Power

lightning

Power is energy flowing! It is measured as energy per unit of time

1 Watt = 1 Joule of energy per second

1 W = 1 J/s

Power Formulas

power drink
No, not that type of Power Formula!

Power is the rate of energy per time:

P = Energy Time

Example: 3000 J of energy is used in 20 seconds, what is the power?

P = 3000 J 20 s = 150 W

We can also change the formula this way:

  P = Energy Time
Energy is also Force × Distance, so:   P = Force × Distance Time
Distance/Time is Velocity, so:   P = Force × Velocity

guys pull

Example: These guys pull with a combined force of 4000 N, and manage a velocity of 0.4 m/s, what power are they using?

P = 4000 N x 0.4 m/s
= 1600 N m/s
= 1600 W

Hang on! Is N m/s (Newton meter per second) really the same as W (Watt)?

In fact all these are the same:

    Power is:
Start with:   N m/s
A Newton is kg m/s2:   kg m/s2 m/s
Simplify:   kg m2/s3
A Joule is kg m2/s2:   J/s
A Watt is J/s:   W

Horsepower

1 horsepower (hp) is 745.7 W (or 746 W for electrical motors)

Example: Sam's car has a 200 hp engine. What is that in watts?

200 × 745.7 = 149,140 W = 149 kW (to nearest kW)

What is a kiloWatt Hour?

1 kWh is the Energy used when the Power is 1 kW for 1 hour.  

bar radiator

This is a little 1 kW radiator. How much energy does it use in an hour?

So in an hour it uses:

1000 J/s × 3600 s = 3,600,000 J = 3.6 MJ

And so 1 kWh (kiloWatt hour)  is 3.6 MJ of Energy