Understanding Electrical 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. It is governed by Ohm's law, which states that the current (I) through a conductor is directly proportional to the voltage (V) across it, for a given resistance (R):

V = IR

Factors Affecting Resistance

• Length: Resistance is directly proportional to the length of the conductor. Longer wires have higher resistance.
• Cross-sectional Area: Resistance is inversely proportional to the cross-sectional area. Thicker wires have lower resistance.
• Material: Different materials have different resistivities (ρ), a property that determines how resistive a material is.
• Temperature: Resistance increases with increasing temperature for most metallic conductors.

Electrical Energy and Power

Electrical energy is the energy transferred by an electric current through a component. The electrical power (P) dissipated in a resistor is given by:

P = IV = I2R = V2/R

Worked Example

Problem: A 12 V battery is connected to a 3 Ω resistor. Find the current, voltage drop across the resistor, and power dissipated.

Solution:

1. Using V = IR, I = V/R = 12/3 = 4 A
2. The voltage drop across the resistor is 12 V (same as the battery)
3. Power dissipated = I2R = 42 × 3 = 48 W

Efficiency in Electrical Systems

The efficiency of an electrical system is the ratio of useful power output to total power input. Energy is lost due to heating effects from resistance, so practical systems are never 100% efficient.

Efficiency = (Useful power output) / (Total power input) × 100%