Ch. 15 - EnergyCh. 15 - Energy

I. Energy and Work (p.124-131) Energy and Work Energy Conservation of Energy

Energy and Work Energy and Work

What is energy? Whenever work is

done, energy is transformed or transferred from one system to another

When is work done?

Energy is the ability to do work.

REMEMBER: Work is done only when an object moves.

BUT energy can be present in an object or a system when nothing is happening.

HOWEVER it can only be observed when it is transferred from one object or system to another.

How are energy and work related?

Energy and Work Energy and Work

Because the amount of energy transferred is measured by how much work is done – energy and work are expressed in the same unit.

Joules (J) is the unit of Energy

SI Unit of Energy

Energy and WorkEnergy and Work

Work transfer of energy through motion force exerted through a distance

W = Fd

Distance must be in same direction of force!

W: work (J)F: force (N)d: distance (m)

1 J = 1 N·m

Brett’s backpack weighs 30 N. How much work is done on the backpack when he lifts it 1.5 m from the floor to his back?

GIVEN:

F = 30 N

d = 1.5 m

W = ?

WORK:

W = F·d

W = (30 N)(1.5 m)

W = 45 J

FWd

Energy and Work Energy and Work

A dancer lifts a 40 kg ballerina 1.4 m in the air and walks forward 2.2 m. How much work is done on the ballerina during and after the lift?

GIVEN:

m = 40 kg

d = 1.4 m - during

d = 2.2 m - after

W = ?

WORK:

W = F·d F = m·a

F =(40kg)(9.8m/s2)=392 N

W = (392 N)(1.4 m)

W = 549 J during lift

No work after lift. “d” is not in the direction of the force. F

Wd

Energy and Work Energy and Work

Potential EnergyPotential Energy

Potential Energy is energy that is stored.

You can’t see it but you know it’s there.

AKA – Energy of Position

Types of Potential Energy Types of Potential Energy Type of Energy ExampleGravitational Potential Energy (GPE)Energy stored due to position (objects that are above Earth’s surface such as apples in an apple tree).

Chemical EnergyEnergy stored in chemical bonds such as food or fuel.

Energy stored in chemical bonds such as food or fuel.

Elastic Energyenergy stored by something that can stretch or compress such as a rubber band or spring.

Gravitational Potential EnergyGravitational Potential Energy

Depends on mass

and height.GPE = m g h

Or

GPE = mass x free-fall acceleration x height (mg = weight in Newtons)

Mass (m) = kg; Gravity (g) = 9.8 m/s2; Height (h) = m

AKA - GPE

Potential Energy (PE) stored energy cannot be seen depends on position or configuration of an object

• Which boulder has greater gravitational PE?

• What other ways can an object store energy?

Gravitational Potential Energy Gravitational Potential Energy

ExampleExampleA 65 kg rock climber ascends a cliff.

What is the climber’s gravitational potential energy at a point 35 m above the base of the cliff?

Given:

m = 65kg

h = 35 m

g = 9.8m/s

GPE = ?

2

GPE = mgh

GPE = 65 x 9.8 x 35

GPE = 22,295 J

Kinetic EnergyKinetic Energy is the energy of motion.

Kinetic EnergyKinetic Energy

Kinetic Energy (KE) energy in the form of motion depends on mass and velocity

80 km/h

50 km/h

80 km/h

80 km/h truck

50 km/h motorcycle

• Which has the most KE?

• Which has the least KE?

Kinetic Energy Kinetic Energy

KE = ½ mass x velocity

OR

KE = ½ m v

mass (m) = kg

velocity (v) = m/s

2

2

AKA = KE

Note:

Kinetic energy depends more

on speed than on mass.

ExampleExampleWhat is the kinetic energy of a

44kg cheetah running at 31 m/s?

Given:

m = 44 kg

v = 31 m/s

KE = ?

KE = ½ (44) x (31)2

KE = 22 x 961

KE = 21142 J

KE = ½ m v 2

Forms of EnergyForms of Energy Forms of Energy:

Each of these forms of energy can be converted into other forms of energy.

Kinetic Potential Energy in FieldsMechanical Thermal Chemical

Electrical Electromagnetic Nuclear

ENERGY

motion of electric charges

bonding of atoms

motion of objects

internal motion of particles

changes in the nucleus

The ability to cause change.

MECHANICAL

ELECTRICALCHEMICAL

NUCLEAR

THERMAL

joules (J)

Energy Energy

Energy Energy

Electrical energy: results from the flow of charged particles or electrons. Electric charges can exert forces that do work.

Chemical Energyis the energy stored in chemical bonds – when the bonds are broken, the released energy can do work.

Energy Energy

Mechanical Energy the energy associated with the motion or position of an object. The sum of potential and kinetic energy in a system (Usually involves movement of an object)

Nuclear Energy: energy stored in atomic nuclei – nuclear fission releases energy by splitting nuclei apart, nuclear fusion releases energy by combining 2 nuclei into a larger nuclei.

Thermal Energy: energy given off as heat (friction). The total potential and kinetic energy of all the microscopic particles in

an object.

Energy Energy

Electromagnetic Energy: a form of energy that travels through space in the form of waves (visible light and X-rays are examples).

Energy ConversionsEnergy Conversions

The process of changing energy from one form to another.

Law of Conservation of EnergyLaw of Conservation of Energy

Energy can not be created nor destroyed, it can only be changed.

Energy can be transferred to another object/system or to another form.

Conservation of EnergyConservation of Energy

Law of Conservation of Energy Energy may change forms, but it cannot

be created or destroyed under ordinary conditions.

Example: PE KE mechanical thermal chemical thermal

E. Conservation of EnergyE. Conservation of Energy

PE KE

View pendulum animation. View roller coaster animation.

E. Conservation of EnergyE. Conservation of Energy

Mechanical Thermal

View rolling ball animations. View skier animation.

Ch. 5 - EnergyCh. 5 - Energy

II. Thermal Energy Temperature Thermal Energy Heat Transfer

A. TemperatureA. Temperature

Temperature measure of the

average KE of the particles in a sample of matter.

B. Thermal EnergyB. Thermal Energy

Thermal Energy the total energy of the particles in

a material. KE - movement of particles PE - forces within or between

particles due to position. depends on temperature, mass,

and type of substance.

B. Thermal EnergyB. Thermal Energy

Which beaker of water has more thermal energy? B - same temperature, more mass

200 mL

80ºC

A400 mL

80ºC

B

C. Heat TransferC. Heat Transfer

Heat thermal energy that flows from

a warmer material to a cooler material.

Like work, heat is... measured in joules (J) a transfer of energy

C. Heat TransferC. Heat Transfer

What are 3 types of heat transfer?

Conduction Convection Radiation

D. ConductionD. Conduction

Transfer of heat as a result of direct contact

E. ConvectionE. Convection

F. RadiationF. Radiation

Summary Summary

QuizQuiz

C. Heat TransferC. Heat TransferWhy does A feel hot and B feel cold?

80ºC

A

10ºC

B

Heat flows from A to your hand = hot. Heat flows from your hand to B = cold.

C. Heat TransferC. Heat Transfer

Specific Heat (Cp)

amount of energy required to raise the temp. of 1 kg of material by 1 degree Kelvin

units: J/(kg·K)or J/(kg·°C)