Class 10 Electricity Notes

Notes Previous Years Questions Important Questions

Electricity Notes

Charge(Q):-

Electric charge is the basic physical property of matter that causes it to experience a force when kept in an electric or magnetic field.

  • It can be positive or negative.
  • S.I. unit – Coulomb (C)

Quantisation of Charge:-

Every charge is an integral multiple of a basic charge i.e., charge on an electron(e).

  • Q=ne
  • Number of electrons(n) = Q/e
  • Charge on an electron (1e) =1.6 x 10-19 C

Electric Circuit:-

A continuous and closed path of an electric current is called an electric circuit.

Electric current(I):-

The rate of flow of electric charge is called electric current.

  • I = Q/t
  • S.I. unit:- Ampere(A)
  • Conventionally, the direction of flow of electric current is opposite to the direction of flow of electrons
  • The amount of electric current in a circuit is measured by an instrument called ammeter.
  • An ammeter is always connected in series in a circuit.
  • 1 mA = 10-3 A & 1 µA = 10-6 A

1 Ampere:- The electric current flowing through a circuit is said to be 1 ampere when 1 coulomb of charge flows through it per second.

Electric Potential:-

The work done to move a unit charge from infinity to a point is known as electric potential of that point.

Electric Potential Difference:-

The work done to move a unit charge from one point to another point is called potential difference between the two points.

  • V = W/Q
  • S.I. unit:- Volt(V)
  • Potential difference is measured by an instrument called voltmeter.
  • Voltmeter is always connected in parallel across the points between which the potential difference is to be measured.

1 Volt:- The potential difference across the two points of a conductor is said to be 1 volt when 1 joule of work is done to move a charge of 1 coulomb from one point to the other.

Ohm’s Law:-

At constant temperature, the potential difference across the ends of a conductor is directly proportional to the amount of current flowing through it.

Mathematically,
V ∝ I
V = IR where, R is constant of proportionality & is known as resistance.

Circuit diagram and V-I graph for Ohm’s law:-

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Resistance:-

The property of a conductor to oppose the flow of charges through it is called resistance.

  • R = V/I
  • S.I. unit:- Ohm (Ω)

1 Ohm:- The resistance of a conductor is said to be 1 Ω if the potential difference across the ends of a conductor is 1 V and the current flowing through it is 1 A.

Resistor:- A conductor having some appreciable resistance

Variable Resistance/Rheostat:- A component used to regulate current without changing the voltage source

Factors on which resistance of a conductor depends are:-

  • its length (l)
  • its cross-section area (A)
  • nature of its material

Mathematically,
R ∝ l ——–(i)
R ∝ 1/A ——(ii)

On combining (i) & (ii):-
R = ρl / A where, ρ(rho) is constant of proportionality & is known as resistivity

Resistivity/Specific Resistance(ρ):-

The resistance of the conductor of unit length & unit area of cross section is called its resistivity.

  • ρ =RA/l
  • S.I. unit :- ohm meter (Ω m)

On increasing the temperature, both resistance and resistivity of a conductor increases.

Resistors in Series:-

When two or more resistors are connected end to end to each other, then they are said to be connected in series.

In series combination,

  • current flowing is same through each resistor.
  • the potential difference across each resistor is different.

.

Consider current ‘I’ is flowing through the circuit & the potential difference across resistors R1, R2 & R3 be V1, V2, V3 respectively. Let the total potential difference be ‘V’ & equivalent resistance be ‘Rs‘.

On applying Ohm’s law,
V = IRs ; V1 = IR1; V2 = IR2 ; V3 = IR3

Now, V = V1 + V2 + V3
or, IRs = IR1 + IR2 + IR3
or, Rs = R1 + R2 + R3

Resistors In Parallel:-

When two or more resistors are connected together between two points then they are said to be connected in parallel.

In parallel combination,

  • The potential difference is same across each resistor.
  • The current flowing through each resistor is different.

.

Consider potential difference across the resistors be V & the current flowing through resistors R1, R2 & R3 be I1, I2 & I3 respectively. Let the total current flowing through the circuit be I & equivalent resistance be Rp.

By Ohm’s law,
I = V/Rp ; I1 = V/R1 ; I2 = V/R2 ; I3 = V/R3

Now, I = I1 + I2 + I3
or , V/Rp = V/R1 + V/R2 + V/R3
or , 1/Rp = 1/R1 + 1/R2 + 1/R3

Advantages of parallel combination over series combination:-

  • In series combination, current remains same throughout the circuit while in parallel combination, current is different through each device and different components require different currents to operate properly.
  • If one device fails in series circuit then none of the devices works while it will not happen in parallel circuit.
  • The total resistance in a parallel circuit is decreased while the total resistance in series circuit is increased.

Electric Power:-

The amount of electrical energy consumed per unit time in an electric circuit is known as electric power.

P = W/t
Or, P = VQ/t
Or, P = VI = I2R = V2/R

  • S.I. unit of power:- Watt(W)

1 Watt:- Electric power of an appliance is said to be 1 W if 1 A of current flows through it and the potential difference across its ends is 1 V.

Heating Effect of Electric Current:-

W = VQ =VIt

or H = VIt = I2Rt

Joule’s Law of Heating:-

According to Joule’s law of heating, heat produced in a resistor is directly proportional to

  • square of current (H ∝ I2)
  • resistance (H ∝ R)
  • time for which current flows (H ∝ t)

H = I2Rt

  • S.I. unit of energy:- Joule(J)
  • Commercial unit of energy:- kilowatt hour(kW h)

Practical Applications of Heating Effect of Electric Current:-

Appliances based on heating effect of electric current are electric bulb, electric iron, electric toaster, electric oven, electric kettle, electric heater, fuse etc.

  • Tungsten is used as the filament of the electric bulb.
  • The bulbs are filled with chemically inactive gases like nitrogen and argon to prolong the life of the filament.

Conversion of Commercial Unit of Energy to S.I. Unit:-

1 kW h = 1000 W x 3600 s = 3.6 x 106 Ws = 3.6 x 106 J

1 unit = 1 kW h = 3.6 x 106 J

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