Easily Understand Electric Charges and Electric Fields Class 12 Notes

Electric charges and fields are two of the most fundamental concepts in physics, but that doesn’t mean they can be easily understood. To get started with these two ideas, check out these electric charges and electric fields class 12 notes. Then, you will be all set to apply these ideas to related concepts such as electromagnetic induction and electrostatics.

Electric Charges and Electric Fields Class 12 Notes

Electricity is the flow of electric charge. You can think of electric charges as tiny, invisible particles that carry a force. There are two kinds of charges: positive (+) and negative (-). Electricity is created when a difference in charge between two objects creates an electric field. The opposite charges attract each other while like charges repel each other. With these class 12 notes, you will be able to easily understand electric charges and fields

What is Electric Charge?

Electric charge is the measure of the distribution of the electric field. The SI unit of electric charge is coulomb (C). An object either has an equal amount of positive (+) and negative (-) charges, or it has a net positive (+) or negative (-) charge.

If there are an unequal number of both types of charges, then this results in what’s called an unbalanced charge. If there are more positive (+) than negative (-), then this results in what’s called a net positive (+) charge. If there are more negative (-) than +, then this results in what’s called a net negative (-) charge.

Both electrical attraction and repulsion depend on the magnitude of the force between two objects.

How to Measure Electric charge:

One way to measure electric charge is to use a voltmeter. This meter measures the voltage of an electric charge. When the two plates are placed in water, the one that has more electrons will have a stronger negative charge.

The plate with fewer electrons will have a stronger positive charge. The plate that has more electrons will cause the needle on the voltmeter to move towards negative while the other plate will cause it to move towards positive.

If both needles are near zero, then there is no charge between the two plates. If you think about this, it makes sense since if there was a strong enough charge between them, they would attract each other.

Coulomb’s Law

Coulomb’s Law is a statement of the mathematical relationship between two electric charges, Q1 and Q2. Coulomb’s law was first published by Charles-Augustin de Coulomb in 1785. The law states that the magnitude of the electrostatic force (F) between two point charges is proportional to the product of their magnitudes (Q1×Q2) but inversely proportional to the square of the distance (d) between them.

That is, F=k*( q1 * q2 )*(d^2)/(4*pi*epsilon0). In other words, the greater the charge on each particle times its distance from one another, the stronger their mutual repulsion will be. It can also be stated as:

The magnitude of the electrostatic force is equal to k∙Q1∙Q2/(4∙pi∙ε0). With all else being equal, the higher your body has an electrical potential (voltage), then you are going to have a larger push away or pull towards something else with an electrical potential.

Methods of Charging:

  • Charging by Friction
  • Charging by conduction
  • Charging by induction

Charging by Friction:

Charging by friction is when two objects have static electric charges. The object that has the higher charge will attract the object that has a lower charge. 
The first thing to do is to find out what the charges are on both objects.  If one of them has a positive charge, then it would be attracted to an object with a negative charge, but would not be attracted to an object with no charge or a positive charge.
If the two objects have opposite charges, they will repel each other. In order for this to happen, there must be a force pushing them away from each other.
Charging by Conduction:
When two objects come in contact, they can create an electric charge. This is called conduction. When the objects are in contact, electrons can move from one object to the other creating a net transfer of electrons.
The object that loses electrons becomes positively charged while the object that gains electrons becomes negatively charged. his type of charging is most common when metal objects are in contact because metals have loosely-bound electrons. However, this phenomenon also occurs when non-metallic materials touch because these materials have charges on their surface which cause electron flow.

Charging by induction:

The most common method of charging a battery is induction. Let’s take the example of charging a car battery. You can charge the battery by connecting one end of the wire to a power source, such as a wall socket, and touching the other end to the positive terminal on the car battery. The current flow through that wire will cause the copper on the inside of it to become positively charged.

If you touch the negative terminal on the car battery with this now-positively-charged wire, electrons will move into it from there. As a result, both ends of the wire now have equal amounts of positive charges (but opposite polarity).

Electric Field

Electric fields are created by electric charges. The stronger the charge, the stronger the field becomes. If you have an electric charge that’s positive (+), then it will have a negative (-) electric field around it.

If you have an electric charge that’s negative (-), then it will have a positive (+) electric field around it. The way to picture this is to imagine two opposite charges attracting each other because of their opposite fields surrounding them.

For example, if you have a positive charge on one side and a negative charge on the other side as they are going to attract each other.

Electric Field Lines:

The force exerted by an electric field is defined as the force that would be exerted on a charged particle of unit charge if it were placed in the field. This means that it is always perpendicular to the direction of motion.
The electric field lines are directed away from positive charges and towards negative charges. The magnitude of the electric field at any point is inversely proportional to the square of the distance from this point to the nearest charge, which also means that they are inversely proportional to the square of their length.
For example, if you are 100 meters from a charge, you will experience four times less than the strength of the field. If you are 1 meter from a charge (100 times closer), then you will experience 16 times more strength than you would have been at 100 meters away.
However, there is no difference in the strength of the field 10 meters or 20 meters away from a charge. There are three types of electric fields:
  1. Stationary fields: A stationary field is an electric field created by two or more point charges. The strength of the electric field at a given point is proportional to the product of the charges and inversely proportional to the square of the distance from them.
  2. Accelerating fields:  An accelerating field is one that changes its magnitude over time. They can be either positive or negative in direction. An example of an accelerating electric field would be a positive charge that moves farther away from a negative charge. The electric field will get stronger as the distance between the two charges increases.
  3. Rotating fields: A rotating field is a vector field which rotates steadily about a fixed axis. The term is usually used to refer to fields that are generated by rotating charged particles such as electrons or protons, but it can also be applied to fields caused by rotating magnetic dipoles. Rotating electric fields occur when the electric charge density in a given volume is non-uniform.

Electric field due to dipole:

An electric dipole is a separation of positive and negative charges where the charges are equal in magnitude but opposite in sign. An example of an electric dipole is an electron that has been knocked loose from its nucleus. The electron will be negatively charged, while the nucleus will be positively charged. One can also create an electric dipole by separating two equal masses.

A strong electric field is created by the two separated masses. If one mass were moved towards the other, it would feel less of a force as it got closer to the other mass because there would be more attraction between them. However, if one mass were moved away from the other, it would feel more of a force as it got farther away from the other mass because there would be less attraction between them.

Torque on an Electric Dipole in a Uniform Electric Field:

Torques on an electric dipole in a uniform electric field are: torque is proportional to the strength of the electric field, to the product of the length of the dipole and its charge, and to the sine of the angle between the dipole axis and direction of E. The torque always points perpendicular to the plane defined by E.

An electric dipole can be created from a pair of equal but opposite charges at a distance d apart, where d >> R (see figure). The magnitude of the force on each charge is qE=qEd cos(angle) where q is the magnitude of the charge.

The potential difference across the two charges is formula_1 = (k/d)(Q2-Q1), so the total potential difference for this dipole equals formula_2 = kd. The electric flux for this situation equals formula_3 = kd sin(angle), and so we have Faraday’s law for induced emf, which states that formula_4 = -fdI.

If the charge Q is moving relative to E, then there will be both a Lorentz force due to Q moving through space and an electrostatic force due to Q being subject to changes in electric fields. If Q has velocity v towards E, then these forces combine according to Newton’s third law: formula_5 =, where B is magnetic induction and A is electrostatic induction.


The following points are covered in the Electric Charges and Electric Fields Class 12 Notes: electric charge – a quality of matter that produces an electrostatic attraction or repulsion on other electrically charged objects. Electric field – a region surrounding an object in which the electric force can be detected, measured, or computed: field lines – imaginary lines that show the direction of the force exerted by one object to another.

Tagged with: conduction | coulomb law | electric charge | electric field | induction

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