Ch 14, 15, 16

Work, Power, Energy, and

Heat

• Work – transfer of energy

• through motion• a. Force must be

exerted through a distance

Ch 14 WORK AND POWER

• b. Amount of Work depends on:

–1. Amount of applied Force

–2. Distance over which the force is applied

Work

F d

• c. W=Fd

(Force multiplied by distance)

• d. Unit is the Joule (1J = 1Nm)

Work

F d

• e. In order for work to be done:

–1. Something has to move

–2. Motion must be in the direction of the force

• How much work is done when a car is pushed with 600 N of force for a distance of 50m?

• How far will a ball roll if 17J of work was done to push it with 2 N of force?

Power – rate of doing work

a. To increase Power:

–1. increase work done

– 2. do work in less time

b. P=W/t • (Work divided by time)

c. SI unit is Watt

(1W=1J/s)

d. common unit is horsepower (1hp=746W)

Work

P t

• How much power is needed to do 40 Joules of work in 5 seconds?

• How long will it take to do generate 70 Watts of power if I do 4900 Joules of work?

• How much power will I need if I want to do the same amount of work in half the time?

Reference table check

Work

F d

Work

P t

Fd

P t

• You exert a vertical force of 72 N to lift a box a height of 1 meter in a time of 2 seconds. How much power is used to lift the box?

Star questions

• True or False

–Decreasing the amount of time it takes to do work, increases the power

–Motion must occur in order for work to be done

Ch 15-Energy • Energy is the

ability of an object to cause change

• Basic unit is the Joule (J) =Nm

Forms of energy

1. Mechanical = PE+KE

2. Thermal = heat

3. Chemical = wood, gasoline

4. Electrical = lightning, batteries

5. Electromagnetic = visible light, x-rays

6. Nuclear = fission, fusion

Kinetic Energy – energy in the form of motion

•a. Greater mass, more KE

•b. Greater velocity, more KE

2

2mvKE

• A 20-kg kid slides down a slide at a velocity of 2 m/s. What is the kinetic energy of the kid?

Potential Energy – stored energy

a. Amount depends on:

1. Position

2. Shape

Types of PE:Types of PE:

1. 1. Gravitational Gravitational – – related to height related to height above Earthabove Earth’’s s surfacesurface

a. Higher up, a. Higher up, more PEmore PE

b. Greater mass, b. Greater mass, more PEmore PE

2. 2. ElasticElastic – – object that is object that is stretched or stretched or compressedcompressed

3. Chemical – 3. Chemical – found in food or found in food or fuelsfuels

• PE=mgh=Fgh

• SI unit is Joule=J=Nm

PE

mg h

• A potted plant on a 2.5 m high window sill has a mass of 3-kg. What is the potential energy of the plant?

• A man weighing 950N stands at the top of 30m cliff. What is the man’s potential energy?

3 people of equal mass climb the mt using different paths. Which path shows the person gaining the most PE?

Roller coaster

max height

PE

speed

ME

•Energy Conversion – process of changing energy from one form to another

Law of Conservation of Energy states that matter cannot be created nor destroyed

STAR Questions

• List the 6 forms of energy– Mechanical, chemical, electromagnetic, thermal,

electrical, nuclear

• Work depends on what 2 things?– Distance and force

• KE, chemical PE, elastic PE or gravitational PE?– Bouncing ball– Rock at edge of cliff– Glass of Orange Juice– Car battery– Compressed spring

KE

Gravitational PE

Chemical PE

Chemical PE

Elastic PE

30oC 90oC

Heat =energy that flows from higher temps to lower temps

Ch 16-HEAT

What will happen to the temperature in the beakers over time?

Temperature –measure of average KE of the particle in a sample of matter

–a. Increase temp = greater KE

–b. Decrease temp = lower KE

–c. Absolute Zero=0K= KE of all particles cease

Thermal Energy – the total energy of the particles in a material (both KE and PE)

a. More mass at the same temp means greater thermal energy

KE of 3?

TE of 3?

200g 200g 400g

50oC 70oC 70oC

Thermal Expansion & Contraction

a. Hot temps cause materials to expand

b. Cold temps cause materials to contract

Specific Heat (C) – amount of energy needed to raise the temp of 1g of a material by 1oC

• Think of it as “resistance to temperature change”

• The higher the c, the harder it is the change the temp.

• An iron spoon and a silver spoon have the same mass. Which becomes hotter when both are left in hot chocolate for one minute? Pg 476 for c values

• Why does a piece of steel heat up more than an equal mass of wood when both absorb the same energy?

The metal has a lower specific heat than the wood, so its temp increases more as thermal energy is absorbed

Which substance will have the greatest increase in temp when equal masses absorb equal amounts of thermal energy?

• Water (4.18 J/goC)

• Ethyl alcohol (2.43 J/goC )

• Ammonia gas (2.1 J/goC )

• Aluminum (0.90 J/goC )

• Lead (0.46 J/goC )

Answer : A

The same energy was added to each sample. Which material has the highest specific heat?Al, b/c Temp rises the least for a given amt energy added. Smaller ΔT means a larger specific heat

It is impossible to build a It is impossible to build a machine that does nothing machine that does nothing but convert thermal energy but convert thermal energy into useful work. into useful work.

no machine can be 100% efficient, why not?

Energy Efficiency

Thanks Cambridge Physics Outlet for amazing graphics!

Conduction – transfer of energy through matter by direct contact of particles

a. Energy is transferred when particles moving at different speeds collide

b. Occurs in solids, liquids, and gases–1. Solids conduct

better

Convection – transfer of energy by the movement of matter

a. Fluid – anything that flows

b. Occurs in liquids & gases

Radiation –transfer of energy in form of waves

a. Shiny materials reflect radiant energy

b. Dull materials absorb

c. Darker colors absorb more

d. Lighter colors absorb less

STAR Questions

• Decrease temperatures = ___________ KE• Decreased• Hot temperatures cause materials to ______• Expand• The higher the C-specific heat, the _________ it is to

change the temp• Harder• Dull materials ___________ radiant energy• Absorb• Lighter colors reflect ___________ radiant energy• less