CBSE Class 10 Science: Chapter 12 Electricity Important Concepts
Welcome to Part 7 of our 8-part series on Chapter 12, Electricity. This post is your ultimate revision guide, summarizing all the important concepts, definitions, laws, and formulas you need to know for your board exams.
Recommended Books for Deep Practice
Key Concepts & Definitions
1. Electric Charge (Q)
It is the fundamental physical property of matter that causes it to experience a force when placed in an electromagnetic field.
- SI Unit: Coulomb (C)
- It is a scalar quantity.
- Charge of one electron (e): -1.6 × 10⁻¹⁹ C
- Quantization of Charge: Total charge (Q) on a body is always an integral multiple of the charge of an electron. Q = n * e (where 'n' is the number of electrons)
2. Electric Current (I)
It is defined as the rate of flow of electric charge (Q) through a cross-section of a conductor.
I = Q / t- SI Unit: Ampere (A)
- Definition of 1 Ampere: When 1 Coulomb of charge flows through a conductor in 1 second, the current is said to be 1 Ampere.
- Direction: The direction of conventional current is *opposite* to the flow of electrons (i.e., from the positive terminal to the negative terminal).
- Measured by: An Ammeter (always connected in series).
3. Electric Potential & Potential Difference (V)
Potential Difference (V): It is the work done (W) to move a unit charge (Q) from one point to another in an electric circuit.
V = W / Q- SI Unit: Volt (V)
- Definition of 1 Volt: When 1 Joule of work is done to move a charge of 1 Coulomb from one point to another, the potential difference is 1 Volt.
- Measured by: A Voltmeter (always connected in parallel).
4. Ohm's Law
It states that the potential difference (V) across the ends of a metallic conductor is directly proportional to the current (I) flowing through it, provided its temperature remains constant.
V ∝ I or V = IR- R is the constant of proportionality, called Resistance.
- The V-I graph for an ohmic conductor is a straight line passing through the origin.
5. Resistance (R)
It is the property of a conductor to resist or oppose the flow of electric current through it.
- SI Unit: Ohm (Ω)
- Factors affecting Resistance: Resistance of a conductor depends on:
- Length (L): R ∝ L (Resistance increases with length)
- Area of Cross-section (A): R ∝ 1/A (Resistance decreases as the wire gets thicker)
- Nature of Material (ρ): Depends on the material's resistivity.
- Temperature: Resistance of pure metals increases with temperature.
6. Resistivity (ρ)
It is the resistance of a conductor of unit length (1m) and unit cross-sectional area (1m²). It is an intrinsic property of the material.
- SI Unit: Ohm-meter (Ω·m)
- It does *not* depend on the length or thickness of the wire; it only depends on the material and temperature.
- Conductors (e.g., Copper): Have very low resistivity (approx. 1.68 × 10⁻⁸ Ω·m).
- Alloys (e.g., Nichrome): Have high resistivity (approx. 110 × 10⁻⁸ Ω·m) and are used in heating devices.
- Insulators (e.g., Glass, Rubber): Have very high resistivity (approx. 10¹⁰ to 10¹⁴ Ω·m).
7. Combination of Resistors
Resistors in Series:
- Current (I) through each resistor is the same.
- Voltage (V) of the battery is divided (V = V₁ + V₂ + ...).
- Equivalent Resistance (Rs) is the sum of individual resistances.
Resistors in Parallel:
- Voltage (V) across each resistor is the same.
- Current (I) from the battery is divided (I = I₁ + I₂ + ...).
- The reciprocal of the equivalent resistance (Rp) is the sum of reciprocals.
8. Joule's Law of Heating
It states that the heat (H) produced in a resistor is directly proportional to:
- The square of the current (I²)
- The resistance (R)
- The time (t) for which the current flows.
Applications: Electric fuse, electric iron, electric toaster, electric bulb (filament).
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9. Electric Power (P)
It is the rate at which electrical energy is consumed in a circuit.
- SI Unit: Watt (W)
- Definition of 1 Watt: The power consumed by a device that carries 1A of current when operated at a potential difference of 1V.
10. Commercial Unit of Energy
The commercial unit of electrical energy is the kilowatt-hour (kWh), also known as one "unit" on electricity bills.
Definition of 1 kWh: The energy consumed when a device of 1 kilowatt (1000 W) power is operated for 1 hour.
1 kWh = 3.6 × 10⁶ JImportant Formulas at a Glance
| Quantity | Formula | SI Unit |
|---|---|---|
| Electric Charge (Q) | Q = n * e | Coulomb (C) |
| Electric Current (I) | I = Q / t | Ampere (A) |
| Potential Difference (V) | V = W / Q | Volt (V) |
| Ohm's Law | V = IR | - |
| Resistance (R) | R = ρ (L / A) | Ohm (Ω) |
| Resistors in Series (Rs) | Rs = R₁ + R₂ + ... | Ohm (Ω) |
| Resistors in Parallel (Rp) | 1/Rp = 1/R₁ + 1/R₂ + ... | Ohm (Ω) |
| Heat Produced (H) | H = I²Rt | Joule (J) |
| Electric Power (P) | P = VI or P = I²R or P = V²/R | Watt (W) |
| Energy Conversion | 1 kWh = 3.6 × 10⁶ J | - |
Common Circuit Symbols
| Component | Symbol |
|---|---|
| Electric Cell | |
| Battery | |
| Resistor (Fixed) | |
| Rheostat (Variable Resistor) | |
| Ammeter | |
| Voltmeter | |
| Switch (Open) | |
| Switch (Closed) | |
| Electric Bulb |
