IGCSE Physics 0625

4.2.4 Resistance

This section focuses on understanding the relationship between electrical power, current, and voltage. We will recall and apply the formula for electrical power.

Electrical Power

Electrical power is the rate at which electrical energy is converted into other forms of energy, such as heat, light, or motion. It is measured in Watts (W).

The relationship between electrical power (P), current (I), and voltage (V) is given by the following equation:

$$P = I \times V$$

Where:

• P = Electrical power (in Watts)
• I = Current (in Amperes)
• V = Voltage (in Volts)

Using the Power Equation

This equation can be used to calculate any of the three quantities if the other two are known.

To find Power (P): Multiply the current (I) by the voltage (V). $$P = I \times V$$

To find Current (I): Divide the power (P) by the voltage (V). $$I = \frac{P}{V}$$

To find Voltage (V): Divide the power (P) by the current (I). $$V = \frac{P}{I}$$

Example Calculations

Example 1: A light bulb has a resistance of 10 ohms and is connected to a 12V battery. Calculate the power dissipated by the bulb.

Solution:

1. We know the voltage (V = 12V) and the resistance (R = 10 ohms).
2. First, we need to calculate the current (I) using Ohm's Law: $$I = \frac{V}{R} = \frac{12V}{10 \Omega} = 1.2 A$$
3. Now, we can calculate the power (P) using the power equation: $$P = I \times V = 1.2 A \times 12V = 14.4 W$$

Answer: The power dissipated by the bulb is 14.4 W.

Example 2: A resistor with a resistance of 20 ohms is connected across a 5V supply. Calculate the current flowing through the resistor and the power dissipated by it.

Solution:

1. We know the voltage (V = 5V) and the resistance (R = 20 ohms).
2. First, we calculate the current (I) using Ohm's Law: $$I = \frac{V}{R} = \frac{5V}{20 \Omega} = 0.25 A$$
3. Next, we calculate the power (P) using the power equation: $$P = I \times V = 0.25 A \times 5V = 1.25 W$$

Answer: The current flowing through the resistor is 0.25 A and the power dissipated is 1.25 W.

Table Summary

Quantity	Symbol	Unit
Power	P	Watt (W)
Current	I	Ampere (A)
Voltage	V	Volt (V)