6.2 Electrical Energy & Uses
What is Electrical Power?
Power (P) is the rate at which energy is transferred. Electrical power is measured in watts (W).
Power (P) is the rate at which energy is transferred. Electrical power is measured in watts (W).
⚡ Key Concept:
• Power tells us how quickly energy is used
• An appliance with a high power rating transfers a lot of energy every second
• 1 watt = 1 joule per second
The higher the power, the more energy transferred each second.
• An appliance with a high power rating transfers a lot of energy every second
• 1 watt = 1 joule per second
The higher the power, the more energy transferred each second.
💡 Real-Life Examples:
Low Power Appliances (under 100W):
• LED bulb: ~10W
• Phone charger: ~5W
• Laptop: ~50W
High Power Appliances (over 1000W):
• Kettle: ~2000W
• Hairdryer: ~1500W
• Electric heater: ~2500W
• LED bulb: ~10W
• Phone charger: ~5W
• Laptop: ~50W
High Power Appliances (over 1000W):
• Kettle: ~2000W
• Hairdryer: ~1500W
• Electric heater: ~2500W
Basic Power Formula
$P = \frac{E}{t}$
Power = Energy ÷ Time
Using Current & Voltage
$P = I \times V$
Power = Current × Voltage
The Power Equations Triangle
We can calculate power using different combinations of current (I), voltage (V), and resistance (R).
• Current (I)
• Voltage (V)
• Current (I)
• Resistance (R)
• Voltage (V)
• Resistance (R)
We can calculate power using different combinations of current (I), voltage (V), and resistance (R).
⚡ The Four Power Equations:
$$P = \frac{E}{t}$$ (Power from energy and time)
$$P = I \times V$$ (Power from current and voltage)
$$P = I^2 \times R$$ (Power from current and resistance)
$$P = \frac{V^2}{R}$$ (Power from voltage and resistance)
$$P = I \times V$$ (Power from current and voltage)
$$P = I^2 \times R$$ (Power from current and resistance)
$$P = \frac{V^2}{R}$$ (Power from voltage and resistance)
Example 1: Using P = IV
A lamp draws a current of 0.5 A when connected to a 230 V supply. What is its power?
Step 1: Write down what you know
$I = 0.5$ A, $V = 230$ V
Step 2: Use the formula
$P = I \times V$
$P = 0.5 \times 230$
$P = 115$ W
Answer: The lamp has a power of 115 W
Step 1: Write down what you know
$I = 0.5$ A, $V = 230$ V
Step 2: Use the formula
$P = I \times V$
$P = 0.5 \times 230$
$P = 115$ W
Answer: The lamp has a power of 115 W
Example 2: Using P = I²R
A current of 3 A flows through a 20 Ω resistor. Calculate the power dissipated.
Step 1: Write down what you know
$I = 3$ A, $R = 20$ Ω
Step 2: Use the formula
$P = I^2 \times R$
$P = 3^2 \times 20$
$P = 9 \times 20 = 180$ W
Answer: The resistor dissipates 180 W
Step 1: Write down what you know
$I = 3$ A, $R = 20$ Ω
Step 2: Use the formula
$P = I^2 \times R$
$P = 3^2 \times 20$
$P = 9 \times 20 = 180$ W
Answer: The resistor dissipates 180 W
Example 3: Using P = V²/R
A 230 V heater has a resistance of 26.5 Ω. What is its power rating?
Step 1: Write down what you know
$V = 230$ V, $R = 26.5$ Ω
Step 2: Use the formula
$P = \frac{V^2}{R}$
$P = \frac{230^2}{26.5}$
$P = \frac{52900}{26.5} = 1996$ W ≈ 2000 W
Answer: The heater is rated at about 2000 W (2 kW)
Step 1: Write down what you know
$V = 230$ V, $R = 26.5$ Ω
Step 2: Use the formula
$P = \frac{V^2}{R}$
$P = \frac{230^2}{26.5}$
$P = \frac{52900}{26.5} = 1996$ W ≈ 2000 W
Answer: The heater is rated at about 2000 W (2 kW)
When to use P = IV
When you know:• Current (I)
• Voltage (V)
When to use P = I²R
When you know:• Current (I)
• Resistance (R)
When to use P = V²/R
When you know:• Voltage (V)
• Resistance (R)
🧮 Power Calculator:
Enter any two values to calculate power:
🎯 Power Equation Practice:
W
W
Calculating Energy Transferred
Rearranging the power equation gives us the energy equation.
Rearranging the power equation gives us the energy equation.
⚡ Energy Formula:
$$E = P \times t$$
Energy (J) = Power (W) × Time (s)
Important:
• Energy is measured in joules (J)
• For household energy, we use kilowatt-hours (kWh)
• 1 kWh = 1000 W × 3600 s = 3,600,000 J
Energy (J) = Power (W) × Time (s)
Important:
• Energy is measured in joules (J)
• For household energy, we use kilowatt-hours (kWh)
• 1 kWh = 1000 W × 3600 s = 3,600,000 J
Example 1: Energy in Joules
A 60 W light bulb is left on for 5 minutes. How much energy does it use?
Step 1: Convert time to seconds
$t = 5 \times 60 = 300$ s
Step 2: Calculate energy
$E = P \times t$
$E = 60 \times 300$
$E = 18000$ J = 18 kJ
Answer: The bulb uses 18,000 J (18 kJ) of energy
Step 1: Convert time to seconds
$t = 5 \times 60 = 300$ s
Step 2: Calculate energy
$E = P \times t$
$E = 60 \times 300$
$E = 18000$ J = 18 kJ
Answer: The bulb uses 18,000 J (18 kJ) of energy
Example 2: Energy in kWh (Electricity Bills)
A 2 kW heater is used for 3 hours. If electricity costs 30p per kWh, what is the cost?
Step 1: Calculate energy in kWh
$E = P \times t$
$E = 2 \text{ kW} \times 3 \text{ hours} = 6$ kWh
Step 2: Calculate cost
Cost = $6 \times 30p = 180p = £1.80$
Answer: It costs £1.80 to run the heater
Step 1: Calculate energy in kWh
$E = P \times t$
$E = 2 \text{ kW} \times 3 \text{ hours} = 6$ kWh
Step 2: Calculate cost
Cost = $6 \times 30p = 180p = £1.80$
Answer: It costs £1.80 to run the heater
ENERGY METER
0.000
kWh
Run for:
hours
🎯 Energy Calculation Practice:
Two Types of Current
There are two ways electricity can flow: direct current (DC) and alternating current (AC).
There are two ways electricity can flow: direct current (DC) and alternating current (AC).
Direct Current (DC)
→→→→
Current flows in one direction only
Supplied by batteries and cells
Alternating Current (AC)
∿∿∿∿
Current constantly changes direction
Supplied by mains electricity
⚡ UK Mains Electricity:
• Type: Alternating Current (AC)
• Frequency: 50 Hz (changes direction 50 times per second)
• Voltage: About 230 V
The mains voltage is dangerous and can kill.
• Frequency: 50 Hz (changes direction 50 times per second)
• Voltage: About 230 V
The mains voltage is dangerous and can kill.
| Feature | DC (Direct Current) | AC (Alternating Current) |
|---|---|---|
| Direction | One way only | Changes back and forth |
| Source | Batteries, solar cells | Power stations, mains |
| UK Voltage | 1.5V, 9V, 12V (typical) | 230V |
| Uses | Phones, torches, cars | Home appliances, lighting |
🎯 AC or DC Quiz:
The Three-Core Cable
Most mains appliances are connected with a three-core cable containing three wires.
Most mains appliances are connected with a three-core cable containing three wires.
🟤 Live Wire (Brown)
Voltage: 230 V
Carries the high voltage from the supply
⚠️ DANGEROUS
🔵 Neutral Wire (Blue)
Voltage: 0 V
Completes the circuit
Returns current to supply
🟢🟡 Earth Wire (Green & Yellow)
Voltage: 0 V
Safety wire only
Protects from electric shock
💡 How the Earth Wire Protects You:
1. The earth wire connects to the metal casing of an appliance
2. If the live wire accidentally touches the casing...
3. A large current flows through the earth wire (path of least resistance)
4. This blows the fuse or trips the circuit breaker
5. The electricity is cut off, preventing electric shock
Without the earth wire: You could become the path to ground and receive a fatal shock.
2. If the live wire accidentally touches the casing...
3. A large current flows through the earth wire (path of least resistance)
4. This blows the fuse or trips the circuit breaker
5. The electricity is cut off, preventing electric shock
Without the earth wire: You could become the path to ground and receive a fatal shock.
🛡️ Fuses - Your Circuit Protectors:
A fuse is a safety device containing a thin wire that melts if the current gets too high, breaking the circuit.
A fuse is a safety device containing a thin wire that melts if the current gets too high, breaking the circuit.
3A
Low power appliances
5A
Medium power appliances
13A
High power appliances
Example: Choosing the Right Fuse
A hairdryer is rated at 1500 W and operates on 230 V mains. What fuse should be used?
Step 1: Calculate the current
$I = \frac{P}{V} = \frac{1500}{230} = 6.5$ A
Step 2: Choose the fuse
The fuse must be slightly higher than the normal operating current.
Available fuses: 3A, 5A, 13A
Answer: Use a 13A fuse (the next size up from 6.5A)
Step 1: Calculate the current
$I = \frac{P}{V} = \frac{1500}{230} = 6.5$ A
Step 2: Choose the fuse
The fuse must be slightly higher than the normal operating current.
Available fuses: 3A, 5A, 13A
Answer: Use a 13A fuse (the next size up from 6.5A)
⚠️ Electrical Safety Rules:
• Never touch electrical appliances with wet hands
• Don't overload sockets with too many plugs
• Replace damaged cables immediately
• Never poke anything into sockets
• Switch off appliances before unplugging
• Never touch electrical appliances with wet hands
• Don't overload sockets with too many plugs
• Replace damaged cables immediately
• Never poke anything into sockets
• Switch off appliances before unplugging
🧮 Fuse Calculator:
Enter the appliance's power rating to find the correct fuse:
Power:
W at 230V
🎯 Wire Identification Quiz: