⚡ Electricity
From Electric Charge to Current, Potential Difference and Resistance
Understand Electricity instead of simply memorising formulae.
⚡ Enter the World of Electricity
Think of a bulb glowing in your room, a mobile phone charging, a fan rotating or a television working. All these familiar situations involve the flow of electric charge.
But three questions immediately arise:
How much charge?
Electric charge is the quantity of electricity transferred.
How fast?
Current tells us how quickly charge flows.
What causes the flow?
Potential difference provides the electrical push.
What opposes it?
Resistance opposes the flow of current.
🔹 1. Electric Charge
Electric charge is a fundamental property of matter responsible for electrical effects. The SI unit of electric charge is the coulomb (C).
When charge passes through a cross-section of a conductor, an electric current is produced.
🔹 2. Electric Current
| Symbol | Meaning | SI unit |
|---|---|---|
| I | Current | ampere (A) |
| Q | Charge | coulomb (C) |
| t | Time | second (s) |
🧠 Current Simulator
Change the charge and time to see how current changes.
Try this: If the same charge flows in less time, what happens to current?
🔹 3. One Ampere – What Does It Mean?
If a charge of 1 coulomb passes through a cross-section of a conductor in 1 second, the current is 1 ampere.
Therefore, a current of 2 A means that 2 coulombs of charge pass through the conductor every second.
🔹 4. Potential Difference
Charges do not simply start flowing without a cause. An electrical source such as a cell provides an electrical push.
| Symbol | Quantity | SI unit |
|---|---|---|
| V | Potential difference | volt (V) |
| W | Work done | joule (J) |
| Q | Charge | coulomb (C) |
🔋 A Simple Electrical Picture
The cell provides potential difference, the conductor provides a path, and the bulb uses electrical energy.
🔹 5. Resistance
The SI unit of resistance is ohm (Ω).
From this relation:
Higher resistance
For the same potential difference, the current is smaller.
Lower resistance
For the same potential difference, the current is larger.
🔹 6. Ohm's Law
For a conductor at constant temperature, the current flowing through it is directly proportional to the potential difference across its ends.
V = IR
This is called Ohm's law.
📈 V–I Relationship
If resistance remains constant, increasing the potential difference increases the current proportionally.
| Potential difference V | Current I |
|---|---|
| 1 V | 0.5 A |
| 2 V | 1 A |
| 3 V | 1.5 A |
| 4 V | 2 A |
🧪 Resistance Simulator
Observe carefully: when voltage increases while resistance remains constant, current increases.
🔹 7. Factors Affecting Resistance
The resistance of a wire depends mainly on:
Length
Resistance increases with increase in length.
Area of cross-section
Resistance decreases when the cross-sectional area increases.
Material
Different materials offer different resistance.
Temperature
Resistance of metallic conductors generally changes with temperature.
🧠 Length vs Resistance
| Wire | Length | Resistance |
|---|---|---|
| A | 1 m | R |
| B | 2 m | 2R |
| C | 3 m | 3R |
🧠 Think Before You Answer
Situation 1
Two wires are made of the same material. Wire A is twice as long as wire B and has the same thickness. Which has greater resistance?
Answer: Wire A.
Situation 2
Two wires have the same length and material. One wire is thicker. Which offers less resistance?
Answer: The thicker wire.
Situation 3
Why are conducting wires generally made from metals?
Because metals contain many free electrons and generally offer relatively low resistance.
🧮 Solved Numerical 1
A charge of 0.5 C passes through a cross-section of a conductor in 5 s. Find the current.
I = 0.5 / 5
I = 0.1 A
🧮 Solved Numerical 2
A current of 1.5 A flows through a conductor for 2.0 s. Find the charge passing through it.
Q = 1.5 × 2
Q = 3 C
🧮 Solved Numerical 3
A starter motor of a car is switched on for 0.8 s, and a charge of 24 C passes through the coil. Calculate the current.
I = 24 / 0.8
I = 30 A
🧮 Solved Numerical 4
In transferring 1.5 C charge through a wire, 9 J of work is done. Find the potential difference.
V = 9 / 1.5
V = 6 V
🧮 Solved Numerical 5
A cell of potential difference 12 V is connected to a bulb. The resistance of the filament when it glows is 24 Ω. Find the current drawn.
I = 12 / 24
I = 0.5 A
🧮 Solved Numerical 6
A bulb draws a current of 1.5 A at 6.0 V. Find the resistance of its filament.
R = 6 / 1.5
R = 4 Ω
🧮 Solved Numerical 7
A current of 0.2 A flows in a wire of resistance 15 Ω. Find the potential difference across its ends.
V = 0.2 × 15
V = 3 V
⚡ The Three Golden Formulae
Current
I = Q/t
Potential Difference
V = W/Q
Ohm's Law
V = IR
🧠 Common Concept Traps
Current → rate of flow of charge
Charge → quantity of electricity transferred
Potential difference → work done per unit charge
Resistance → opposition to current
🎯 25 MCQs – Exam Preparation
🌈 Rainbow Mindmap – Electricity
Use the 7-step Rainbow Method to decode the chapter.
Terms
Q, I, V, R
Current & Push
Circuit
Formulae
Numericals
Concept
📝 Quick Revision
| Quantity | Formula | SI unit |
|---|---|---|
| Current | I = Q/t | ampere (A) |
| Potential difference | V = W/Q | volt (V) |
| Resistance | R = V/I | ohm (Ω) |
| Ohm's law | V = IR | — |
🎯 Exam Strategy
Step 1 → Write the given quantities.
Step 2 → Write what is required.
Step 3 → Select the formula.
Step 4 → Substitute values with units.
Step 5 → Calculate carefully.
Step 6 → Write the answer with the correct SI unit.
💡 The Big Idea
Electricity becomes much easier when you see the relationship between the four major quantities:
Charge
How much electric charge is transferred?
Current
How quickly is charge flowing?
Potential Difference
What electrical push is available?
Resistance
How strongly is the flow opposed?
Understand the relationship → Use the formula → Solve the problem.
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