What is energy?
- Energy is defined as the capacity to do work
- Energy can be thought of as an accounting device
- Energy is never destroyed or lost, only converted
- word coined by Aristotle meaning work within
- Energy = power * time
- Distance is a useful analogy
What is power?
- Power is how quickly we are able to consume or convert energy
- Measure in energy per unit time
- The rate at which energy is delivered.
- Speed is a useful analogy
A Note on Power Units
- To ask for power per unit time (like watts per second) is almost always due to a conceptual error.
- This error is likely because “per second” is part of the watt’s definition
- Knots for speed is a good analogy since you wouldn’t say knots per second
Power and Energy
Power is the rate of conversion of energy.
This means that energy is the product of a power and a time.
E = P \cdot t
Usually, we make the estimation that the power is constant over the time period.
When energy is in joules and time is in seconds, power is in watts.
1 \textrm{watt} = 1 \frac{\textrm{Joule}}{\textrm{sec}} 1 \textrm{joule} = 1\ \textrm{watt} \cdot 1\ \textrm{sec}
When power is in kilowatts and time is in hours, the energy is in kilowatt-hours.
1\ \textrm{kilowatt-hour} = 1\ \textrm{kilowatt} \cdot 1\ \textrm{hour}
Energy to Power
If I convert 1000 joules of energy in 10 seconds, how many watts of power is this?
Power to Energy
If we have a 50 watt laptop running for a short 100 second video we find the energy used by
E = P \cdot t
E = 50\ \textrm{watt} \cdot 100\ \textrm{sec} = 5000\ \textrm{joules}
Energy Units
- Joule
- SI Unit. One Newton-Meter.
- Kilowatt-Hour
- Energy consumed by 1 kW load over one hour
- Calorie
- Energy to heat one gram of water one degree Celsius
- Kilo-calorie
- One thousand calories. Used in food energy content.
- British Thermal Unit (BTU)
- Energy to heat one pound of water by one degree Fahrenheit
- Therm
- 10^5 BTU
- found on energy bills for natural gas
Power Units
- Watt
- One joule per second
- Horsepower
- 746 watts