8th Class Science Electricity NCERT Summary - Electricity

NCERT Summary - Electricity

Category : 8th Class




  • Charge is the fundamental quantity of electricity.
    • The classical study of electricity is generally divided into many areas:
  • Electrostatics: It deals with phenomena due to attractions or repulsions of electric charges but not dependent upon their motion.
  • Electric Current: The study of the forms of energy associated with the flow of electric charge.
  • Electromagnetism: The study of the forces acting between electrically charged particles in motion.
  • Electric Charge: (Often just called charge) It is of two types, i.e. positive (+) and negative \[\left( - \right)\] charges.
  • The term neutral does not refer to third type of charge, but to the presence in a region of positive and negative charges in equal amount.


Methods of Charging

  • Friction: The frictional charging process results in a transfer of electrons between the two objects that are rubbed together.
  • Conduction: It involves touching a negatively charged object to a neutral object. Upon contact, electrons move from the negatively charged object to the neutral object.
  • Induction: If a negatively charged object is used to change a neutral object by induction, then the neutral object will acquire a positive charge and vice-versa.


Electrical Properties of Materials

  • Conductors: Charge moves easily through
  • Metals
  • Electrolytes (ionised liquids)
  • Plasmas (ionised gases)
    • Insulators: Charge does not move easily through
  • Non-metals (pure water, organics, gases, ...)
  • Semiconductors: Sometimes behave as a conductor and sometimes as an insulator.
  • Metalloids (silicon, germanium, doped metals, ...)
    • Superconductors: The perfect conductor that offers no resistance below critical temperature.
    • Many substances become superconductors when they are below some critical temperature.
    • The SI unit of charge is the coulomb (C).


Coulomb's Law

The force between two point charges is directly proportional to the product of magnitude of each charge \[\mathbf{(}{{\mathbf{q}}_{\mathbf{1}}}\mathbf{,}\,{{\mathbf{q}}_{\mathbf{2}}}\mathbf{)}\] and inversely proportional to square of the separation between their centres (r) and directed along the line connecting their centres (r’). This relationship is known as Coulomb's Law.


Electric Current

  • Electric current is the rate at which charge flows through a surface.
    • As a scalar quantity it has magnitude only.
    • The symbol for current is I.
  • The SI unit of current is the ampere (A) which is named after the French scientist Andre-Marie Ampere.
  • Since charge is measured in coulombs and time is measured in seconds, an ampere is the same as a coulomb per second.

Q = It

I = Ampere

Q = Coulomb

t = Second



  • Resistance is the property of a conductor to resist the flow of charges through it. Its SI unit is ohm \[(\Omega )\].
  • Resistivity is a characteristic property of the material. It is a measure of material's ability to oppose electric current. Its SI unit is \[\Omega -M\] (ohm-metre).
  • Resistivity depends on the nature of the material not on its dimensions as resistance.

Resistance\[(R)\,=\,\rho \frac{L}{A}\]

where    \[\rho =\]resistivity of cross section,

L = Length

A = Area of cross section


Ohm's Law

  • A conductor through which a current / if flowing and V is the potential difference between its ends. Then Ohm's law states that \[V\propto I\] or V = RI, where constant of proportionality R is called the resistance of the conductor.
  • The resistance of a given conductor depends on the material it is made of, and on its dimensions. For a given material, the resistance is inversely proportional to the cross-sectional area, for example, a thick copper wire has lower resistance than an otherwise-identical thin copper wire. Also, for a given material, the resistance is proportional to the length, for example, a long copper wire has higher resistance than an otherwise-identical short copper wire.
  • Conductors have positive temperature coefficient of resistance, that is, their resistance increases with the increase in temperature.
  • While for insulators and semi-conductors, the resistance decreases with increase in temperature, i.e., they have negative temperature coefficient of resistance.
  • Superconductors are conductors that can be made to conduct electricity without any resistance at all, so they do not lose energy or generate heat.
  • A semiconductor is neither a good conductor nor a good insulator, but somewhere in between them. The conductive capability of a semiconductor can be permanently changed by introducing impurities through a process known as 'doping' or it can be temporarily modified by the application of an electric field.
  • A lightning conductor is installed on a building to divert lighting away from the structure by providing a direct path to the ground.
  • Filaments in light bulbs are made of tungsten because the metal has a high melting point which makes it remain solid at high temperature. The metal glows brightly when electricity is passed through it and it is also capable of passing electric current continuously for a long time.
  • There are two different ways in order to measure electricity, currents and voltages.
  • Devices such as ammeters and voltmeters, which are based on the galvanometer, a device used to detect small currents, are used to measure electric current,


Static Electricity

  • Static electricity is the result of an imbalance between negative and positive charges in an object.
  • The charge remains until it is able to move away by means of an electric or electrical discharge. Static electricity is named in contras: with current electricity which flows through wires or other conductors and transmits energy.
  • The rubbing of certain materials against one another can transfer negative charges, or electrons. For example, if a person rubs his shoe on the carpet, his body collects extra electrons. The electrons cling to his body until they can be released. As he reaches and touches his furry dog, he will get a shock. It is only the surplus electrons that gets released from you to your unsuspecting pet.


Current Electricity

  • Current electricity is the form of electricity which makes all of our electronic gizmos possible. This form of electricity exists when charges are able to constantly flow. As opposed to static electricity where charges gather and remain at rest, current electricity is dynamic, charges are always on the move. We'll be focusing on this form of electricity throughout the rest of the tutorial.



  • In order to flow, current electricity requires a circuit: a closed, never ending loop of conductive material. A circuit could be as simple as a conductive wire connected end-to-end, but useful circuits usually contain a mix of wire and other components which control the flow of electricity. The only rule when it comes to making circuits is they can't have any insulating gaps in them.
  • If you have a wire full of copper atoms and want to induce a flow of electrons through it, all free electrons need somewhere to flow in the same general direction. Copper is a great conductor, perfect for making charges flow. If a circuit of copper wire is broken, the charges can't flow through the air, which will also prevent any of the charges flowing towards the middle from going anywhere.


Electric Power

  • Power is the rate at which the work is done. It is the work and time ratio. Mathematically, it is computed using the following equation:


Power = Work/Time

  • Like current, power is a rate quantity. Its mathematical formula is expressed on a per time basis.




  • Mechanical Power is commonly measured in "horsepower." One horsepower is equivalent to approximately 746 Watts.
  • Electrical Power is almost always measured in "watts" and it can be calculated by the formula \[P\,=\,VI.\]Where, \[V\,=\]Potential difference and I = amount of electricity.
  • The standard unit of voltage is therefore the joule per coulomb, a unit that has its own name, the volt (V):
    • 1 Volt = 1 Joule/Coulomb
    • Watt = Joule/second
    • 1 kWh = 3.6 \[\times \] 1000000 joules

(mega = 1000000)

So, 1 kWh = 3.6 mega joules

Or, 1 kWh = 3.6 MJ

  • Volt is the unit of measurement for potential difference while Watt is a unit of measurement for power.
  • An ampere, or amp (A or I, for current), is the amount of current in a circuit, while voltage (V) is the strength of the current as it flows through the circuit and watts (W) are the total electrical power released by circuit per second. One watt is equal to one volt multiplied by one amp, which can also be expressed as

1 W = 1 V \[\times \] 1 A

  • Power is the rate at which energy is added to or removed from a circuit by a battery or a load. Current is the rate at which charge moves past a point on a circuit. And the electric potential difference across the two ends of a circuit is the potential energy difference per charge between those two points. In equation form, these definitions can be stated as:



            I = electric current

            Q = electric charge

            T = Time



  • A transformer is a device that changes (transforms) an alternating potential difference (voltage) from one value to another value be it smaller or greater using the principle of electromagnetic induction.
  • Michael Faraday built the first transformer, although he used it only to demonstrate the principle of electromagnetic induction and did not foresee the use to which it would eventually be put.
    • The transformer is based on two principles: first, that an electric current can produce a magnetic field (electromagnetism), and second that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction).
  • Changing the current in the primary coil changes the magnetic flux that is developed.
    • The changing magnetic flux induces a voltage in the secondary coil.
  • Transformers require a varying flux to work. They are, therefore, perfect for AC Power, but do not work at all for DC power, which would keep the flux constant.


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Notes - Electricity

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