Principles of Energy Principles of Energy Conversions and Conversions and ThermodynamicsThermodynamics

Section 3.0Section 3.0

3 types of Systems3 types of Systems• Open: a system in which

matter and energy are exchanged – Ex. Earth

• Closed: A system in which only energy is exchanged with the surroundings, no matter– Ex. A can of soup

• Isolated: Does not exchange matter or energy with the surroundings

The 1The 1stst Law of Thermodynamics Law of Thermodynamics

• Energy cannot be created nor destroyed; only transformed.

• The total amount of energy, including heat, in a system, is transformed into an equal amount of some other form.

• The amount of energy put into a system must equal the amount of mechanical energy + the amount of heat lost.

Perpetual Motion Machine Perpetual Motion Machine

• NON EXISTANT YET• We’ve come close.

Theoretically, if the energy put into a machine could be converted 100% to mechanical energy and if no energy is converted to other forms, than it should operate indefinitely.

The 2The 2ndnd Law of Thermodynamics Law of Thermodynamics

• Nothing is 100% efficient!– heat will always be formed

• Heat always flows from naturally from hot objects to cold objects.

• NEVER from cold to hot!

The mass of an object is a measure of the amount of matter that makes it up.

The mass of an object has nothing to do with the amount of gravity.

An object will have the same mass on Earth as it does on the moon, or even in a region where there is no gravity.

Mass is a scalar quantity, and is measured in kilograms (kg).

The weight of an object is the gravitational force exerted on it by a large body (usually Earth).

Weight is a force, and is therefore a vector quantity and measured in newtons (N).

Only PUNKS symbolize

weight with a W. So, don’t do it!!

gF mg���������������������������� gF

��������������= weight (N)

m = mass (kg)

g��������������

= acceleration due to gravity (m/s2)

On Earth, g = 9.81 m/s2

The difference between mass and weight . . .

Potential energy is energy stored in an object because of its state or position.

Examples include:

elastic potential energy

chemical potential energy

In this course, we will focus on gravitational potential energy.

pE mgh

pE = gravitational potential energy (J)

m = mass (kg)

g = acceleration due to gravity (m/s2)

h = height above reference (m)

The reference point is usually whatever the object will hit if

it is dropped.

When held above the table top, we see that the apple does not have a lot of gravitational potential energy.

Move the apple horizontally so that the ground is now the reference point and the apple has a lot more gravitational potential energy.We are NOT creating energy when we do this.

Instead, we are just changing the proportion of Ep versus other forms of energy.

The point is: the gravitational potential energy can be set to zero at any point you choose.

Only CHANGES in potential energy can be measured!!

h1

h2

Practice Problems p. 174

1) A child with a mass of 25.0 kg is at the top of a slide in an amusement park. If the vertical height of the slide is 4.00 m, calculate the gravitational potential energy of the child relative to the ground.

981 J

2) An 800-g bird has 47.0 J of gravitational potential energy when it is perched high up in a tree. Calculate the bird’s vertical height from the ground.

5.99 m

3) A hanging sign is 3.00 m above the ground and has 1.47 x 103 J of gravitational potential energy. Calculate the mass of the sign.

49.9 kg

Any moving object that has mass, has kinetic energy.

212KE mv

KE = kinetic energy (J)

m = mass (kg)

v = speed (m/s)

Kinetic energy is energy due to motion.

The kinetic energy of an object varies directly as its mass.

KE m

So, for example, if you double the mass of an object, its kinetic energy will double.

The kinetic energy of on object varies directly as the square of its speed.

2 KE v

This is a “direct squared” relationship!

So, for example of you double the speed of an object, its kinetic energy will increase by a factor of four.

2 2KE

4KE

Practice Problems p. 179

4) Calculate the kinetic energy of an electron with a mass of 9.11 x 10-31 kg moving at a uniform speed of 2.00 x 105 m/s.

1.82 x 10-20 J

5) A small toy moving horizontally at a uniform speed of 2.2 m/s has a kinetic energy of 18 J. Calculate the mass of the toy.

7.4 kg

6) A baseball with a mass of 300 g has a kinetic energy of 304 J. Calculate the speed of the baseball.

45.0 m/s

7) A moving toy with a mass of 7.4 kg has a kinetic energy of 18 J. Calculate the speed of the toy.

2.2 m/s

Questions 32-49 pages 199-212, even questions

Read pages 205-210 and reflect on the different types of potential energy.