Oct. 28 – Energy and Power

Energy and Power

Learning Goals: Understand how to calculate power and energy used for different devices.

Success Criteria: You can successfully solve word problems involving power.

Today I discussed how we measure energy using Joules (J).  One Joule is not a lot of energy.  It takes about 1 J to lift your textbook 10 cm.

Normally we use kilojoules: 1 kJ = 1000 J.

Household devices use even more energy, so we use kWh (kilowatt hours):

• 1 kWh = 3 600 kJ = 3 600 000 J

This label can be found on the freezer above.  It uses 512 kWh every year to operate this freezer.

Homework: Find one of these Energuide labels at home.  Look on appliances such as fridge, stove or washer/dryers.  Bring back the following tomorrow,

Appliance: (fridge, stove, washer/dryer, etc.)
Brand: (Maytag, GE, LG, etc...)
kWh per year: ###

Warning: don't try to move your fridge or stove to look for the label!  If you can't find it, look on another appliance.  Stay safe!

Power

Power refers to how fast energy is used.  It is measured in units of Joules per second or Watts.

• 1 J/s = 1 W

To calculate power, we use the equation:

Where

• P = power (Watts)
• E = energy (Joules)
• t = time (seconds)

Example: Usain Bolt uses 15 kJ of energy to run 100 m in 9.58 s.  How much power does he use?

Vide: Usain Bolt breaking the world record.

Given: E = 15 kJ, t = 9.58 s, distance = 100 m

Required: How much power, P?

Analyze: P = E / t

Solve: P = 15 kJ / 9.58 s

           P = 1.57 kJ/s

           P = 1.57 kW

Statement: Usain Bolt uses 1.57 kW of power to run 100 m.

Example: a mantis shrimp punches with 15 J of energy in 2.7 ms.  How much power does the mantis shrimp use to punch?

Here's more information on the Mantis Shrimp.

And some Vidoes!  

• Mantis Shrimp Destorys Clam
• World's Deadliest

G: E = 15 J, t = 2.7 ms = 0.0027 s

R: How much power, P?

A: P = E/t

S: P = 15 J / 0.0027 s

    P = 5555.5555555 W

    P = 5600 W

    P = 5.6 kW

S: The shrimp uses 5.6 kW to punch.  Five times more powerful than Usain Bolt!

Example: How much energy does a  60 W light bulb use in 1 hour?

G: P = 60 W, t = 1 hour = 3600 s

R: How much energy?

A: E = P t

S: E = 60 W x 3600 s

     E = 216000 J

     E = 216 kJ

S: The light bulb uses 216 kJ in one hour.

Handout with more practice: Power and Efficiency Problems

Homework:

• Find the Energuide labels
• Work on your presentation.