Efficiency in Energy Transfer

Key Concepts

• Energy Transfer: Energy can be transferred from one form to another.
• Device Efficiency: Most devices are not 100% efficient, meaning not all input energy is converted to useful output energy.

Efficiency Calculation

Definitions

• Useful Energy Output: Desired energy form (e.g., light in a lamp).
• Waste Energy: Non-useful energy form (e.g., heat in a lamp).

Efficiency Equation

• For energy:

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

• For power:

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

Example: Lamp Efficiency

• Incandescent Bulb:

  • Input Energy: 300 J

  • Useful Output: 45 J

  • Efficiency:

    Efficiency = 45 J / 300 J = 0.15 (15%)

• LED Bulb:

  • Input Energy: 300 J

  • Useful Output: 225 J

  • Efficiency:

    Efficiency = 225 J / 300 J = 0.75 (75%)

• Comparison: LED lamp is 5 times more efficient than the incandescent bulb.

Example: Microwave Efficiency

• Given:

  • Efficiency: 70% (0.7 as decimal)
  • Total Power Input: 800 W

• Calculate Useful Power Output:

  Useful Power Output = Efficiency × Total Power Input = 0.7 × 800 W = 560 W

Common Pitfalls

• Incorrect Division: Dividing the wrong way can result in efficiency greater than 1 or 100% which is unreasonable.
• Energy Conservation: Energy can neither be created nor destroyed, only transferred.

Special Cases

• Devices like electric heaters where thermal energy is the intended useful output can approach 100% efficiency.