Rubbing Does Not Create Charge, It Only Moves Electrons
Learn why rubbing two bodies leaves one positive and the other negative, how like and unlike charges behave, why metals conduct while plastics do not, and how static electricity differs from the current in a wire.
Why does a comb pick up paper only after you run it through dry hair?
Because the rubbing has moved electrons from your hair onto the comb, leaving the comb with an excess of negative charge.
Nothing was created. The electrons existed before, sitting in the hair; friction simply transferred some of them. Count the charge on the comb and the charge left on the hair and they are equal and opposite — the pair together is exactly as neutral as it was.
That is the whole idea of charging by friction, and it explains why the hair becomes charged too, even though nobody notices. This page covers the third part of the ICSE Class 8 Physics chapter on electricity: how bodies acquire charge, how charges act on each other, why some materials conduct, and how static electricity differs from current.
Nothing was created. The electrons existed before, sitting in the hair; friction simply transferred some of them. Count the charge on the comb and the charge left on the hair and they are equal and opposite — the pair together is exactly as neutral as it was.
That is the whole idea of charging by friction, and it explains why the hair becomes charged too, even though nobody notices. This page covers the third part of the ICSE Class 8 Physics chapter on electricity: how bodies acquire charge, how charges act on each other, why some materials conduct, and how static electricity differs from current.
How does rubbing charge a body, and which one goes positive?
An atom has a positive nucleus and negatively charged electrons around it, and in a neutral body the two exactly balance.
The outer electrons are the only part that can move. The nucleus is heavy and locked in place, so charging is always a story about electrons and never about protons.
The rule: the body that loses electrons becomes positively charged; the body that gains them becomes negatively charged. Which body loses depends on how tightly each material holds its outer electrons.
The standard pairs, worth memorising:
- Glass rod rubbed with silk — glass becomes positive, silk negative.
- Ebonite or plastic rod rubbed with fur or wool — the rod becomes negative, the fur positive.
- Comb rubbed on dry hair — comb negative, hair positive.
- A balloon rubbed on hair — balloon negative, hair positive.
Charge comes in whole numbers of electrons. The charge on one electron has magnitude , so the number of electrons making up one coulomb is
One coulomb is therefore an enormous amount of charge — which is why rubbed bodies carry only tiny fractions of it.
Worked example. A plastic rod gains electrons by rubbing. Its charge is
That is nanocoulomb — a billionth-scale charge, from fifty billion electrons.
The conservation point. Since the silk gained exactly the electrons the glass lost, the two charges are equal in size and opposite in sign. Charge is never created or destroyed, only transferred, which is why rubbing produces charge is a misleading way to say it.
Why dry weather matters. On a humid day the thin film of moisture on every surface conducts the charge away as fast as it forms, so the comb refuses to work. Static electricity is a cold-dry-weather phenomenon for exactly that reason.
The outer electrons are the only part that can move. The nucleus is heavy and locked in place, so charging is always a story about electrons and never about protons.
The rule: the body that loses electrons becomes positively charged; the body that gains them becomes negatively charged. Which body loses depends on how tightly each material holds its outer electrons.
The standard pairs, worth memorising:
- Glass rod rubbed with silk — glass becomes positive, silk negative.
- Ebonite or plastic rod rubbed with fur or wool — the rod becomes negative, the fur positive.
- Comb rubbed on dry hair — comb negative, hair positive.
- A balloon rubbed on hair — balloon negative, hair positive.
Charge comes in whole numbers of electrons. The charge on one electron has magnitude , so the number of electrons making up one coulomb is
One coulomb is therefore an enormous amount of charge — which is why rubbed bodies carry only tiny fractions of it.
Worked example. A plastic rod gains electrons by rubbing. Its charge is
That is nanocoulomb — a billionth-scale charge, from fifty billion electrons.
The conservation point. Since the silk gained exactly the electrons the glass lost, the two charges are equal in size and opposite in sign. Charge is never created or destroyed, only transferred, which is why rubbing produces charge is a misleading way to say it.
Why dry weather matters. On a humid day the thin film of moisture on every surface conducts the charge away as fast as it forms, so the comb refuses to work. Static electricity is a cold-dry-weather phenomenon for exactly that reason.
What happens when two charged bodies are brought near each other?
Like charges repel; unlike charges attract. This is the law of electrostatic attraction and repulsion, and it is the fundamental rule of the chapter.
So:
- Positive and positive — repel
- Negative and negative — repel
- Positive and negative — attract
The force acts at a distance, with no contact needed, which makes it one of the non-contact forces met in the chapter on force.
Testing it. Suspend a charged glass rod from a thread. Bring a second charged glass rod near it and the suspended one swings away — both are positive, so they repel. Bring a charged ebonite rod near instead and it swings towards it, since ebonite is negative.
The important asymmetry. A charged body also attracts an uncharged one. Bring the charged comb near small bits of paper and the paper jumps to it, even though the paper has no net charge at all. The comb's charge briefly pulls the paper's own charges slightly out of balance, and the nearer, opposite charges are attracted more strongly than the farther, like ones are repelled.
This gives a precise test. Repulsion is the sure test of charge. Attraction is not, because an uncharged body is attracted too. So to prove a body is charged — and to find its sign — you must observe repulsion, not attraction.
That distinction is asked often and is easy to get wrong, because attraction is the effect you actually see first.
So:
- Positive and positive — repel
- Negative and negative — repel
- Positive and negative — attract
The force acts at a distance, with no contact needed, which makes it one of the non-contact forces met in the chapter on force.
Testing it. Suspend a charged glass rod from a thread. Bring a second charged glass rod near it and the suspended one swings away — both are positive, so they repel. Bring a charged ebonite rod near instead and it swings towards it, since ebonite is negative.
The important asymmetry. A charged body also attracts an uncharged one. Bring the charged comb near small bits of paper and the paper jumps to it, even though the paper has no net charge at all. The comb's charge briefly pulls the paper's own charges slightly out of balance, and the nearer, opposite charges are attracted more strongly than the farther, like ones are repelled.
This gives a precise test. Repulsion is the sure test of charge. Attraction is not, because an uncharged body is attracted too. So to prove a body is charged — and to find its sign — you must observe repulsion, not attraction.
That distinction is asked often and is easy to get wrong, because attraction is the effect you actually see first.
Why do metals conduct electricity while plastics do not?
Because metals contain free electrons and plastics do not.
In a metal, the outermost electron of each atom is held so loosely that it breaks away and wanders through the whole piece of metal. These free electrons belong to no particular atom, and a block of metal is best pictured as a lattice of fixed positive ions in a sea of drifting electrons.
Apply a potential difference and those free electrons all drift in one direction, and that drift is the electric current.
In an insulator every electron is firmly bound to its own atom. There is nothing free to drift, so no current flows however hard you push.
Conductors — all metals, best in silver and copper, then aluminium and iron. Also graphite (the one non-metal that conducts well), human body, earth, and acidic, basic or salty solutions including impure water.
Insulators — plastic, rubber, glass, dry wood, paper, dry air, porcelain, ebonite, pure distilled water.
Where the distinction is used. A wire is copper inside and plastic outside: the copper carries the current and the plastic keeps it there. A screwdriver for electrical work has a steel shaft and a thick rubber handle. Electric poles carry their lines on porcelain insulators.
The boundary case that matters for safety. Pure water is an insulator, but ordinary water is not. Tap water, sweat and rain all contain dissolved salts, which make them conduct well enough to be dangerous. This is precisely why wet hands and electrical switches must never meet, and it is the reason the safety rules of the previous part exist.
And a link back to charging. A charged conductor cannot hold its charge if you are touching it — the charge simply flows away through your body to earth. That is why charging by friction is demonstrated with insulating rods of glass, ebonite or plastic, and why a metal rod held in the hand cannot be charged this way at all.
In a metal, the outermost electron of each atom is held so loosely that it breaks away and wanders through the whole piece of metal. These free electrons belong to no particular atom, and a block of metal is best pictured as a lattice of fixed positive ions in a sea of drifting electrons.
Apply a potential difference and those free electrons all drift in one direction, and that drift is the electric current.
In an insulator every electron is firmly bound to its own atom. There is nothing free to drift, so no current flows however hard you push.
Conductors — all metals, best in silver and copper, then aluminium and iron. Also graphite (the one non-metal that conducts well), human body, earth, and acidic, basic or salty solutions including impure water.
Insulators — plastic, rubber, glass, dry wood, paper, dry air, porcelain, ebonite, pure distilled water.
Where the distinction is used. A wire is copper inside and plastic outside: the copper carries the current and the plastic keeps it there. A screwdriver for electrical work has a steel shaft and a thick rubber handle. Electric poles carry their lines on porcelain insulators.
The boundary case that matters for safety. Pure water is an insulator, but ordinary water is not. Tap water, sweat and rain all contain dissolved salts, which make them conduct well enough to be dangerous. This is precisely why wet hands and electrical switches must never meet, and it is the reason the safety rules of the previous part exist.
And a link back to charging. A charged conductor cannot hold its charge if you are touching it — the charge simply flows away through your body to earth. That is why charging by friction is demonstrated with insulating rods of glass, ebonite or plastic, and why a metal rod held in the hand cannot be charged this way at all.
How is static electricity different from current electricity?
Static electricity is charge at rest on the surface of a body. Current electricity is charge in motion through a conductor.
Setting them side by side:
- State of charge. Static: at rest, accumulated. Current: flowing continuously.
- Material. Static: produced on insulators, since a conductor would lose it. Current: flows through conductors.
- Source. Static: friction, or contact and induction. Current: a cell, battery or generator maintaining a potential difference.
- Duration. Static: temporary, leaking away in moments, especially in damp air. Current: continues as long as the circuit and the source last.
- Location. Static: resides on the surface of the body. Current: passes through the whole conductor.
- Quantity. Static: extremely small charges, as the nanocoulomb example showed. Current: far larger charge flows — one ampere is one coulomb every second.
- Use. Static: photocopiers, spray painting, chimney smoke precipitators. Current: every appliance in a house.
They are the same charge, differently arranged. This is the key point rather than a list to memorise. Electrons on a rubbed comb and electrons drifting through a copper wire are identical particles carrying identical charge — one lot is stuck in place and the other is on the move.
And one can become the other. Touch a charged comb to a metal object connected to earth and the accumulated charge drains away as a brief current. Static discharge is a short-lived current, which is what a spark actually is.
Putting the scale in perspective. The rubbed rod above carried . A current of moves every second — more than a hundred million times that charge, every second. The everyday drama of static electricity involves vanishingly little charge; it simply arrives all at once.
Setting them side by side:
- State of charge. Static: at rest, accumulated. Current: flowing continuously.
- Material. Static: produced on insulators, since a conductor would lose it. Current: flows through conductors.
- Source. Static: friction, or contact and induction. Current: a cell, battery or generator maintaining a potential difference.
- Duration. Static: temporary, leaking away in moments, especially in damp air. Current: continues as long as the circuit and the source last.
- Location. Static: resides on the surface of the body. Current: passes through the whole conductor.
- Quantity. Static: extremely small charges, as the nanocoulomb example showed. Current: far larger charge flows — one ampere is one coulomb every second.
- Use. Static: photocopiers, spray painting, chimney smoke precipitators. Current: every appliance in a house.
They are the same charge, differently arranged. This is the key point rather than a list to memorise. Electrons on a rubbed comb and electrons drifting through a copper wire are identical particles carrying identical charge — one lot is stuck in place and the other is on the move.
And one can become the other. Touch a charged comb to a metal object connected to earth and the accumulated charge drains away as a brief current. Static discharge is a short-lived current, which is what a spark actually is.
Putting the scale in perspective. The rubbed rod above carried . A current of moves every second — more than a hundred million times that charge, every second. The everyday drama of static electricity involves vanishingly little charge; it simply arrives all at once.
Exam tip
Exam tip: repulsion is the only sure test of charge
If a question asks how to prove a body is charged, answer repulsion. Attraction proves nothing, because a charged body attracts an uncharged one too. This single distinction appears in almost every paper.
Always explain charging in terms of electron transfer: the body that loses electrons becomes positive, the one that gains them becomes negative. Never say protons moved — they do not.
Memorise the standard pairs. Glass with silk: glass positive. Ebonite or plastic with fur or wool: rod negative. Comb on dry hair: comb negative.
State that the two charges are equal and opposite, since charge is only transferred, never created.
When asked why a metal rod held in the hand cannot be charged by rubbing, say the charge flows away to earth through the conducting body — and that insulating rods are used instead.
Mention dry conditions whenever a static experiment is described. Moisture in the air conducts the charge away.
For a comparison question, answer in pairs across static and current — state of charge, material, source, duration — rather than describing only one of them.
And keep pure water insulates, ordinary water conducts ready. It is both an exam answer and a safety rule.
Always explain charging in terms of electron transfer: the body that loses electrons becomes positive, the one that gains them becomes negative. Never say protons moved — they do not.
Memorise the standard pairs. Glass with silk: glass positive. Ebonite or plastic with fur or wool: rod negative. Comb on dry hair: comb negative.
State that the two charges are equal and opposite, since charge is only transferred, never created.
When asked why a metal rod held in the hand cannot be charged by rubbing, say the charge flows away to earth through the conducting body — and that insulating rods are used instead.
Mention dry conditions whenever a static experiment is described. Moisture in the air conducts the charge away.
For a comparison question, answer in pairs across static and current — state of charge, material, source, duration — rather than describing only one of them.
And keep pure water insulates, ordinary water conducts ready. It is both an exam answer and a safety rule.
Did you know
Why does a charged comb bend a thin stream of water?
Turn a tap down until the water runs in a fine thread, then bring a comb rubbed on dry hair close to it without touching. The stream bends visibly towards the comb.
The water carries no net charge, so this is the attraction of an uncharged body — the same effect as picking up bits of paper, but far more striking because the water is free to move continuously.
A water molecule has its positive and negative ends slightly separated, so in the comb's presence the molecules turn to point their positive ends towards it. With all of them lined up, the attraction outweighs any repulsion and the whole stream leans over.
What makes this a good demonstration is what it rules out. Nobody can claim the water was charged, because it came straight from an earthed metal tap. The bending must therefore be the attraction of a neutral body — which is exactly why attraction can never be used as a test for charge.
The water carries no net charge, so this is the attraction of an uncharged body — the same effect as picking up bits of paper, but far more striking because the water is free to move continuously.
A water molecule has its positive and negative ends slightly separated, so in the comb's presence the molecules turn to point their positive ends towards it. With all of them lined up, the attraction outweighs any repulsion and the whole stream leans over.
What makes this a good demonstration is what it rules out. Nobody can claim the water was charged, because it came straight from an earthed metal tap. The bending must therefore be the attraction of a neutral body — which is exactly why attraction can never be used as a test for charge.
Key takeaways
Static electricity and charge: quick revision
- Charging by friction transfers electrons only. The body that loses electrons becomes positive; the one that gains them becomes negative. Protons never move.
- Standard pairs: glass with silk — glass positive; ebonite or plastic with fur — rod negative; comb on dry hair — comb negative.
- Charge is conserved: the two bodies end up with equal and opposite charges, so the pair stays neutral overall.
- One electron carries , so one coulomb is electrons. A rod gaining electrons holds .
- Like charges repel, unlike charges attract — a non-contact force.
- A charged body also attracts an uncharged one, so repulsion is the only sure test of charge.
- Conductors have free electrons — metals, graphite, earth, the human body, salty or impure water. Insulators have none — plastic, rubber, glass, dry wood, dry air, pure water.
- Pure water insulates; ordinary water conducts — which is the reason for every wet-hands warning.
- A metal rod held in the hand cannot be charged by rubbing, because the charge drains to earth; insulating rods are used instead.
- Static electricity: charge at rest, on insulators, from friction, temporary, on the surface, very small. Current electricity: charge flowing, through conductors, from a cell or generator, sustained, throughout the conductor, far larger.
- They are the same charge in different states, and a spark is static charge becoming a brief current.
- Static electricity needs dry conditions; humidity conducts the charge away.
Try a set of questions where you have to name the sign on both bodies of a rubbed pair, and one on why repulsion is the real test — those two ideas carry most of this chapter.
- Standard pairs: glass with silk — glass positive; ebonite or plastic with fur — rod negative; comb on dry hair — comb negative.
- Charge is conserved: the two bodies end up with equal and opposite charges, so the pair stays neutral overall.
- One electron carries , so one coulomb is electrons. A rod gaining electrons holds .
- Like charges repel, unlike charges attract — a non-contact force.
- A charged body also attracts an uncharged one, so repulsion is the only sure test of charge.
- Conductors have free electrons — metals, graphite, earth, the human body, salty or impure water. Insulators have none — plastic, rubber, glass, dry wood, dry air, pure water.
- Pure water insulates; ordinary water conducts — which is the reason for every wet-hands warning.
- A metal rod held in the hand cannot be charged by rubbing, because the charge drains to earth; insulating rods are used instead.
- Static electricity: charge at rest, on insulators, from friction, temporary, on the surface, very small. Current electricity: charge flowing, through conductors, from a cell or generator, sustained, throughout the conductor, far larger.
- They are the same charge in different states, and a spark is static charge becoming a brief current.
- Static electricity needs dry conditions; humidity conducts the charge away.
Try a set of questions where you have to name the sign on both bodies of a rubbed pair, and one on why repulsion is the real test — those two ideas carry most of this chapter.