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

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
  • Quad
    • One quadrillion (10^{15}) BTU

Common Energy Conversions

  • 1 BTU = 1055 J
  • 1 kWh = 3.6 MJ

Power Units

  • Watt
  • Horsepower