Electricity – Study Notes

ELECTRICITY

1. Electric Current (I)

Flow of electric charge in a conductor.
Rate of flow: I = Q / t
1 Ampere = 1 Coulomb / 1 sec
Direction: Conventionally from Positive (+) to Negative (-). (Opposite to electron flow).
Ammeter: Measures I. Connect in SERIES. Low resistance.

2. Potential Difference (V)

Work done to move a unit charge from one point to another.
Electric pressure difference.
V = W / Q
SI Unit: Volt (V). (1 V = 1 Joule / 1 Coulomb).
Voltmeter: Connect in PARALLEL. High resistance.
Maintained by cell/battery.

Standard Circuit

Remember: Ammeter counts charge (Series). Voltmeter checks difference (Parallel).

3. Ohm’s Law & Resistance

At constant temperature, potential difference (V) is directly proportional to current (I).
V ∝ I → V = I R
Resistance (R): Property to oppose current. SI Unit: Ohm (Ω).
1 Ohm: If 1A flows through a wire with 1V across it.
Rheostat: Variable resistance device to control current.

Ohmic Conductor

4. Factors Affecting Resistance

Length (l): Directly proportional (R ∝ l). Double length = Double R.
Area (A): Inversely proportional (R ∝ 1/A). Thicker wire = Less R.
Nature of Material: Resistivity (ρ).
Temperature: R increases with Temp (for metals).
R = ρ (l / A)
Resistivity (ρ): Characteristic property of material. Unit: Ωm.
- Metals: Low ρ (Good conductors).
- Alloys: Higher ρ than constituent metals. Don’t oxidize easily.
- Insulators: Very high ρ.

5. System of Resistors

A) Series Combination

Joined end-to-end.
Current (I): Same in all resistors.
Voltage (V): Sum of individual voltages ($V = V_1 + V_2…$).
Rs = R1 + R2 + R3…
Total R > Largest individual R (Maximum Resistance).

B) Parallel Combination

Connected between two points.
Voltage (V): Same across all.
Current (I): Sum of branch currents ($I = I_1 + I_2…$).
1/Rp = 1/R1 + 1/R2…
Total R < Smallest individual R (Minimum Resistance).
Used in homes so appliances work independently!

6. Heating Effect of Current

Electrical energy is dissipated as heat in a resistor.
Joule’s Law of Heating:
- Heat (H) ∝ Square of current (I²).
- Heat (H) ∝ Resistance (R).
- Heat (H) ∝ Time (t).
H = I² R t

Practical Applications:
1. Bulb: Filament gets hot & emits light. Uses Tungsten (High MP: 3380°C). Filled with Nitrogen/Argon.
2. Fuse: Safety device. Wire of low MP (Pb-Sn). Melts if I > rating. Connect in Series.
3. Heater/Iron: Uses Nichrome (Alloy). High resistance, doesn’t burn (oxidize) at high temp.

7. Electric Power (P)

Rate at which electrical energy is consumed.
P = V I
Or P = I²R Or P = V²/R
SI Unit: Watt (W). 1W = 1V × 1A.

Commercial Unit of Energy: Kilowatt-hour (kWh) or ‘Unit’.
Power × Time.
1 kWh = 1000 W × 3600 s
1 kWh = 3.6 × 10⁶ Joules