Electrical Energy, Power and Resistance in A Level Physics

Electrical Energy, Power and Resistance This topic covers the fundamental principles of electrical energy, power, and resistance in circuits. Understanding thes...

Electrical Energy, Power and Resistance

This topic covers the fundamental principles of electrical energy, power, and resistance in circuits. Understanding these concepts is crucial for analyzing and designing electrical systems.

Electrical Resistance and Ohm's Law

Resistance is a measure of the opposition to the flow of electric current in a conductor. It is denoted by the symbol R and is measured in ohms (Ω). Ohm's law states that the current (I) flowing through a resistor is directly proportional to the voltage (V) across it, with the resistance (R) being the constant of proportionality:

V = IR

Factors Affecting Resistance

Length: The resistance of a conductor is directly proportional to its length. 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 determine their intrinsic resistance. Resistivity is a material property measured in ohm-meters (Ω⋅m).
Temperature: The resistance of most metallic conductors increases with increasing temperature due to increased vibrations of atoms.

Electrical Energy and Power

Electrical energy is the capacity to do work or transfer energy in an electrical circuit. It is measured in joules (J) or kilowatt-hours (kWh). The electrical power (P) is the rate at which energy is transferred or dissipated, measured in watts (W).

P = IV or P = I2R or P = V2/R

Worked Example

Problem: A 120 V electrical appliance has a power rating of 1.2 kW. What is the current drawn by the appliance?

Solution:

Given: V = 120 V, P = 1.2 kW = 1200 W
Using P = IV, we can rearrange to find I = P/V
I = 1200 W / 120 V = 10 A

Efficiency in Electrical Systems

Efficiency is a measure of how much useful energy output is obtained from a given energy input. In electrical systems, efficiency accounts for energy losses due to resistance, heat dissipation, and other factors. It is calculated as:

Efficiency = (Useful Output Energy) / (Total Input Energy)