Lecture Notes: Calculating and Increasing Efficiency of Energy Transfer

Introduction

• Objective: By the end of the lecture, you should be able to:
  • Calculate the efficiency of energy transfer.
  • Describe ways to improve energy transfer efficiency (only for higher-tier students).

Types of Light Bulbs

• Incandescent Bulb: Old-style, being phased out but still found in some buildings.
• LED Bulb: Modern, energy-efficient, increasingly popular in the UK.

Understanding Efficiency

• Definition: Efficiency measures the fraction of energy input into an appliance that is transferred to useful forms.
• Equations:
  • Efficiency = Useful output energy transfer / Total input energy transfer
  • Efficiency = Useful power output / Total power input
  • Note: Equations not given in exams; must be memorized.

Example Calculation

• Problem: Electric hob transfers 15,000 J of thermal energy; 12,000 J is transferred to water.
  • Useful output: 12,000 J
  • Total input: 15,000 J
  • Calculated Efficiency: 0.8 (or 80% if converted to a percentage)
• Important Points:
  • Efficiency can be expressed as a decimal or percentage (multiply by 100).
  • Incorrect calculations result in efficiency > 1 or > 100%, which is impossible.

Practice Problem

• Scenario: Electric current delivers 60 watts to a bulb, 3 watts transferred to light.
  • Efficiency = 3 W / 60 W = 0.05 or 5%
• Observation: Inefficiency mainly due to conversion of electrical energy to wasted thermal energy.

Improving Efficiency

• Pan and Hob Example:
  • Inefficiencies occur due to thermal energy loss to air.
  • Solutions:
    • Use a pan with a wider base and a lid to reduce unwanted thermal transfer.
    • Use of an electric kettle where heating element is inside the water.
  • Material Considerations:
    • Metal conducts heat well, leading to more energy loss.
    • Plastics have lower thermal conductivity, better for reducing energy loss.

Conclusion

• Key Outcomes:
  • Calculation of energy transfer efficiency.
  • Understanding methods to increase efficiency (for higher-tier students).

Additional Resources

• More practice questions available in the revision workbook.