IGCSE Physics - 4.2.4 Resistance

This section focuses on understanding and applying the concept of resistance in electrical circuits. We will review the relationship between voltage, current, and resistance, and learn how to calculate resistance using the appropriate formula.

Understanding Resistance

Resistance is the opposition to the flow of electric current in a material. Materials with high resistance impede the flow of current, while those with low resistance allow current to flow easily. Resistance is measured in Ohms (Ω).

Factors Affecting Resistance

The resistance of a material depends on several factors:

• Material: Different materials have different inherent resistances. For example, copper has low resistance, while rubber has high resistance.
• Length: The longer the conductor, the higher the resistance.
• Cross-sectional area: The greater the cross-sectional area, the lower the resistance.
• Temperature: In most metals, resistance increases with temperature.

The Relationship Between Voltage, Current, and Resistance

Ohm's Law describes the relationship between voltage (V), current (I), and resistance (R). The fundamental equation relating these quantities is:

$$R = \frac{V}{I}$$

Where:

• R = Resistance (measured in Ohms, Ω)
• V = Voltage (measured in Volts, V)
• I = Current (measured in Amperes, A)

This equation states that the resistance of a conductor is directly proportional to the voltage applied across it and inversely proportional to the current flowing through it.

Using the Equation for Resistance

To use the equation $R = \frac{V}{I}$, you need to know the voltage across a component and the current flowing through it. Here's how to apply it:

1. Identify the voltage (V) and the current (I) in the circuit.
2. Substitute these values into the formula $R = \frac{V}{I}$.
3. Calculate the resistance (R). Remember to include the units (Ohms).

Example

A resistor has a voltage of 12V applied across it and a current of 0.5A flowing through it. Calculate the resistance of the resistor.

Using the formula: $R = \frac{V}{I} = \frac{12}{0.5} = 24 \Omega$

Therefore, the resistance of the resistor is 24 Ohms.

Resistance in Series and Parallel Circuits

When resistors are connected in series and parallel, the total resistance is calculated differently. These calculations are beyond the scope of this section but are important for understanding more complex circuits.

Circuit Type	Total Resistance
Series	$R_{total} = R_1 + R_2 + R_3 + ...$
Parallel	$\frac{1}{R_{total}} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} + ...$

This concludes the section on resistance. Remember the key equation: $R = \frac{V}{I}$ and how to apply it to solve problems.