1. Energy & Transformations
Power & Efficiency

2. Power Power: The rate of doing work or transferring energy.
Measured in Watts (W)
1 W = 1 J / s (joule per second)
It is given by:
Watts are commonly used to measure the power used by electrical appliances.
Where:
P=power (W)
W = work (J)
∆E = Energy transferred (J)
∆t= time interval (s)

3. Example 1
A runner transforms 3.5 x 105 J of energy running in the New York City Marathon. If it takes 3.25h to run, what is the runner’s power? Express in Watts & Kilowatts

4. Example #2
A 45kg student climbs a flight of stairs 4m high in 25 seconds. Determine
The potential energy of the student at the top of the stairs.
The student’s power.

5. Efficiency
Efficiency: The ratio of the useful energy output of a device to the required input energy.
Typically expressed as a percentage (%).
It is given by:

6. Example #3
An incandescent light bulb requires 1500 J of electrical energy to produce 150 J of light energy.
Write the energy transformation equation.
Calculate the efficiency of the light bulb.
Draw the energy flow diagram.

7. Example #4
A 125 W ceiling fan produces 4500 J of wind energy in 48 seconds. Determine:
The power output of the ceiling fan
The efficiency of the ceiling fan
The energy flow diagram