Power, Work, & Energy

2019 Period 2

Power, Work, & Energy

Power

Power is the rate of changing energy, at which electrical energy is converted into

another form

P=ΔE/Δt

Work

Work is the product of the net force and the distance an object moves in the

direction of the force

W=ΔE---work is change in energy

W=F*d

Work Questions

●

Is lifting weight considered work?

●

Is ice skating or the act of sliding across ice work?

Energy

Energy is the capability to affect a change in a defined system or the capacity

for doing work, it is measured in Joules

There are two types of energy: potential and kinetic

Potential energy: stored energy in a position or state

Equation PE= m*g*h

Kinetic Energy: energy an object has in motion

Equation KE= ½ *m*v

Question

Performing work against a moving object will result in a change of what kind of

energy?

a.

Gravitational Potential Energy

b.

Kinetic Energy

c.

Stored Spring Potential Energy

d.

Chemical Energy

Relationships

●

Energy is the capability to do work, however, work causes changes in energy

●

Performing work against a force will result in a change in energy

Conservation of Energy

Conservation

The Law of Conservation of Energy states that the total energy of a system remains constant.

Conserving energy means that if we know the total energy at one time, we already know the total energy at any other time.

In other words, total energy is constant no matter what. It can transform into something else such as heat but it will always stay constant.

Kinetic Energy

Kinetic Energy is the energy of motion

The equation for calculating kinetic energy (KE) is:

KE = (½)(m)(v)

KE: Kinetic Energy (measured in Joules [J])

m: Mass (measured in kilograms [kg])

Kinetic Energy Calculation Example

A car weighing 800 kg is driving at a speed of 35 m/s.

Determine the kinetic energy.

KE:(½)(m)(v)^2

KE:(½)(800)(35)^2

Potential Energy

Potential Energy is the energy stored

The equation for calculating potential energy (PE) is :

PE = mgh

M - mass, in kg

G - acceleration due to gravity

Potential Energy Calculation Example

An object weighing 50 kg is stationary on top of a 70 meter

hill. Determine the potential energy present.

PE = mgh

PE = (50)(10)(70)

Practice Problem

Practice Problem Cont.

Using the Conservation Law we can determine the

energies of each point by comparing it to Point A.

Point A shows that the total energy adds up to 100

Using this, we just subtract the known energy from each of the points from 100.

Point B: Potential Energy=50

Point C: Potential Energy=0

Impulse and Momentum

What is momentum?

The definition of momentum is the measure of inertia in motion. It has units of

kg m/sec.

Momentum = mass X velocity

p = m X v

Units:

●

Momentum- kg m/sec

●

Mass- kg

Practice Problem

What is the momentum of an 4 kg apple traveling at 3 meters per second?

p = m X v

p = (4kg)(3m/s)

p = 12 kgm/s

Think About It

What has more momentum: a 500 kg truck moving at 0 m/s, or a 5 kg skateboard

moving at 2 m/s?

p

= (500kg)(0m/s)

p

= 0 kgm/sec

p

= (5kg)(2m/s)

p

= 10 kgm/sec

The skateboard has more momentum

What is Impulse?

Impulse is a change in momentum. ( p = impulse) It has units of Nsec.

Impulse = Force x time

J = F x t

Units:

●

Impulse - N*sec

●

Force - N

●

Time - Seconds

Practice Problem

A 500kg truck is moving at 20 m/s. When you hit the breaks the truck will come to

a complete stop is 2 seconds. What is the the impulse and force from the breaks ?

Impulse = P

Impulse =Pfinal - pintial

Impulse = 0 - 20*500

Impulse = -10,000N*s

Collisions

What is a collision and what are the different types?

Collision - Crash or a bump // The meeting

of two objects in which each exerts a force

upon the other, causing the exchange of

energy or momentum

Elastic collision - Collisions where the

objects bounce and do not change shape

(ex. Bouncy balls )

CHART

1) Sketch

a) Choose a positive direction

2) MVP chart (fill in known

information)

3) Calculate momentum

(mass x velocity)

4) Set both sides equal and

solve

a) Sides set equal because momentum is conserved!

A stationary 500 kg cart is hit at 50 m/s by a 400 kg cart. After the

collision the 50 kg cart is moving at 10 m/s. What is the velocity of the

400 kg cart after the collision?

20,000=400v+5,000

-5,000 from both sides

15,000 = 400v

400 kg cart

(Before) 500 kg cart(Before) 400 kg cart (After) 500 kg cart (After)

Mass 400 kg 500 kg 400 kg 500 kg

Velocity 50 m/s 0 m/s v 10 m/s

Momentum 20,000 0 400v 5,000

15,000 = 400v

A 280 kg yellow cart is heading towards a 400 kg blue cart at 10 m/s to

the right. The blue cart is going towards the yellow cart at 20m/s to the

left . In a perfectly inelastic collision how fast are the carts going after

the collision and in what direction?

-2,800 + 8000 = 680 v

5,200 = 680 v

Divide both sides by 680

v = 7.65 m/s to the left

Yellow cart Blue cart Bellow cart

Mass 280 400 680

Velocity -10 20 v

Momentium -2,800 8,000 680 v

Two toy cars are moving towards each other. The blue car is 2 kg with a

velocity of 8 m/s to the left. The red car is 5 kg moving at 3 m/s to the

right and the collide elastically. Find two equations that can be used to

solve the end velocities.

Blue (before) Red (before) Blue (after) Red (after)

Mass 2 5 1 5

Velocity -8 3 V_b V_r

Momentum -16 15 V_b 5 V_r

-1 = V_b + 5V_r V_b = 5V_r + 1

KE = ½ (2) (-8)2 _{+ ½ (5) (3)}2 _{= ½ (2) (V}

b)2 + ½ (5) (Vr)2

½ (2) (64) + ½ (5) (9) = V_b2 _{+ 2.5 V} r2

64 + 48/2 = V 2 _{+ 2.5 V} 2

64 + 22.5 = V_b2 _{+ 2.5 V} r2

CHARGES

Charges

-

Two types of charges:

-

PROTONS (positive), +e,

1.6X10^-19 C

-

ELECTRONS (negative), -e,

-1.6x10^-19 C

-

Flow of electrons=electricity

-

Measured in Coulombs (C)

Charges Cont.

-

Charges exert electromagnetic

forces on other charges,

described by Coulomb’s Law.

-

Electron mass<Proton mass

Coulomb’s Law

32

SYMBOL

MEANING

UNIT

ABBR.

F

FORCE

NEWTON

N

r

DISTANCE

METER

m

Q

CHARGE

COULOMB

C

k

COULOMB’S

Coulomb’s Law Cont.

-

It describes force between 2 charges.

-

Charges (Q’s) are different signs, F= (-), force will be

attractive.

-

Charges (Q’s) are same signs, F= (+), force will be

repulsive.

Coulomb’s Law Practice

1)

A piece of Styrofoam has a charge of -0.004 C and is placed 3.0 m from a piece of

salt with a charge of -0.003 C. How much electrostatic force is produced?

34

a)

F=[(9x10^9 Nm^2/C^2)(-0.004C)(-0.003 C)]/(3^2)

F=108000/9

The Act of Charging/Charge Distribution

Neutral Charges

- Neutral is the state of having no excess charge

- Neutral objects still have charges inside them but the positive equals the negative

Transfer of Charge

- Transfer of charge is usually the

movement of electrons

- Since electrons are neither created nor destroyed that

implies Conservation of Charge

Types of Charging

Charging By Contact

One charged object comes in contact with a neutral object to make

two balanced objects.

Types of Charging

37

Charging By Friction

Two neutral objects rub together, one of

the objects takes electrons from the

Types of Charging

Charge Polarization

A charged object is brought close to

the surface of a neutral object

causing the charges in the neutral

object to separate.

Types of Charging

Charging By Induction

A charged object is brought close to

two conductors then once you remove

the conductors the initial charge of

the object is removed.

Review Questions

1. Two balloons are electrically charged with a positive charge.

-

If you double one of the charges, how does the force of the balloon change?

40

The one that hasn’t been doubled feels a greater

attraction toward the one with the larger

Review Questions

41

- If you double the separation between the two charges, how does the

force change?

The force gets smaller by 1/4.

- If you change all the positive charges to negative charges, how does the

force change?

Review Questions

2. A neutrally charged bat hits a neutrally charged ball, this results in a

positively charged bat and a negatively charged ball, what kind of charging

is this?

42

Some More Review Questions

3. What will the direction and magnitude of the force be on the

positive charge?

43

-5

0 1 3 5 7 9 7

F= 9*10

_{*(5)(7)/(4)^2}

= 1.97 * 10

_{N to the left}

Electric Fields and

Equipotential Lines

Uno Queso Lechuga Por Favor

Starting off with the basics of electric

field

lines

there are 2 different types of charges. There are

positive and negative

charges

.

Electric

field

lin

es

start from a positive charge and they end at a

negative charge.

Electric

field

lin

es that start

from the same type of charge will repel each

other. (Positive and positive or negative and

Literally the one normal slide….like that one family

member...trust me, we all got one

Strengths/Weaknesses:

1. The amount of charges can affect the

strength of an electric field

WW2 was just Actors Doing a Better Scene of WW1

Equipotential Lines:

*Megalovania starts to play*

Conductors: Are a material or device that

conducts or transmits heat, electricity, or

sound.

Voltage & Electric Energy

What is Voltage?

●

V, or voltage, is related to a charge’s position in an electric field.

●

Voltage is usually called the electric potential but it is

NOT

the

electric potential energy. Voltage is used to calculate the energy

Visualizing the Voltage

●

Highest voltage at positive charges

●

Lowest voltage at negative charges

●

Positive charges “fall down” the voltage

- Positive charges will move from a high V to low V

●

Negative charges “fall up” the voltage

+

-+

-+

-+

-Practice Problems

A cube has a voltage of 50 V and a triangular pyramid has a voltage of 30 V. There’s

a wire connects them.

If you place a Positive charge between two objects, how would the Positive charge travels?

If you place a Negative charge between two objects, how would the Negative charge

travels?

A. From cube to triangular pyramid

A. From triangular pyramid to cube

Electric Potential Energy

●

Electric Potential Energy Equation:

PE

_elec

= QV

- PE

is the electric potential energy ( Joules )

- Q are the charges ( coulombs )

Practice Problems

A charged baseball has a charge of 7 C and a voltage of 10 volts.

A. Determine the electric potential.

PE

= QV

= ( 7 )( 10 )

= 70 Joules

b. 70 Joules

a. 10 volts

Practice Problems

A charged particle has a charge of -4x10^-2 C. It was initially located in

a position that feels 25 volts. Then it moved to the final position that

feels 20 volts. Calculate the electric potential energy experienced by

the particle.

PE

= QV

Equipotential Lines

●

Describe regions in space at a particular voltage.

1 V

2 V

3 V

- For negative particles, its voltage would came from small V to large V along the equipotential lines. - For positive particles, its voltage would came from

Practice Problems

A ball with a charge of 3 C is located at

position “B”.

Calculate the charge in energy to move the

ball to position “A”.

Calculate the charge in energy to move the

ball to position “C”.

A: 0 J because same voltage

A: -6J

Ohm’s Law

Ohm’s Law

● The equation Current = Voltage/Resistance is called Ohm’s Law.

● Units of measurements Ampere = Volt/Ohm

● As resistance increases, current decreases (Indirectly proportional)

Formula

Current=Voltage/Resistance

I=V/R

Current

● The rate of flow of electric charge through an area ● Symbol: I

● Unit of Current: Amperes, Amps (A)

● Amperes are the movement of current in a circuit.

● Current is defined as the flow of positive charges

Voltage

● Voltage is related to a charge’s position in an electric field

● Voltage is electric potential or the potential to do work

● Voltage is used to calculate the energy

● When voltage increases, more energy is available to push electrons through obstacles. The more energy available to push electrons, the more electrons pushed through the obstacles.

● Symbol: V

Resistance

● Resistance is a measure of how much a device obstructs current

● Resistance = Obstacle (Ex. Stepping on a hose when water is flowing through it)

● Symbol:Ω

● Unit of resistance: ohms

Basics of Circuits

Ohm’s law can be used to solve simple circuit problems

to find the current, voltage or the resistance of the

circuit.

Basic Circuit Problem #1

Find the current of this circuit problem.

Step 1: Use Ohm’s Law

Step 2: Plug in the values for each variable in Ohm’s Law

Step 3: Solve

Work to solve problem: 1) I = V/R

Circuits

Series vs Parallel

Parallel

-

Placed parallel, with multiple paths

-

Total current is sum of currents

-

Voltage is all the same

Series

-

Objects are placed in a row, with

only one path

-

Total current is all the same

-

Total voltage is sum of voltages

Parallel

Equivalent

Resistance

No matter the complexity, any network

of resistors can be analyzed by finding

an equivalent resistance

For a circuit in a series,

R

= R

+ R

+ R

+ …

and the R

> any single R

For a circuit in parallel,

Current Schematic Notation

Wires

Resistor

Example: lightbulbs

Flow of Current and Electrons

-

Positive charges are bound in place. Only electrons move in a conductor.

-

Flow of current is positive to negative

Types of Current

-

Direct Current: Current that flows in one ongoing direction, and are more

likely to damage electronics

-

Alternating Current: Current that switches between two directions, which

Magnetism

Basic Information

●

Magnetism comes from moving

_charges

●

Each Magnet is a dipole with a

north pole and south pole

●

Magnetism comes from atoms

themselves in material being

magnets , they atoms align into

clusters called magnetic domains

●

If current and magnetic field are

parallel, the force is zero , so it has

F = B I L

F - Force ( in Newtons) [N]

B - Magnetic Field (in Teslas) [T]

I- Current (in Amps) [A]

Right Hand Rules

1st Right Hand Rule :

●

Direction of current = thumb

●

External Magnetic Field = fingers

●

Force = Palm

2nd Right Hand Rule :

●

For a current carrying wire , grab the

wire so your thumb points in the

direction of the wire

Real life Application

alternating causes the coil to spin and thus causing an electric motor

Questions

1.)A straight wire that is 2 m long and is carrying a current of 4.0 A and is at right angles to a magnetic field with a strength of 0.7 T. What is the force on the wire?

a. 5.6 N

b. 5.6 W c. .056 N d. .056 W

2.)What causes pieces of iron to be magnets ? a. Applying heat

b. Aligned Magnetic Domains

c. A lot of force

3.) A 15 N force pushes to the right on a 0.5 m long wire that carries 8 A of current downwards. a. Determine the direction of the magnetic field.

( Into the page) w/ respect to paper

Stellar Properties

By Blair Stephenson, Leo Sandoval, Vivian Crowl

●

Brightness

●

Temperature

●

Mass

●

Parallax

●

Spectroscopy

●

HR diagrams

Brightness of a Star

● A star’s brightness is defined by its magnitude

○

Apparent magnitude

■ The lower the magnitude (negative), the brighter the star

■ ex) -26.27 is brighter than -4

○

Absolute magnitude

■ The best way to compare brightness of stars

Q1: Star temperature

Which of the following is correct:

A. The hottest stars are red

B. Blue stars emit energy in short wavelengths C. Colder stars emit short wavelengths

Star temperature

The temperature of a star is important because it can describe what is being burned as fuel, and help determine the age of a star. The more mass a star has, the hotter the star will be and the heavier elements it will end up burning. When a star is blue it burns hottest and emit shorter light wavelengths. Red stars are on the cooler side, and the emit shorter light wavelengths.

Heavier elements undergo fusion at even higher core temperatures. When a star is burning helium, it’s around 100 million K; Carbon burns at 500 million K; Neon burns at 1.2 billion K; Oxygen at 1.5 billion K; Silicon at 3 billion K.

The surface temperature of a star is determined by its color!

Mass of a star

The size of a star is measured in a unit called AU or astronomical unit. An astronomical unit (A.U.) is the average distance between the Earth and the Sun, which is about 93 million miles or 150 million kilometers.

We can determine the mass of a star easily by looking at two stars that are orbiting each other and by us looking at their speed and period we can then determine the mass of both in relationship with its gravitational pull

Q2: Distance in space

The most common method of

measuring distance in space is . .

A.

Parsecs (ps)

B.

Radar

How distance is measured in space

Most objects in space are too far away to measure using conventional methods

that are popular on Earth. Instead, parallax, radar, and cepheid variables.

●

Radar:

Parallax:

○ Requires 2 stars, uses the geometry between Earth’s orbit around our Sun and another star

○ Uses relative positioning of distant objects and a changing angle between stars

Q3: Spectroscopy

Which of the following about spectroscopy is correct?

A. The brightness of a star is key when finding its chemical composition

B. The elements that make up stars can emit light at multiple wavelengths

Spectroscopy:

I. Light is refracted through a prism to find the different visual wavelengths that a star emits (electromagnetic radiation)

II. The spectra of the Sun and stars exhibits bright and dark lines called Fraunhofer lines

A. You can match up these lines with the known spectra of certain elements

III. Each element that makes up a star emits/absorbs light only at specific wavelengths

IV. Through this process, the chemical composition of stars can be determined

In a nutshell: You can find what elements make up a star by examining the light that those elements emit or absorb.

Redshift

Definition: when an object’s motion relative to earth changes, the location of the spectral lines shift.

● The lines shift towards the red side if the object is moving away.

● The lines shift towards the blue side if object is moving towards earth.

What is an HR Diagram?

First Things First: Human Resources? Hummus and Radishes?

No! HR stands for Hertzsprung Russell Diagram, after the

scientists who developed it

Definition: a scatter plot used by astronomers that allows them

to categorize stars based on color, luminosity (brightness),

temperature, and stage of ‘life’

Anatomy of an HR Diagram

Y-axis: Luminosity (brightness)

X-axis: Temperature

Left to Right: Color (blue, white, yellow,

orange, red)

Top to Bottom: Biggest brightness to least

bright, generally

Type/Groups of Stars

● Super Giants - The largest stars. They can be found at the top of the chart in both red and blue, and are the ones that have enough mass to collapse into a black hole.

More Stars

● Main Sequence - This is the most common and “average” star. The main sequence is the long strip of stars of all color running through the middle of the diagram. When stars are created, this is where the start out. However, stars starting at different points on the main sequence will go through different life stages, depending on their mass.

● White Dwarves - White dwarves are

Predicting Star Placement

Now that you know how HR Diagrams work, you can plot a star and predict

a lot of information about it.

Answer: Main Sequence

The star that was described is

none other than Sirius A, the

brightest star we can see. It is a

blue, mid temperate, main

sequence star.

Color: Blue

Temperature: 9,400 K

Luminosity: 25.4 solar luminosity

The Life Cycle of

Stars + The Big

BaNG theory

Life Cycle of Stars

⋆

Stars form from nebulae

⋆

Gravity pushes gas together

_→

pressure increases the

temp.

→

fusion occurs

→

a star is born

⋆

REMINDER

→

a nebulae is a collection of gases + dust

floating in space

All stars are born in the main sequence, powered by the

fusion rxns burning hydrogen in their cores

⋆

Stars

Δ

over time

→

smaller the star, longer the

lifespan

⋆

Ending of a star depends on its mass + temperature

⋆

Stars ends up “dead”- meaning they don’t have fusion

Life Cycle of Stars pt. 2

⋆

NEBULAE CREATED OUT OF STELLAR EXPLOSIONS CAN

CREATE NEW STARS!!

⋆

SMALL MASS STARS

→

lower temp., ergo burn slower +

relatively dimmer

⋆

reddish-end of color spectrum

⋆

typically ends in explosion

→

white dwarf

⋆

still burns hot in core- initial temp. still present

⋆

LARGE MASS STARS

_→

medium temp., burn faster

+brighter than small stars

⋆

Between red + orange on color spectrum

⋆

ends in supernova explosion

→

neutron star

⋆

Initial temp. still present in core

Life Cycle of Stars pt. 3

←

OVERVIEW OF LIFE CYCLE

HR DIAGRAM

↑

⋆

SUPER LARGE MASS STARS

→

high

temp., burns fast + bright

⋆

Blue colored stars

⋆

Goes supernova

→

becomes a black

Black Holes

⋆

Formation: massive stars run out of fuel

→

collapse; mass is compressed into a

tiny space

⋆

Size: R = 2Gm/c

⋆

Force from initial star = force of

black hole

⋆

Features:

⋆

Gravitational redshift effect

⋆

Gravitational time dilation

111

* Event horizon

* Singularity

Big bang Theory: Evidence

1) More distant obj.s appear younger

→

implies universe changes over time

2) Ratios of H + He coincide with those

resulting from a nuclear rxn

→

suggest nuclear explosion occurred

3) Cosmic microwave bckg.

→

something happened to the ENTIRE

universe

4) Light from every obj. is redshifted

→

The End!