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 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

Power Units

• Watt
• Horsepower