QFocus

QFocus pt 2

QFocus pt 3•You can design, build, and

refine a device that works to convert one form of energy into another form of energy

• Label questions as open or close ended questions

• Change three from open ended to close ended or vise versa.

•Pick 3 questions that you want to be answered by the time you are able to build and explain this device.

•Choose 2 questions to share with the class.•Why did you choose

these 2 questions?

Energy and Work

Energy

• in the next 3 minutes, think and discuss as many examples of energy and types of energy you can think of.

Types of energy

• Electrical Energy

• Chemical Energy

• Thermal Energy

• Mechanical Energy

• There are many different types of energy in the world.

• Each of these can be transferred and transformed into other types of energy for different uses.

Ex. Nuclear Power Plant

• In the next 3 minutes, discuss with your group and write down all the ways money exists in the world, how it is stored and used.

“Money as energy” model

• Cash or debit card = kinetic energy– Energy due to motion. Kind of like cash, is readily

usable.• Bank/savings account = potential energy– energy stored because of where the mass is

located, e.g. gravitational potential energy or magnetic potential energy. Like savings, it is there, but must be transferred in order to be used.

• Getting a paycheck or making payments = Work

WORK• SCIENTIFIC WORK—energy transferred by

applying a force on an object over a certain displacement. Work, like energy, is measured in Joules

W = F Δx

1 joule = 1 Nm = 1 kg m2 / s2

Work (cont.)• Energy can be transferred into an object (+

work)

• or energy can be transferred from an object (- work)

Back to energy

• Since work is the flow of energy, you can define energy as:–Energy is the ability to do work.

Dropping a golf ball• What is doing work on the golf ball as it is

lifted up? How is the work being done?• What is doing the work as the golf ball is

falling down? How is the work being done?• What does the golf ball do to the pin

when it hits it? Use physics vocabulary.• What happens to the amount of energy

the golfball has when you change the height you drop it at? How do you know?

Potential Energy

• It is the amount of energy stored based on the location of the object.

• It is equal to the amount of work that it takes to place the object in that location.

Discuss with your group

• Can we use potential energy to do work? [provide evidence and reasoning]

• [you can look back at your notes for help, or think about the demonstration we just did]

Gravitational Potential Energy

• The energy stored due to the height of an object

• PEg = mgh

m = mass, g = acceleration due to gravityh = height

Defining the height

• When we talk about PE, we use the difference in PE from one position to another.

• For gravitational PE, we need to define a position we will consider to be zero height

Ex.

• A 50 kg person is on a balcony 4 m above the ground. Then the person goes up on the roof of the building, which is 10 m above the ground. What is the person’s gravitational potential energy relative to the balcony?

Elastic Potential Energy

• Based on how much you stretch or compress and elastic object (rubber band, spring, bungee cords, etc.)

• Again, there will be some zero position where the elastic object will be neither stretched or compressed (in equilibrium).

example of a spring

Kinetic Energy

• Kinetic energy is the energy due to the motion of the object.

The equation for Kinetic Energy is:

KE = (1/2) m * v 2

m = mass, v = velocity

Power

• Work does not take into account the time it takes to cause a displacement by applying a force.

• The work can be done quickly or slowly, and you will get the same amount of work done.

Power

• Power is the rate at which the work is done.

• P = work / time

• Units = J/s = Watts = W

Horsepower

•Horsepower is another unit for power

•1 hp = about 750 Watts

kiloWatt hour• Energy companies use the unit kW

hour to define a unit of energy.

• They use this energy to calculate your energy bill

• 1 kW hr = [1 kW] x [1 hr] = [1 kW] x [3600 s] = [1 kJ/s] x [3600 s] = 3600 kJ

Work and Kinetic Energy

• How can you make an object speed up?

• What happens to the energy of an object when it speeds up or slows down?

Work - Kinetic Energy Theorem

• The work you put into, or the work an object does will change the kinetic energy of the object:

• W = ΔKE = KEf – KEi = (1/2)mvf2 –

(1/2)mvi2

Ex 1

• A 500 kg car is moving at 2 m/s. If a tow truck pulls the car and does 1000 J of work, what is the new velocity of the car?

Temperature, Heat, and Internal Energy

Thermal Energy

• Thermal energy is energy possessed by an object or system due to the movement of particles within the object or the system.

Temperature

• Temperature - measure of the average molecular kinetic energy of a substance

• Heat - transfer of energy due to difference of temperature– Objects do not contain heat– Flows from higher to lower temperature objects

Thermal Equilibrium

• Thermal Equilibrium - objects in thermal contact reach the same temperature

Internal Energy

• Total of all energies inside a substance or system– Kinetic– Potential

• A substance does not contain heat, but contains energy.

Discuss, decide, and provide evidence and reasoning

• Compared to a giant iceberg, a hot cup of tea has

• (A) more internal energy and higher temperatures(B) higher temperature but less internal energy(C) a greater specific heat and more internal energy(D) none of the above

• Temperature is a measure of an object's• (A) average molecular kinetic energy

(B) average molecular potential energy (C) total molecular kinetic energy(D) total molecular potential energy

• Heat flows from an object with a high• (A) internal energy to an object

with a lower internal energy(B) temperature to an object at a lower temperature(C) both of the above since the two phrases are equivalent

• If only the potential energy of the molecules in a body is increased, the absolute temperature of the substance• (A) increases

(B) decreases(C) remains the same

•Could the thermal energy of a bowl of hot water equal that of a bowl of cold water?

Different types of systems• There are three different types of systems we

can define:1. Open system: In this system, matter and

energy can flow freely in and out of the system.

2. Closed system: Here, only energy can flow freely in and out, but the matter will stay inside the system.

3. Isolated system: For this, both the matter and energy stay inside the system and cannot flow freely in or out.

What type of system is this? Why?

What type of system is this? Why?

The Law of conservation of energy

Law of conservation of energy

• In an isolated system, the total amount of energy remains the same.

• That means:

Etotal initial = Etotal final

For mechanical energy

• mechanical energy only involves KE and PE, so from the law of conservation of energy:

KEinitial + PEinitial = KEfinal + PEfinal

• If I drop the mass from a pendulum, will my face be safe? Why?