Imagine a world without the electricity. You have no phones, no lights, and no fans. Your life would stop completely. Electricity is one of the most important discoveries of modern life. It helps in homes, schools, hospitals, and many things.
- Electric Charge (Q)
- Electric Current (I)
- Potential Difference (V)
- Ohm’s Law
- Resistance (R) and Resistivity (ρ)
- Combination of Resistors
- Heating Effect of Current
- Electric Power (P)
- Household Circuits: Why Parallel Connection?
- Quick Revision Table
- Solved Numerical
- Deep Questions
- Final Exam Tips
- Conclusion
In this chapter, we will study electric charge, electric current, circuits, resistance, heating effect, and power electricity.
1. Electric Charge (Q)
The electric charge is a property of the matter. The objects will experience force in an electric field.
Types of the Charge:
A Positive Charge = Proton
A Negative Charge = Electron
SI Unit:
Coulomb (C)
Charge of One Electron:
e = -1.6 × 10⁻¹⁹ C
Formula:
Q = ne
Where:
Q is the total charge
n = Number of the electrons
e = the Charge of one electron
2. Electric Current (I)
The electric current is the rate of the flow of an electric charge through a conductor.
Formula:
I = Q / t
Where:
I = Current
Q = Charge
t = Time
SI Unit:
Ampere (A)
Direction of Current:
Conventional Current = Positive to Negative
Electron Flow = Negative to Positive
Measuring Instrument:
Ammeter (connected in series)
3. Potential Difference (V)
The charges need energy to move the circuit. This pushing is called potential difference or voltage.
Definition:
The work done is moving a unit charge from one point to another.
Formula:
V = W / Q
Where:
V = Voltage
W = Work done
Q = Charge
SI Unit:
Volt (V)
Measuring Instrument = Voltmeter (it’s connected in parallel)
4. Ohm’s Law
At constant temperature, the current flowing through a conductor is directly proportional to the potential difference across its ends. This is known as Ohm’s Law.
Where:
V = Voltage
I = Current
R = Resistance
5. Resistance (R) and Resistivity (ρ)
The Resistance opposes the flow of the current.
Factors Affecting Resistance:
1. Length (L):
More length = More resistance
2. Area (A):
More thickness = Less resistance
3. Material:
Depends on conductor type
4. Temperature:
When itWhen it increases with the heat
SI Unit:
Resistance = Ohm (Ω)
Resistivity = Ω m
6. Combination of Resistors
A. Series Combination
It’s connected one after another.
The current is the same in all the resistors. the resistors.
The voltage is divided.
Formula:
Rs = Rs = R1+ R2+ R3
B. Parallel Combination
It connects with the same two points.
The Voltage was the same in all resistors
The current is divided. divided.
7. Heating Effect of Current
When a current passes through the resistance wire, then heat is produced. produced.
Joule’s Law of Heating:
H = I2Rt
Where heat depends on:
Square of current
Resistance
Time
For Uses:
- Electric heater
- Iron
- Geyser
- Bulb filament
- Fuse wire
8. Electric Power (P)
The electric power is the rate at which electrical energy is consumed. It is known as electric power. power.
Pointwise: Pointwise:
• It shows how fast electrical energy is used.
• Symbol of electric power = P
• It is used in bulbs, fans, heaters, motors, etc.
• SI unit of power = Watt (W)
• Commercial unit = kilowatt-hour (kWh)
• Formula:
• P = VI
Other formulas:
• P = I^2R
• 1 kWh = 3.63.6 × 10⁶ JJ
• 1 kWh = (1 Unit)
9. Household Circuits: Why Parallel Connection?
The homes use parallel circuits because:
1. Each appliance works independently. independently.
2. The Same voltage to all the appliances
3. If one bulb fuses, then the others are still working. working.
10. Quick Revision Table
| Quantity | Symbol | Formula | SI Unit |
|---|---|---|---|
| Charge | Q | Q = ne | Coulomb |
| Current | I | I = Q/t | Ampere |
| Voltage | V | V = W/Q | Volt |
| Resistance | R | R = V/I | Ohm |
| Resistivity | ρ | ρ = RA/L | Ωm |
| Power | P | P = VI | Watt |
| Heat | H | H = I²Rt | Joule |
11. Solved Numerical
Question:
The electric bulb is connected to a 220 V supply. The current is 0.50 A. Then a fine power.
Given:
V = 220 V
I = 0.50 A
Formula:
P = VI
Solution:
P = 220 \times 0.50 = 110W
Answer:
Power = 110 Watt
Deep Questions
Q1. Why is electric power called the rate of doing electrical work?
Answer: Because it tells how much electrical energy is used per second by a device.
Q2. Why does a heater consume more power than an LED bulb?
Ans : A heater converts electrical energy
into heat and needs more energy per second than an LED bulb.
Q3. If voltage increases and resistance remains constant, what happens to power?
Ans: Power increases because
Q4. Why are high-power appliances connected with thick wires?
Ans: The high-power The high-power appliances draw more current, so they use thick wires to avoid overheating. overheating.
Q5. Why is kilowatt-hour used in homes instead of joules? joules?
Ans: Joule is a very small unit. Kilowatt-hour is larger and easier for billing electricity.
Q6. Why does an electric bulb get hot when it is switched on??
Answer: Some electrical energy can change into heat along with light.
Q7. How does resistance affect electric power when current is constant?
Ans: The power increases with resistance because
P = I^2R
Q8. Why do electricity bills depend on energy used, not only power rating?
Answer: Because total bills depend on how much power is used and for how long it is used.
Q9. Why is a 100 W bulb brighter than a 40 W bulb generally?
Ans: It uses more electrical power per second, and it usually produces more light.
Q10. Why shouldn’t several power appliances be operated simultaneously from a single socket?
Ans: They may draw excess current, causing an overheat or a fuse trip.
Conclusion
Electricity is the important chapter for your exams. It has formulas, numericals, diagrams, and practical uses. If you understand this concept clearly. Then this chapter becomes a very, very easy and scoreable concept.
Do daily practice and revise the formulas again and again.