Current Electricity – Study Notes
(Designed for PSU‑GATE physics candidates, beginner‑friendly and free of heavy jargon)
1. What is Current?
- Current is the flow of electric charge.
- Think of it like water flowing through a pipe.
- Water flow → Electric current (measured in amperes (A)).
- A higher flow means more charge passes per second.
Tiny Example
A 1‑ampere (1 A) current is like 1 litre of water passing a point every second.
2. Voltage – The “Push”
- Voltage (V) is the push that forces charge to move.
- Analogy: A water tower’s height gives water pressure.
- Higher tower → higher pressure → more water flow.
- Voltage is measured in volts (V).
Tiny Example
A 12‑V battery pushes electrons just like a 12‑metre high water tower pushes water.
3. Resistance – The “Traffic Jam”
- Resistance (R) is how much a material slows down charge flow.
- Analogy: Narrow streets slow down cars.
- Measured in ohms (Ω).
Tiny Example
A thin wire (high R) is like a narrow road; a thick wire (low R) is like a wide highway.
4. Ohm’s Law – The Basic Relationship
-
Ohm’s Law links voltage, current, and resistance.
-
Formula:
[ V = I \times R ]
- V = voltage (V)
- I = current (A)
- R = resistance (Ω)
-
Rearranged forms:
- ( I = \frac{V}{R} )
- ( R = \frac{V}{I} )
Tiny Example
If a 9‑V battery is connected to a 3‑Ω resistor, the current is
[ I = \frac{9,V}{3,\Omega} = 3,A ]
5. Series and Parallel Circuits
| Configuration | How components are arranged | How voltages/currents add |
|---|---|---|
| Series | Components one after another | Currents are the same through each; voltages add up |
| Parallel | Components side by side | Voltages across each are the same; currents add up |
Tiny Example – Series
Two 2‑Ω resistors in series: total resistance = 4 Ω.
Tiny Example – Parallel
Two 2‑Ω resistors in parallel: total resistance is
[ \frac{1}{R_{\text{eq}}} = \frac{1}{2} + \frac{1}{2} \implies R_{\text{eq}} = 1,\Omega ]
6. Kirchhoff’s Laws – Simple Rules for Complex Circuits
| Law | What it says | Everyday Analogy |
|---|---|---|
| KCL (Current Law) | At a junction, the sum of currents entering equals the sum leaving. | Cars entering a roundabout split into the same number leaving. |
| KVL (Voltage Law) | Around any closed loop, the sum of voltage rises equals the sum of voltage drops. | Water pumped around a closed loop: total pressure added equals pressure lost. |
7. Power and Energy
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Power (P) is the rate of doing work or using energy.
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Formula:
[ P = V \times I ]
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In terms of resistance:
[ P = I^2 \times R = \frac{V^2}{R} ]
-
Measured in watts (W).
Tiny Example
A 12‑V battery supplying 2 A gives
[ P = 12,V \times 2,A = 24,W ]
8. Electromotive Force (EMF) – The “Battery Push”
- EMF is the voltage produced by a source (like a battery).
- It’s the “push” that starts the current.
- Measured in volts.
Tiny Example
A 9‑V battery has an EMF of 9 V, which pushes electrons through a circuit.
9. Quick Review Checklist
- Current = charge flow (A).
- Voltage = push (V).
- Resistance = obstacle (Ω).
- Ohm’s Law: (V = I \times R).
- Series: same current, voltages add.
- Parallel: same voltage, currents add.
- Kirchhoff: conserve current at junctions, conserve voltage in loops.
- Power: (P = V \times I).
10. Quick Practice Problem
Problem:
A 5‑V supply is connected to a 10‑Ω resistor.
- Find the current.
- Find the power dissipated in the resistor.
Solution:
- (I = \frac{V}{R} = \frac{5,V}{10,\Omega} = 0.5,A).
- (P = V \times I = 5,V \times 0.5,A = 2.5,W).
Tip for GATE – Remember to keep all units consistent (V, A, Ω, W) and double‑check whether components are in series or parallel before applying Ohm’s Law or Kirchhoff’s rules.
Happy studying!