Notes For All Chapters Science Class 10 CBSE
1. Introduction to Electricity
Electricity is a controllable and convenient form of energy used in homes, schools, hospitals, industries, etc.
It is essential for lighting, heating, operating machines, communication, and numerous modern applications.
This chapter deals with:
- Electric current and circuits
- Potential difference
- Resistance and Ohm’s Law
- Series and parallel circuits
- Heating effect of current
- Electric power
2. Electric Current and Circuit
Electric current: Flow of electric charges (electrons) through a conductor.
- Example: In a torch, the battery provides charges which flow through the bulb when the switch is ON.
Electric circuit: A continuous and closed conducting path for current.
Current stops if the circuit is broken or switch is OFF.
Direction of current: Conventionally opposite to electron flow (from positive to negative terminal).
Formula for Electric Current:
I=\(\frac{Q}{t}\)
I: Current (ampere, A)
Q: Charge (coulomb, C)
t: Time (seconds, s)
1 Coulomb (C) ≈ charge of \(10^{18}\)
1 Ampere = 1 Coulomb/1 second.
Small units:
- 1 milliampere (mA) = \(10^{-3}\)A
- 1 microampere (µA) = \(10^{-6}\)A
Ammeter: Measures current, connected in series with the circuit.
3. Electric Potential and Potential Difference
Charges flow due to potential difference (voltage), like water flows due to pressure difference.
Potential difference: Work done to move a unit charge between two points.
V = \(\frac{W}{Q}\)
Unit: Volt (V)
1 volt = 1 joule per coulomb (1 J/C)
Voltmeter: Measures potential difference, connected in parallel.
4. Circuit Diagram and Symbols
Circuit diagrams are simplified drawings using symbols for components.
Key symbols include:
- Cell, Battery
- Plug key (open/closed)
- Wire joint / crossing wires
- Bulb, Resistor (R), Rheostat
- Ammeter, Voltmeter
5. Ohm’s Law
Ohm’s Law: At constant temperature, potential difference (V) across a conductor is directly proportional to current (I) through it.
V∝I ⇒ V = IR
R: Resistance (Ω), property of conductor to oppose current.
R = \(\frac{V}{I}\)
1 ohm (Ω): Resistance when 1 V potential difference produces 1 A current.
Inverse relation: I = \(\frac{V}{R}\)
Rheostat: Device to vary current by changing resistance.
6. Factors Affecting Resistance
Resistance depends on:
Length (l): R ∝ l
Area of cross-section (A): R∝ \(\frac{1}{A}\)
Material: Different materials have different resistivity.
- R = ρ\(\frac{l}{A}\)
ρ: Resistivity (Ω m) – characteristic property of material.
Metals: Low resistivity (\(10^{-8}-10^{-6}\)Ωm)→ good conductors.
Insulators: High resistivity (\(10^{12}-10^{17}\)Ωm).
Applications of resistivity:
- Alloys have higher resistivity → used in heating devices.
- Tungsten used in bulbs.
- Copper, aluminium used for transmission lines.
7. Resistance of a System of Resistors
1. Resistors in Series
- Same current through all resistors.
- Total potential difference is sum across each resistor.
- Rs = R1+R2+R3
Equivalent resistance increases.
Applications:
- Used when higher resistance is needed.
2. Resistors in Parallel
- Same potential difference across all resistors.
- Total current is sum of individual currents.
\(\frac{1}{Rp}\) = \(\frac{1}{R1}\)+\(\frac{1}{R2}\)+\(\frac{1}{R3}\)+…….
Equivalent resistance decreases.
Advantages:
- Devices receive required current.
- Failure of one component does not break the circuit.
- Used in domestic wiring.
8. Heating Effect of Electric Current
Electrical energy supplied is partly converted into work and partly into heat.
If circuit is resistive, all energy converts to heat
- H = VIt
Using Ohm’s Law:
- H = \(I^2\) Rt
(Joule’s Law of Heating)
Heat produced is:
- ∝ \(I^2\) (current squared)
- ∝ R (resistance)
- ∝ t (time)
9. Applications of Heating Effect
Electric heater, iron, toaster, kettle, oven: Convert electrical energy into heat.
Electric bulb: Filament (tungsten) gets heated and emits light.
Fuse: Safety device that melts when current exceeds a limit, breaking the circuit.
- Fuse ratings: 1A, 2A, 3A, 5A, 10A etc.
10. Electric Power
Electric power (P): Rate of electrical energy consumption.
P = VI = \(I^2\)R = \(\frac{V^2}{R}\)
Unit: Watt (W) – 1 A current at 1 V.
Larger unit: Kilowatt (kW) = 1000 W
Electric Energy (E):
- E = P × t
Commercial Unit: kilowatt-hour (kWh) = 3.6 × 10⁶ J
1 kWh = Energy used by 1 kW appliance in 1 hour.
Important Points to Remember
Current: I = \(\frac{Q}{t}\)
Potential Difference: = \(\frac{W}{Q}\)
Resistance: R = \(\frac{V}{I}\)
Series: Rs = R1+R2+…
Parallel: \(\frac{1}{Rp}\) = \(\frac{1}{R1}\)+\(\frac{1}{R2}\)+\(\frac{1}{R3}\)+…….
Heat: H = \(I^2\) Rt
Power: P = VI = \(I^2\)R = \(\frac{V^2}{R}\)
Energy: E = Pt
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