Static Electricity
A plastic comb rubbed through dry hair may attract small pieces of paper. In winter, removing synthetic clothes may also produce a faint crackling sound or a tiny spark. These effects are caused by static electricity.
To understand the Class 10 Electricity chapter well, it is important to understand stationary charge, transfer of charge and the force acting between charges.
👉 What is Static Electricity?
Static electricity is the phenomenon in which an imbalance of positive or negative electric charge develops on an object and the charge remains accumulated there for some time.
Here, “static” does not mean that charge can never move. It means that the charge is not flowing continuously as electric current through a closed circuit. If a suitable path becomes available, the accumulated charge may quickly discharge.
Example: Static charge can remain for some time on a rubbed comb, balloon, plastic scale or synthetic clothing.
👉 How Does an Object Become Charged?
Under ordinary conditions, an object is approximately electrically neutral because its total positive and negative charges balance each other. If electrons are transferred from one object to another, this balance is disturbed.
- If an object gains electrons, it becomes negatively charged.
- If an object loses electrons, it becomes positively charged.
Important
In ordinary static electricity phenomena, protons do not move from one object to another. The main process is the transfer of electrons.
👉 Conservation of Charge
When two objects are rubbed together, one object may lose some electrons and the other may gain those electrons. One object therefore acquires positive charge and the other an equal amount of negative charge.
Thus, new charge is not created; charge is transferred from one object to another and the total charge remains conserved.
This is a simple example of the law of conservation of charge.
👉 Interaction Between Charges
Electric charge is of two types: positive and negative.
| Nature of Charge | Interaction |
|---|---|
| Like charges (+,+ or −,−) | Repel each other |
| Unlike charges (+,−) | Attract each other |
Coulomb's law is used to calculate the magnitude of this attractive or repulsive force.
👉 Main Methods of Charging an Object
An object can mainly be charged in three ways.
1. Charging by Friction
When two different materials are rubbed together, electrons may be transferred between them. One object loses electrons and becomes positively charged, while the other gains electrons and becomes negatively charged.
Example: A plastic comb or balloon rubbed through dry hair can attract small pieces of paper.
Remember: Friction does not itself “create” charge. It provides an opportunity for electrons to transfer.
2. Charging by Contact / Conduction
When a charged conductor touches a neutral conductor, charge can redistribute between them. After separation, the previously neutral conductor may remain charged.
3. Charging by Induction
When a charged object is brought close to a neutral conductor without touching it, the charges inside the conductor rearrange. This phenomenon is called electrostatic induction.
With suitable earthing, a conductor can also be given a lasting charge by induction.
👉 Behaviour of Static Charge on Conductors and Insulators
| Property | Conductor | Insulator |
|---|---|---|
| Movement of charge | Charge carriers can move comparatively easily | Movement of charge carriers is very limited |
| Static charge | Can redistribute rapidly over the surface | Can remain locally trapped for a longer time |
| Examples | Copper, aluminium, iron | Glass, rubber, dry plastic |
This is why static charge is easily observed on plastic combs and balloons, especially in dry weather.
👉 Electrostatic Induction in Simple Terms
Suppose a positively charged rod is brought near a neutral metallic conductor. The free electrons in the conductor are attracted toward the end nearer the rod. The near end therefore becomes relatively negative and the far end relatively positive.
The rod has not touched the conductor, yet charge has rearranged inside the conductor. This is electrostatic induction.
This idea is important because a charged object can sometimes attract an electrically neutral object as well.
👉 Earthing
Earth may be treated as a very large reservoir of charge. If a conductor is connected to Earth through a conducting path, electrons can flow to Earth or come from Earth. This connection is called earthing.
Earthing provides a safe path for excess charge.
Everyday use: Earthing the metal body of electrical appliances is important for safety, although this is discussed in more detail under current electricity and domestic circuits.
👉 Electroscope
An electroscope can be used to test whether an object is charged. In a traditional gold-leaf electroscope, metal leaves or thin foils move apart when they acquire similar charges because like charges repel.
It is a basic instrument for detecting static charge.
👉 Coulomb's Law
For two stationary point charges, the magnitude of the electrostatic force:
- is directly proportional to the product of the magnitudes of the charges,
- is inversely proportional to the square of the distance between them,
- acts along the straight line joining the two charges.
Therefore,
and,
Hence,
or,
where,
- = electrostatic force between the two charges,
- = magnitudes of the charges,
- = distance between the charges,
- = proportionality constant depending on the medium.
Attraction or Repulsion?
- For like charges, the force is repulsive.
- For unlike charges, the force is attractive.
Using in the formula gives the magnitude of the force. The nature of the force is decided from the signs of the charges.
👉 Coulomb's Law in Vacuum
In vacuum,
Therefore,
where is the permittivity of free space.
Approximately,
So in vacuum, or approximately in air for many school-level problems,
may be used.
👉 Coulomb's Law in a Medium
If the permittivity of a medium is , then
If the relative permittivity or dielectric constant is , then
Therefore,
and for the same two charges at the same distance,
Thus, the greater the relative permittivity, the smaller the Coulomb force between the same charges under the same conditions.
Note: Older books often use for relative permittivity or dielectric constant. The notation is also widely used in modern texts.
👉 Important Results from Coulomb's Law
1. If One Charge is Doubled
Keeping everything else unchanged,
so doubling one charge doubles the force.
2. If Both Charges are Doubled
So the force becomes four times.
3. If the Distance is Doubled
So the force becomes one-fourth.
4. If the Distance is Tripled
So the force becomes one-ninth.
👉 Unit of Electric Charge
The SI unit of electric charge is coulomb, symbol C.
The magnitude of elementary charge is
The dimensional formula of charge is
Additional Information
In the CGS-ESU system, the unit of charge is statcoulomb (statC). Approximately,
For most Class 10 numerical problems, using SI units is more convenient.
👉 Solved Example 1
Two point charges and are placed apart in vacuum. Find the magnitude of the force between them.
Given,
According to Coulomb's law,
Therefore, the force is 0.6 N. It is repulsive if the charges are alike and attractive if they are unlike.
👉 Solved Example 2
The force between two charges is 16 N. If the distance between them is doubled, what is the new force?
According to Coulomb's law,
When the distance is doubled,
Therefore, the new force is 4 N.
👉 Solved Example 3
The force between two charges in vacuum is 12 N. If they are placed at the same distance in a medium of relative permittivity , find the force.
Therefore, the force in the medium is 4 N.
👉 Static Electricity vs Current Electricity
| Static Electricity | Current Electricity |
|---|---|
| Charge remains accumulated on an object | Charge carriers undergo an organised flow |
| Commonly observed through friction, contact or induction | Usually observed in a closed circuit with a potential difference |
| Can discharge suddenly | Can flow continuously in a circuit |
| Example: rubbed comb, balloon | Example: battery-powered circuit |
👉 Static Electricity in Everyday Life
⭐ Comb and paper: A plastic comb rubbed through hair may attract small pieces of paper.
⭐ Balloon and hair: After a balloon is rubbed through hair, strands of hair may move toward the balloon.
⭐ Synthetic clothes: In dry weather, removing synthetic clothes may produce crackling sounds or tiny sparks.
⭐ Small shock from a car or metal door: Static charge accumulated on the body may suddenly discharge and produce a small shock.
⭐ Lightning: Large-scale charge separation in clouds can lead to a sudden large electrical discharge. This is a natural large-scale example of static electricity.
👉 Why is Static Electricity Less Noticeable on Humid Days?
Dry air behaves as a comparatively good insulator, so charge accumulated on an object may remain there for some time. In humid air, a thin moisture layer may form on surfaces and allow charge to leak away gradually.
Therefore, static shocks are often more noticeable on dry winter days and less noticeable in humid conditions.
👉 Try It Yourself (Quick Practice)
- Why do we say that no new charge is created when an object becomes charged by friction?
- A negatively charged rod is brought near a neutral metal sphere. How will the charge in the sphere rearrange?
- The force between two charges is 20 N. What will it become if the distance is tripled?
- If one charge is doubled and the other tripled, with distance unchanged, by what factor will the force change?
- The force between two charges in vacuum is 18 N. What will it be in a medium of relative permittivity 6?
- What is the nature of Coulomb force for like and unlike charges?
Exam tip: In Coulomb's law numerical problems, first convert microcoulomb or nanocoulomb to coulomb and distance to metre. Then use .
👉 Common Mistakes to Avoid
- “Charges do not move at all in static electricity.” Accumulated charge may redistribute or discharge when a suitable path is available.
- “Friction creates new charge.” Usually electrons are transferred between objects and total charge is conserved.
- “A charged object can attract only another charged object.” Through induction, a charged object can also attract a neutral object.
- “Doubling the distance halves the force.” Coulomb force varies as , so doubling the distance makes the force one-fourth.
- “Changing the medium does not affect Coulomb force.” The force depends on the permittivity of the medium.
- “The sign of q₁q₂ alone should be used as the value of force.” For the magnitude, using is convenient; decide attraction or repulsion separately from the signs.
👉 Quick Revision
- Static electricity is associated with net electric charge accumulated on an object.
- Objects generally become charged by gaining or losing electrons.
- Total charge is conserved.
- Like charges repel and unlike charges attract.
- Objects can be charged by friction, contact and induction.
- Earthing provides a conducting path between an object and Earth for excess charge.
- Coulomb's law: .
- In vacuum, .
- Doubling the distance reduces Coulomb force to one-fourth.
- Increasing relative permittivity reduces the force between the same charges.
- The SI unit of electric charge is coulomb (C).
