Understanding Energy, Power and Resistance in A Level Physics

Electrical Resistance and Ohm's Law

Electrical resistance is a measure of the opposition to the flow of electric current in a conductor. The relationship between current, voltage, and resistance is described by Ohm's law: V = IR, where V is the potential difference (voltage) across the conductor, I is the current flowing through it, and R is the resistance.

Factors Affecting Resistance

• Length: Resistance is directly proportional to the length of the conductor. Longer conductors have higher resistance.
• Cross-sectional Area: Resistance is inversely proportional to the cross-sectional area of the conductor. Thicker conductors have lower resistance.
• Material: Different materials have different resistivities, which is a measure of a material's ability to resist current flow. Metals like copper and aluminum have low resistivity, while insulators like rubber and plastic have very high resistivity.
• Temperature: For most materials, resistance increases with increasing temperature. This is due to increased vibrations of atoms, which scatter the flow of electrons.

Electrical Energy and Power

Electrical energy is the energy transferred by an electric current. The power dissipated in a resistor is given by: P = IV or P = I²R or P = V²/R. The unit of power is the watt (W).

Worked Example

Problem: A 120 Ω resistor is connected to a 12 V battery. Calculate the current, power dissipated, and energy transferred in 5 minutes.

Solution:

• Given: R = 120 Ω, V = 12 V
• Using V = IR, I = V/R = 12/120 = 0.1 A
• Power dissipated, P = VI = 12 × 0.1 = 1.2 W
• Energy transferred in 5 minutes = Power × Time = 1.2 × (5 × 60) = 360 J

Efficiency in Electrical Systems

The efficiency of an electrical system is the ratio of useful power output to total power input, expressed as a percentage. Inefficiencies arise due to energy losses, such as heat dissipation in resistors. Improving efficiency is crucial for energy conservation and reducing operating costs.