Class 10 Electricity NotesNotes Previous Years Questions Important Questions
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–).
- Number of electrons(n) = Q/e
- Charge on an electron (1e–) =1.6 x 10-19 C
A continuous and closed path of an electric current is called an electric circuit.
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.
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.
At constant temperature, the potential difference across the ends of a conductor is directly proportional to the amount of current flowing through it.
V ∝ I
V = IR where, R is constant of proportionality & is known as resistance.
Circuit diagram and V-I graph for Ohm’s law:-
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
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
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.
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