Electric Power - CBSE Class 10 Science (Physics)

1. What is Electric Power?

Electric Power is the rate at which electrical energy is consumed or produced in any electrical device or circuit.
In simpler words, it tells how fast work is done using electricity in a device.

Important Points:

When you use an electric fan, the speed at which it uses electrical energy is its power.
Power helps us compare which device uses more electricity.

Examples:

An electric bulb uses energy quickly to produce light. The rate at which it uses energy is its power.
A microwave oven uses more power than a fan because it does more work in the same time.

2. Formulas for Electric Power

There are multiple formulas to calculate electric power. These formulas are based on what quantities you know.

a. Basic Formula (Energy and Time)

P = \frac{W}{t}

P = Power (in Watt, W)
W = Work done or energy used (in Joule, J)
t = time taken (in seconds)

Examples:

If a fan uses 720 J energy in 60 seconds, its power = 720 J / 60 s = 12 W
If a toaster uses 1000 J in 10 seconds, power = 1000 J / 10 s = 100 W

b. Using Voltage and Current

P = V \times I

P = Power (in W)
V = Potential difference (in volts)
I = Current (in amperes)

Examples:

An iron works at 220 V and 5 A: P = 220 × 5 = 1100 W
A bulb uses 12 V and 0.5 A: P = 12 × 0.5 = 6 W

c. Using Current and Resistance

P = I^2 \times R

I = Current (in A)
R = Resistance (in Ω)

Examples:

A heater draws 2 A and its resistance is 50 Ω: P = (2)² × 50 = 4 × 50 = 200 W
A coil uses 0.2 A, resistance is 400 Ω: P = (0.2)² × 400 = 0.04 × 400 = 16 W

d. Using Voltage and Resistance

P = \frac{V^2}{R}

V = Voltage (in V)
R = Resistance (in Ω)

Examples:

A device has 230 V across 115 Ω: P = (230)² / 115 = 52900 / 115 = 460 W
A resistor of 50 Ω with 100 V: P = (100)² / 50 = 10000 / 50 = 200 W

3. Units of Electric Power

Main SI Unit: Watt (W)
Bigger units: Kilowatt (kW), Megawatt (MW)
- 1 kW = 1000 W
- 1 MW = 1,000,000 W = $10^6$ W

Examples:

A geyser marked "2 kW" uses 2,000 W.
A power station generating 250 MW—this means it produces 250,000,000 W.

4. Unit of Electrical Energy

Watt hour (Wh): 1 Wh = 1 W used for 1 hour
Kilowatt hour (kWh): 1 kWh = 1,000 Wh = 3.6 × $10^6$ J (joules)
1 kWh = 1 unit (seen in the electricity bill)

Examples:

100 W TV runs for 10 hours: energy = 100 W × 10 h = 1,000 Wh = 1 kWh (1 unit)
500 W device running for 4 hours: E = 500 × 4 = 2,000 Wh = 2 kWh

5. Summary Table of Power Formulas

Formula	When to Use
$P = V \times I$	You know Voltage and Current
$P = I^2R$	You know Current and Resistance
$P = \frac{V^2}{R}$	You know Voltage and Resistance

6. Key Points to Remember

Electric Power shows how quickly electrical energy is used or produced.
1 kilowatt (kW) = 1000 watts (W)
Electrical energy consumed (in kWh) = Power (kW) × Time (in hours)
1 kWh = 3,600,000 J (3.6 × $10^6$ J)
You pay electric bills based on consumption in kWh (units).

Example Problems

Example 1: (Using V and I)

A fan works at 220 V and draws 0.5 A.
Power = V × I = 220 × 0.5 = 110 W

Example 2: (Using I and R)

Resistor: 20 Ω, Current: 5 A.
Power = $I^2R = 5^2 × 20 = 25 × 20 = 500\,W$

Example 3: (Using V and R)

Resistor: 100 Ω, Voltage: 200 V.
Power = $P = \frac{(200)^2}{100} = 40000 / 100 = 400\,W$

Example 4: (kWh to Joule Conversion)

Device: 2 kW for 3 hours = 6 kWh.
Energy = 6 × 3.6 × $10^6 = 21.6×10^6$ J.

Example 5: (Electricity Bill)

1.5 kW heater for 2 hours/day, 30 days.
Energy/day: 1.5 × 2 = 3 kWh
Total: 3 × 30 = 90 kWh (units)
Cost at ₹6/unit: 90 × 6 = ₹540

Fun Activity: Find Your Home Appliance Power!

Objective:
Check how much power and energy your favorite home appliance uses.

Materials Needed:

List of appliances (with watts written on them)
Calculator
Pen and notebook

Steps:

Note down the power rating (“W” or “kW”) of any appliance (e.g., TV, fridge).
List approximate hours you use it daily.
Calculate energy in kWh:
Energy = Power (in kW) × Time (hours)
Find the cost for a month if 1 kWh = ₹8.

Observations:

Devices used longer or with higher power use more energy.
You can see which appliances add more to the electricity bill!
Comparing a LED bulb (9 W) to a tube light (40 W) shows why LEDs are energy-saving.

Scenario Based Questions

Scenario: You see that two bulbs of 60 W and 100 W are running at the same voltage.
- Question: Which bulb consumes more energy in 5 hours and why?
- Answer: The 100 W bulb consumes more energy because it has a higher power rating. In 5 hours:
  60 W bulb: 0.06 kWh × 5 = 0.3 kWh
  100 W bulb: 0.1 kWh × 5 = 0.5 kWh.
Scenario: Your parents ask you to help save electricity in the house.
- Question: What suggestions can you give using your knowledge of electric power?
- Answer: Use appliances with lower power ratings, turn off lights and devices when not needed, and switch to LED lights to reduce energy use and save money.
Scenario: Your friend buys a 200 W mixer and uses it 30 minutes every day.
- Question: How much energy will it use in a month (30 days)?
- Answer:
  Daily use in hours = 0.5 h
  Energy/day = 200 W × 0.5 h = 100 Wh = 0.1 kWh
  In 30 days: 0.1 × 30 = 3 kWh
Scenario: An electrician connects an electric heater to a 230 V supply. Its resistance is 115 Ω.
- Question: What is the current and power used by the heater?
- Answer:
  Current, $I = \frac{V}{R} = \frac{230}{115} = 2\,A$
  Power, $P = V \times I = 230 \times 2 = 460\,W$
Scenario: You have to choose between a 500 W iron and a 1000 W iron. Both will be used for 1 hour.
- Question: Which will use more electricity and cost more in your bill? By how much?
- Answer:
  500 W iron uses 0.5 kWh; 1000 W iron uses 1 kWh per hour.
  The 1000 W iron will use more electricity and costs double for the same time.

Enjoy Learning!

Remember: Understanding electric power helps save energy, money, and the environment. Have fun asking questions and trying calculations at home!