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:-

.

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 ‘R_s‘.

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

Now, V = V₁ + V₂ + V₃
or, IR_s = IR₁ + IR₂ + IR₃
or, R_s = 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 R_p.

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

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