grade 8 Electric Power & Energy

Electric Power & Energy

Electric power is the work electricity does per second; a higher wattage makes a bulb brighter. The formula is P = V × I, so raising voltage or current raises power. Energy is power times time, and 1 kWh on your bill is 1,000 W used for one hour. Change voltage and current and bulb brightness shifts; change power and hours and the energy total appears.

What is Electric Power?

💡 Power = the rate at which electricity does work

①60W vs 100W bulb — higher W = brighter

②Power (P) = work done by electricity per second

③Increasing voltage or current increases power

Power and Bulb Brightness

Voltage V10V

Current I3A

Power Formula

P = V × I

Power (W) = voltage (V) × current (A)

Using Ohm's Law

P = I²R = V²/R

Substituting V = IR lets you compute power from resistance

Energy — Total Consumption

Power P60W

Hours5h

Energy Formula

W = P × t = V × I × t

Energy (Wh) = power (W) × time (h)

💡 What is kWh?

①1 kWh = 1,000 Wh — billing unit

②Using a 1000 W heater for 1 hour = 1 kWh

③Your bill's "consumption" is kWh

Joule (J) and Calorie (cal)

Unit Conversion

1 Wh = 3,600 J, 1 cal = 4.2 J

W = P × t (J if seconds); convert to cal for heat

Joule Heating

Q = 0.24 × I² × R × t

Heat generated when current passes through a resistor (cal)

Summary

Power & Energy (Core)

P = VI, W = Pt = VIt

Power = energy per second; energy = total consumption

🎯 Exam Key Points

①Remember P = VI = I²R = V²/R

②Energy units: Wh, kWh, J (1Wh = 3,600J)

③Bulb brightness ∝ power (W)

④Electricity bills based on energy (kWh)

⑤Same power but longer time → more energy (cost)

Examples and Unit Test

Example 1

An appliance has 2 A flowing from a 220 V source. What is its power consumption in watts?

Power is found with P = V × I (power = voltage × current).

P = 220 V × 2 A = 440 W.

▸ 440 W

Power P = V × I, with the unit W (watt).

Example 2

If a 1,000 W heater is used for 2 hours, what is the electrical energy used, in kWh?

Electrical energy is the product of power and time (W = P × t).

1,000 W × 2 h = 2,000 Wh = 2 kWh. (1 kWh = 1,000 Wh)

▸ 2 kWh

Electrical energy = power × time, and 1 kWh = 1,000 Wh.

Unit-test style

Which statement about electric power and energy is correct?

①Power is found with P = V ÷ I

②Electrical energy is the product of power and time used

③The smaller the wattage (W), the brighter the bulb

④1 kWh is 100 Wh

⑤The unit of power is V (volt)

▸ ② Electrical energy is the product of power and time used

Electrical energy is W = P × t, found by multiplying power by the time used.

Power is P = V × I (①), a larger wattage is brighter (③), 1 kWh = 1,000 Wh (④), and the unit of power is W (watt) (⑤).

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Ohm's Law

Magnetic Field

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