Current Electricity

Every time you switch on a light, charge a phone, or use an electric heater during cold Kathmandu winters, you are using current electricity. While electrostatics deals with charges at rest, current electricity studies charges in motion.

Electric Current

Current is the rate of flow of electric charge through a conductor.

I = Q/t (unit: Ampere, A)

Conventional current flows from positive to negative terminal (opposite to electron flow).

Ohm's Law

The current through a conductor is directly proportional to the potential difference across it, provided the temperature remains constant.

V = IR

where V is voltage (volts), I is current (amperes), and R is resistance (ohms).

Resistance and Resistivity

Resistance depends on the material and dimensions of the conductor:

R = rho L / A

where rho is resistivity (ohm metre), L is length, and A is cross-sectional area.

Resistors in Series and Parallel

Series combination: Current is the same through all resistors.

R_total = R1 + R2 + R3 + ...

Parallel combination: Voltage is the same across all resistors.

1/R_total = 1/R1 + 1/R2 + 1/R3 + ...

Worked Example

Three resistors of 2 ohm, 3 ohm, and 6 ohm are connected in parallel. Find the equivalent resistance.

Solution:

• 1/R = 1/2 + 1/3 + 1/6 = 3/6 + 2/6 + 1/6 = 6/6 = 1
• R = 1 ohm

Kirchhoff's Laws

Junction Rule (KCL): The sum of currents entering a junction equals the sum of currents leaving it. (Conservation of charge)

Loop Rule (KVL): The sum of all potential differences around any closed loop is zero. (Conservation of energy)

Wheatstone Bridge

A Wheatstone bridge is a circuit used to measure unknown resistance precisely. It is balanced when:

P/Q = R/S

At balance, no current flows through the galvanometer.

Power in Electric Circuits

P = VI = I²R = V²/R (unit: Watt, W)

Worked Example: A 60 W bulb operates at 220 V. Find the current drawn and resistance.

• I = P/V = 60/220 = 0.27 A
• R = V²/P = (220)²/60 = 48400/60 = 806.7 ohm

Nepal Connection

Nepal's hydroelectric power stations (like Kulekhani and Upper Tamakoshi) generate electricity. Understanding circuit analysis helps electrical engineers design efficient power distribution systems across the country.

Key Takeaways

• Ohm's law: V = IR is the foundation of circuit analysis
• Series: resistances add up; Parallel: reciprocals add up
• Kirchhoff's laws are based on conservation of charge and energy
• Wheatstone bridge is balanced when P/Q = R/S