1. What Electric Power Really Means
Whenever an electrical device works—like a bulb glowing or a fan rotating—it uses energy per second. This rate at which electrical energy is consumed or supplied is called electric power.
Higher power means the device uses or delivers more energy per second.
1.1. Definition of Power
Electric power is the rate at which electrical energy is converted into other forms such as heat, light or mechanical work.
\( P = \dfrac{W}{t} \)
2. Power in Terms of Voltage and Current
Since electrical devices operate on voltage and current, power can also be expressed using these quantities.
2.1. Power Formula
\( P = VI \)
This tells us the power consumed depends on both the potential difference applied and the current flowing.
2.1.1. Using Ohm’s Law in Power
Using \( V = IR \), power can also be written as:
\( P = I^2 R \)
\( P = \dfrac{V^2}{R} \)
These forms are useful depending on which quantities are known.
3. What Electrical Energy Means
When electrical power acts over a period of time, it results in electrical energy. This is the total amount of work done or total energy consumed.
Energy is simply power multiplied by time.
3.1. Formula for Electrical Energy
\( W = Pt \)
If power remains constant over time, this relation gives the total energy used.
3.1.1. Energy Using Current and Resistance
Using power formulas:
\( W = I^2 R t \)
\( W = V I t \)
4. Units of Power and Energy
The standard unit of power is the watt (W). One watt means one joule of energy used per second.
Larger units:
- kilowatt (kW) = 1000 W
- megawatt (MW) = 106 W
4.1. Unit of Energy
Energy is commonly measured in joules (J). For household electricity billing, a different unit is used:
1 kilowatt-hour (kWh) = energy consumed by a 1 kW appliance in 1 hour.
\( 1\,\text{kWh} = 3.6 \times 10^6\,\text{J} \)
5. Power Consumption in Daily Appliances
Electrical appliances are rated by their power. For example, a 60 W bulb uses 60 joules of energy every second, while a 1500 W heater uses much more energy per second.
The rating helps choose devices depending on how much energy they will consume.
5.1. Small Example
A 1000 W iron working for 30 minutes consumes energy:
\( W = P t = 1000 \times 1800 = 1.8 \times 10^6\,\text{J} \)