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Energy Efficiency

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Energy Efficiency
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Students will learn how to properly think about efficiency and how to calculate the efficiency of electrical devices.

Key Equations

 P = V \cdot I ; Power in electricity is the voltage multiplied by the current

E = P \cdot \Delta t \; ; the electrical energy used is equal to the power dissipated multiplied by the time the circuit is running

 Eff = \frac{P_{out}}{P_{in}} ; Efficiency is the Power out divided by the Power input

Assuming same time periods:  Eff = \frac{E_{out}}{E_{in}} = \frac{Work}{E_{in}}

Guidance

Conservation of Energy & Electrical Efficiency:

Electrical energy is useful to us mostly because it is easy to transport and can be easily converted to or from other forms of energy. Of course, conversion involves waste, typically as heat.

Electrical energy consumed can be determined by multiplying power by time (E = P \Delta t) . Recall the equations for mechanical and thermal energy/work (PE=mgh, KE=1/2mv^2, Q=mc \Delta T) . An important idea is the efficiency of an electrical device: the fraction of electrical energy consumed that goes into doing useful work (E_{out}/E_{in}) , expressed as a percentage.

Example 1

You use a 100 W electric motor to lift a 10 kg mass 5 m, and it takes 20 s.

The electrical energy consumed is E_{elec} = P \cdot t = (100 \ W) (20 \ s) = 2000 \ J

The work done is against gravity, so we use PE = mgh = (10 \ kg) (10 \ m/s^2) (5 \ m) = 500 \ J

The efficiency is E_{out}/E_{in} = (500 \ J) / (2000 \ J) = 0.25 = 25\% \ \text{efficient}

Watch this Explanation

Time for Practice

  1. The useful work a light bulb does is emitting light (duh). The rest is “wasted” as heat.
    1. If a standard incandescent 60 W bulb is on for 1 minute and generates 76 J of light energy, what is its efficiency? How much heat energy is produced?
    2. If the efficiency of a CFL ( c ompact fl uorescent) bulb is 20%, how many joules of light energy will a 60 W bulb produce?
  2. Most microwave ovens are 1000 W devices. You heat 1 cup (8 fl. oz., or 30 ml) of room temperature water (20^\circ C) for 30 seconds in a microwave.
    1. What current does the oven draw?
    2. If the water heats up to 90^\circ C , what is the heating efficiency of the oven?
  3. The 2010 Toyota Prius (m=1250 \ kg) battery is rated at 201.6 V with a capacity of 6.5 Ah.
    1. What is the total energy stored in this battery on a single charge?
    2. If you used the battery alone to accelerate to 65 mph one time (assuming no friction), what percentage of the battery capacity would you use?

Answers to Selected Problems

  1. a. 2.1%, 3524 J b. 720 J
  2. a. 8.3A b. 29%
  3. a. 4.7 \times 10^6 \ J b. 11.2%

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