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# Problems in estimating power, energyand efficiency

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1. A typical photovoltaic cell delivers 4.2×10&#8722;3 W of electric energy when illuminated with 0.12 W of light energy. What is the efficiency of the cell?

2. An "energy bar" contains 27 g of carbohydrates. If the energy bar was his only fuel, how far could a 68 kg person walk at 5 km/hr?

3. The resistance of an exercise bike is often provided by a generator; that is, the energy that you expend is used to generate electric energy, which is then dissipated. Rather than dissipate the energy, it could be used for practical purposes.

A. A typical person can maintain a steady energy expenditure of 400 W on a bicycle. Assuming a typical efficiency for the body and a generator that is 90% efficient, what useful electric power could you produce with a bicycle-powered generator?
B. How many people would need to ride bicycle generators simultaneously to power a 180 W TV in the gym?

4. A 72 kg student consumes 2500 Cal each day and stays the same weight. One day, he eats 3500 Cal and, wanting to keep from gaining weight, decides to "work off" the excess by jumping up and down. With each jump, he accelerates to a speed of 3.2 m/s before leaving the ground.

How many jumps must he make? Assume that the efficiency of the body in using energy is 25%

https://brainmass.com/physics/power/problems-estimating-power-energy-efficiency-395792

#### Solution Preview

See attached file.

1. A typical photovoltaic cell delivers 4.2×10−3 W of electric energy when illuminated with 0.12 W of light energy. What is the efficiency of the cell?

Power output

Power input

Power efficiency of the cell is

Usually power efficiency is expressed as a % so

% power efficiency of the cell

2. An "energy bar" contains 27 g of carbohydrates. If the energy bar was his only fuel, how far could a 68 kg person walk at 5 km/hr?

Carbohydrates produce on average 17 kJ/g of energy [2]

So the energy bar contains and equivalent energy of given by

Convert velocity in km/hr to m/s

Kinetic energy is (m is his mass and v his velocity) however as units of velocity are in m/s then this is ...

#### Solution Summary

A number of power efficiency and energy word problems are solved and estimates obtained for power efficiency and energy consumption

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