Exploring Energy, Power and Resistance in A Level Physics

Understanding Electrical Resistance

Electrical resistance is a measure of the opposition to the flow of electric current in a conductor. It is represented by the symbol R and is measured in ohms (Ω). The relationship between resistance, voltage (V) and current (I) is defined by Ohm's law:

V = IR

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 (ρ), which is the resistance per unit length and cross-sectional area. Metals like copper and aluminum have low resistivities, making them good conductors.
• Temperature: The resistance of metallic conductors increases with temperature due to increased vibrations of atoms. Semiconductors and insulators may exhibit different temperature dependencies.

Electrical Energy and Power

Electrical energy is the energy transferred by the flow of electric charge. The power (P) dissipated in a resistor is given by:

P = IV or P = I²R or P = V²/R

The electrical energy (E) transferred in a circuit is the product of power and time:

E = Pt or E = QV

Efficiency in Electrical Systems

Efficiency is the ratio of useful power output to the total power input, expressed as a percentage. In electrical systems, some energy is dissipated as heat due to resistance, reducing efficiency. Minimizing resistance and using conductors with low resistivity can improve efficiency.

Worked Example

Problem: A 12 V battery is connected to a 6 Ω resistor. Calculate the current, power dissipated, and energy transferred in 5 minutes.

Solution:

• Using Ohm's law: I = V/R = 12/6 = 2 A
• Power dissipated: P = IV = 2 × 12 = 24 W
• Energy transferred: E = Pt = 24 × (5 × 60) = 7200 J

Understanding these concepts is crucial for designing and analyzing electrical circuits and systems, as well as optimizing energy efficiency.

For further reading, refer to the OCR A Level Physics specification and resources on BBC Bitesize.