🌟 Class 10 Physical Science – Chapter: Current Electricity – Notes
🔌 1. Electric Current
Definition: The rate of flow of electric charge through a conductor per unit time is called electric current.
Formula: I = Q / t
Where, I = current (Ampere), Q = charge (Coulomb), t = time (second)
SI Unit: Ampere (A)
1 Coulomb: Charge carried by 6.25 × 1018 electrons.
🔄 2. Types of Current
- Direct Current (D.C.): Current that flows in one direction only.
- Alternating Current (A.C.): Current that periodically reverses its direction.
🔋 3. Electric Cell
Definition: A device which converts chemical energy into electrical energy.
EMF of a Cell
Definition: The potential difference between the terminals of a cell in an open circuit is called its Electromotive Force (EMF).
Unit: Volt (V)
📐 4. Ohm’s Law
Law (1826): At constant temperature, the electric current flowing through a conductor is directly proportional to the potential difference across its ends.
Mathematical Form: V ∝ I or V = IR
Where, V = Voltage (Volt), I = Current (Ampere), R = Resistance (Ohm)
Graph: A straight line through the origin for V vs I.
Definition of Resistance:
The resistance of a conductor is the ratio of the potential difference across its ends to the current flowing through it.
SI Unit: Ohm (Ω)
1 Ohm = 1 Volt / 1 Ampere
📏 5. Factors Affecting Resistance
Formula: R = ρ × (l / A)
Where, R = resistance, ρ = resistivity, l = length, A = cross-sectional area
- R ∝ l (longer wire, more resistance)
- R ∝ 1/A (thicker wire, less resistance)
🔃 6. Conductivity and Resistivity
Definition: Conductivity is the reciprocal of resistivity.
Formula: σ = 1 / ρ
SI Unit: mho m-1 or S/m
⚡ 7. Heating Effect of Current (Joule's Law)
Joule’s Law of Heating:
- 1st Law: H ∝ I2 (for constant R and t)
- 2nd Law: H ∝ R (for constant I and t)
- 3rd Law: H ∝ t (for constant I and R)
Combined Formula: H = I2 × R × t
Where: H = heat in joules, I = current, R = resistance, t = time
🔋 8. Electric Power and Energy
Formulae:
- P = VI
- P = I2R
- P = V2 / R
Energy (E) = Power × Time
Units:
- Watt (W), Kilowatt (kW)
- 1 Kilowatt-hour = 3.6 × 106 J (also known as 1 Unit or 1 B.O.T)
🧲 9. Magnetic Effect of Current
Oersted’s Experiment: A current-carrying conductor deflects a nearby magnetic needle, indicating a magnetic field is produced.
Right Hand Thumb Rule: Thumb = current, curled fingers = magnetic field direction
🧲 10. Electromagnet
Definition: A temporary magnet created by passing current through a coil wound around a soft iron core.
⚙️ 11. Motor and Generator
- Electric Motor: Converts electrical energy into mechanical energy.
Law: Fleming’s Left Hand Rule - Electric Generator: Converts mechanical energy into electrical energy.
Law: Fleming’s Right Hand Rule
⚠️ 12. Safety Devices
- Fuse: Thin wire that melts when excessive current flows. Made of 75% lead and 25% tin.
- Earthing: Connecting metal parts of appliances to the earth to prevent shock.
- Short Circuit: Direct connection between live and neutral wires causing high current.
- Overloading: Excessive current drawn due to multiple appliances.
🧪 13. Electrical Instruments
| Instrument | Measures | Connected | Ideal Resistance |
|---|---|---|---|
| Ammeter | Current (I) | In Series | 0 Ω |
| Voltmeter | Voltage (V) | In Parallel | ∞ Ω |
| Galvanometer | Small Current | — | Low |
📌 14. Definitions & Constants
- Electric Charge (e): 1.6 × 10-19 C
- Permittivity of Free Space (ε0): 8.854 × 10-12 C2 N-1 m-2
- Specific Resistance of Copper at 20°C: 1.6 × 10-6 ohm-cm
- CFL: Compact Fluorescent Lamp
- LED: Light Emitting Diode
🧠 15. Summary
- Electric current is flow of charge per unit time.
- Ohm’s law relates voltage, current and resistance.
- Joule’s law explains heating effect of current.
- Motors and generators show energy conversions.
- Safety devices protect against overcurrent.