounding air, quietly and continuously — a slow leak rather than a sudden spark. A rounded knob of the same metal, at the same height, would hold its charge and leak almost nothing.
So the spikes on a building are doing something before any lightning arrives. They are steadily bleeding away the induced charge, working to keep the difference between cloud and building small enough that no flash occurs.
It is the same reason a charged comb attracts paper at its teeth rather than its flat back, and why high-voltage equipment is built with smooth rounded surfaces everywhere the designer does not want charge to escape.
That crowding makes the electric effect at a sharp tip intense enough to pull charge out of the tip and into the surrounding air, quietly and continuously — a slow leak rather than a sudden spark. A rounded knob of the same metal, at the same height, would hold its charge and leak almost nothing.
So the spikes on a building are doing something before any lightning arrives. They are steadily bleeding away the induced charge, working to keep the difference between cloud and building small enough that no flash occurs.
It is the same reason a charged comb attracts paper at its teeth rather than its flat back, and why high-voltage equipment is built with smooth rounded surfaces everywhere the designer does not want charge to escape.
Key takeaways
Induction, electroscopes and lightning: quick revision
- Charging by conduction: contact required, charge is transferred, both bodies end with the same sign, and the charging body loses some. shared with an identical sphere gives each.
- Charging by induction: no contact, charges only rearrange — near face opposite, far face same — and the charging body loses nothing.
- To make induction permanent: rod near, earth the far side, remove the finger first, then the rod. The body is left charged opposite to the rod.
- A gold leaf electroscope has a metal cap, metal rod, insulating plug, two light leaves and a glass jar. Charge reaching both leaves makes them repel and diverge — the law of like charges made visible.
- A pith ball electroscope is simpler: the neutral ball is first attracted, then takes the charge on contact and is repelled.
- On a neutral electroscope, divergence proves the body is charged but not its sign.
- To find the sign, charge the electroscope first: wider divergence means the same sign, collapse means the opposite sign.
- Lightning: friction between droplets and ice inside a cloud separates charge, the cloud base induces the opposite charge on the ground, and when the potential difference is great enough the air conducts and discharges as a flash. Thunder is the explosive expansion of the heated air.
- Flash before sound: for a five-second delay, roughly a kilometre for every three seconds.
- A lightning conductor — spikes, copper strip, buried plate — leaks induced charge from its sharp points and gives any strike a low-resistance path to earth. It does not repel lightning.
Practise writing out the induction sequence in the right order and the sign-detection procedure from memory — both are short, both are asked, and both are lost by getting one step out of place.
- Charging by induction: no contact, charges only rearrange — near face opposite, far face same — and the charging body loses nothing.
- To make induction permanent: rod near, earth the far side, remove the finger first, then the rod. The body is left charged opposite to the rod.
- A gold leaf electroscope has a metal cap, metal rod, insulating plug, two light leaves and a glass jar. Charge reaching both leaves makes them repel and diverge — the law of like charges made visible.
- A pith ball electroscope is simpler: the neutral ball is first attracted, then takes the charge on contact and is repelled.
- On a neutral electroscope, divergence proves the body is charged but not its sign.
- To find the sign, charge the electroscope first: wider divergence means the same sign, collapse means the opposite sign.
- Lightning: friction between droplets and ice inside a cloud separates charge, the cloud base induces the opposite charge on the ground, and when the potential difference is great enough the air conducts and discharges as a flash. Thunder is the explosive expansion of the heated air.
- Flash before sound: for a five-second delay, roughly a kilometre for every three seconds.
- A lightning conductor — spikes, copper strip, buried plate — leaks induced charge from its sharp points and gives any strike a low-resistance path to earth. It does not repel lightning.
Practise writing out the induction sequence in the right order and the sign-detection procedure from memory — both are short, both are asked, and both are lost by getting one step out of place.