Work, Energy and POWER!!!

Section 11.1BC Science Probe 9

What is Work?

• In science, work is defined as the transforming or converting of energy.

• So every time that you flip a switch, or plug something in, work is done! (Do NOT use this as an excuse to say that you have done your WORK at home!!!)

What is Work?

• Because work is a change in energy, it gets the same units as energy: Joules (J)

• What are some examples of work being done?

What is Work?

• Because work is a change in energy, it gets the same units as energy: Joules (J)

• What are some examples of work being done?– A battery in a flashlight producing an electric

current to produce light energy from electrical energy

– A motor where electrical energy is converted into motion

What is Energy?

• In general, we can say that an object has energy (E) if it has the ability to do work.

• There are lots of kinds of energy. Here are some examples…

Type of Energy Description Examples

ElectricalEnergy from the movement of electrons

- Electricity- Lightning

Gravitational potentialEnergy stored in an object because of its height or position

- Waterfall- Wrecking ball

Chemical potentialEnergy stored in chemical bonds

- Natural gas- Petroleum

Elastic potentialEnergy stored in objects that are stretched or compressed

- Springs- Rubber bands

Nuclear

Energy stored in the nucleus of an atom

- Uranium atoms (fission)

- Hydrogen atoms (fusion)

KineticEnergy due to motion - Moving hockey puck

- Falling raindrop

What is Energy?

• We can classify all of these kinds of energy into two major types:

– Potential energy (PE): energy that can be stored in an object

– Kinetic energy (KE): energy that an object has because of its motion

What is Energy?• The different kinds of energy can be converted

into each other.• For example:– If you throw a ball in the air, you give it kinetic energy.– This is converted to gravitational potential energy as

the ball rises (because it has the ability to fall)– At its highest point, all of the ball’s kinetic energy has

been converted to gravitational potential energy– As it falls, the GPE gets converted back to kinetic

energy!– WORK has been done!

What is Energy?

• Kinetic energy (KE) depends on the mass and speed of an object.

• The greater the mass or speed, the greater the KE of the object.

• When an object’s speed changes, work is done that equals that object’s change in KE.

What is Energy?

• Here’s an equation to show it:• W = work• KEfinal = final kinetic energy

• KEinitial = initial kinetic energy

W = KEfinal – KEinitial

What is Energy?

• We can even shorten the equation: W = ΔKE

– Remember that Δ is delta and it shows a change

• So work equals the change in kinetic energy!

What is Energy?

• Since our original definition of work was something like a change into another type of energy, we can shorten the equation one more time

W = ΔE

What is Energy?

• All of the types of energy that we have talked about can be converted from one type of energy into another.

• When this happens, there is no overall loss in energy because as one type decreases, another will increase by the same amount.

What is Energy?

• This gives us the Law of Conservation of Energy:– In any closed system, the total amount of energy

(including potential energy) remains constant

• This means that when you add the amounts of all the different kinds, the total amount of energy will never change.

What is POWER?

• Power (P) is the rate at which energy is transformed, or the rate at which work is done.

– If you walk or run up a hill, you do the same amount of work, but to run up the hill in a short amount of time, you need more power than when you walk up slowly.

What is POWER?

• Power is measured in watts (W)

• One watt is equal to one joule per second (1W = 1J/s)

What is POWER?

• This is how we can calculate power: P = ΔE/Δt

• P = power• ΔE = the amount of energy transformed (in

joules)• Δt = the time interval (in seconds)

What is POWER?

• Since we know that W = ΔE, we can substitute W for ΔE

P = W/ Δt

Assignment

• CYU page 339• # 1, 2, 3, 4, 7, 8, 9