ConcepTest 8.1aConcepTest 8.1a Bonnie and Klyde IBonnie and Klyde I

BonnieBonnieKlydeKlyde

Bonnie sits on the outer rim of a merry-go-round, and Klyde sits midway between the center and the rim. The merry-go-round makes one complete revolution every two seconds.Klyde’s angular velocity is:

a) same as Bonnie’sb) twice Bonnie’sc) half of Bonnie’sd) 1/4 of Bonnie’se) four times Bonnie’s

The angular velocityangular velocity of any point on a solid object rotating about a fixed axis is the sameis the same. Both Bonnie and Klyde go around one revolution (2 radians) every two seconds.

ConcepTest 8.1aConcepTest 8.1a Bonnie and Klyde IBonnie and Klyde I

BonnieBonnieKlydeKlyde

Bonnie sits on the outer rim of a merry-go-round, and Klyde sits midway between the center and the rim. The merry-go-round makes one complete revolution every two seconds.Klyde’s angular velocity is:

a) same as Bonnie’sb) twice Bonnie’sc) half of Bonnie’sd) 1/4 of Bonnie’se) four times Bonnie’s

ConcepTest 8.1bConcepTest 8.1b Bonnie and Klyde IIBonnie and Klyde II

BonnieBonnieKlydeKlyde

a) Klydeb) Bonniec) both the samed) linear velocity is zero

for both of them

Bonnie sits on the outer rim of a merry-go-round, and Klyde sits midway between the center and the rim. The merry-go-round makes one revolution every two seconds. Who has the larger linear (tangential) velocity?

Their linear speedslinear speeds vv will be different since v = Rv = R and Bonnie is located further outBonnie is located further out (larger radius R) than Klyde.

BonnieBonnie

KlydeKlyde

BonnieKlyde V21V

a) Klydeb) Bonniec) both the samed) linear velocity is zero

for both of them

Bonnie sits on the outer rim of a merry-go-round, and Klyde sits midway between the center and the rim. The merry-go-round makes one revolution every two seconds. Who has the larger linear (tangential) velocity?

ConcepTest 8.1bConcepTest 8.1b Bonnie and Klyde IIBonnie and Klyde II

Follow-up:Follow-up: Who has the larger centripetal acceleration? Who has the larger centripetal acceleration?

ConcepTest 8.4ConcepTest 8.4 Using a WrenchUsing a Wrench

You are using a wrench to loosen a rusty nut. Which arrangement will be the most effective in loosening the nut?

a)

c)d)

b)

e) all are equally effective

ConcepTest 8.4ConcepTest 8.4 Using a WrenchUsing a Wrench

You are using a wrench to loosen a rusty nut. Which arrangement will be the most effective in loosening the nut?

a)

c)d)

b)

Since the forces are all the same, the only difference is the lever arm. The arrangement with the largest largest lever armlever arm (case #2case #2) will provide the largest torquelargest torque.

e) all are equally effective

Follow-up:Follow-up: What is the difference between arrangement 1 and 4? What is the difference between arrangement 1 and 4?

ConcepTest 8.6ConcepTest 8.6 Closing a DoorClosing a DoorIn which of the cases shown below is the torque provided by the applied force about the rotation axis biggest? For all cases the magnitude of the applied force is the same.

a) FA

b) FC

c) FD

d) all of them

e) none of them

ConcepTest 8.6ConcepTest 8.6 Closing a DoorClosing a Doora) FA

b) FC

c) FD

d) all of them

e) none of them

In which of the cases shown below is the torque provided by the applied force about the rotation axis biggest? For all cases the magnitude of the applied force is the same.

The torque is: == F d sin F d sin

and so the force that is at 90°90°

to the lever arm is the one that

will have the largest torquelargest torque.

Clearly, to close the door, you

want to push perpendicularperpendicular!!

Follow-up:Follow-up: How large would the force have to be for How large would the force have to be for FFDD??

ConcepTest 8.9ConcepTest 8.9 Moment of InertiaMoment of Inertia

a) a) solid aluminum

b) hollow goldb) hollow gold

c) samec) same

same mass & radius

solid hollow

Two spheres have the same radius and equal masses. One is made of solid aluminum, and the other is made from a hollow shell of gold.

Which one has the bigger moment of inertia about an axis through its center?

ConcepTest 8.9ConcepTest 8.9 Moment of InertiaMoment of Inertia

a) a) solid aluminum

b) hollow goldb) hollow gold

c) samec) same

same mass & radius

solid hollow

Two spheres have the same radius and equal masses. One is made of solid aluminum, and the other is made from a hollow shell of gold.

Which one has the bigger moment of inertia about an axis through its center?

Moment of inertia depends on mass and distance from axis squared. It is bigger for the shell since its mass is located farther from the center.

ConcepTest 8.10ConcepTest 8.10 Figure SkaterFigure Skater

a)a) the samethe sameb)b) larger because she’s rotating larger because she’s rotating

fasterfaster

c)c) smaller because her rotational smaller because her rotational inertia is smallerinertia is smaller

A figure skater spins with her arms extended. When she pulls in her arms, she reduces her rotational inertia and spins faster so that her angular momentum is conserved. Compared to her initial rotational kinetic energy, her rotational kinetic energy after she pulls in her arms must be

ConcepTest 8.10ConcepTest 8.10 Figure SkaterFigure Skater

a)a) the samethe sameb)b) larger because she’s rotating larger because she’s rotating

fasterfaster

c)c) smaller because her rotational smaller because her rotational inertia is smallerinertia is smaller

A figure skater spins with her arms extended. When she pulls in her arms, she reduces her rotational inertia and spins faster so that her angular momentum is conserved. Compared to her initial rotational kinetic energy, her rotational kinetic energy after she pulls in her arms must be

KErot=1/2 I 2 = 1/2 L (used L= I ). Since L is conserved, larger means larger KErot. The “extra” energy comes from the work she does on her arms.

Follow-up:Follow-up: Where does the extra energy come from? Where does the extra energy come from?

ConcepTest 11.1aConcepTest 11.1a Harmonic Motion Ia) 0a) 0

b) b) AA/2/2

c) c) AA

d) 2d) 2AA

e) 4e) 4AA

A mass on a spring in SHM has amplitude A and period T. What is the total distance traveled by the mass after a time interval T?

ConcepTest 11.1aConcepTest 11.1a Harmonic Motion Ia) 0a) 0

b) b) AA/2/2

c) c) AA

d) 2d) 2AA

e) 4e) 4AA

A mass on a spring in SHM has amplitude A and period T. What is the total distance traveled by the mass after a time interval T?

In the time interval time interval TT (the period), the mass goes through one complete oscillationcomplete oscillation back to the starting point. The distance it covers is: The distance it covers is: A + A + A + AA + A + A + A (4 (4AA).).

A mass on a spring in SHM has amplitude A and period T. What is the net displacement of the mass after a time interval T?

a) 0b) A/2c) Ad) 2Ae) 4A

ConcepTest 11.1bConcepTest 11.1b Harmonic Motion II

A mass on a spring in SHM has amplitude A and period T. What is the net displacement of the mass after a time interval T?

a) 0b) A/2c) Ad) 2Ae) 4A

The displacement is x = x2–x1. Since the initial and final positions of the mass are the same (it ends up back at its original position), then the displacement is zero.

ConcepTest 11.1bConcepTest 11.1b Harmonic Motion II

Follow-up:Follow-up: What is the net displacement after a half of a period? What is the net displacement after a half of a period?

A spring can be stretched a distance of 60 cm with an applied force of 1 N. If an identical spring is connected in series with the first spring, how much force will be required to stretch this series combination a distance of 60 cm?

a) 1/4 Nb) 1/2 Nc) 1 Nd) 2 Ne) 4 N

ConcepTest 11.3bConcepTest 11.3b Spring Combination II

A spring can be stretched a distance of 60 cm with an applied force of 1 N. If an identical spring is connected in series with the first spring, how much force will be required to stretch this series combination a distance of 60 cm?

a) 1/4 Nb) 1/2 Nc) 1 Nd) 2 Ne) 4 N

Here, the springs are in series, so each spring is only stretched 30 cm, and only half the force is needed. But also, since the springs are in a row, the force applied to one spring is transmitted to the other spring (like tension in a rope). So the overall applied force of 1/2 N is all that is needed. The combination of two springs in series behaves like a weaker spring!!

ConcepTest 11.3bConcepTest 11.3b Spring Combination II

ConcepTest 11.4ConcepTest 11.4 To the Center of the Earth

A hole is drilled through the center of Earth and emerges on the other side. You jump into the hole. What happens to you?

a) you fall to the center and stopb) you go all the way through and

continue off into space c) you fall to the other side of

Earth and then returnd) you won’t fall at all

You fall through the hole. When you reach the center, you keep going because of your inertia. When you reach the other side, gravity pulls gravity pulls you back toward the centeryou back toward the center. This is Simple This is Simple Harmonic Motion!Harmonic Motion!

ConcepTest 11.4ConcepTest 11.4 To the Center of the Earth

A hole is drilled through the center of Earth and emerges on the other side. You jump into the hole. What happens to you?

a) you fall to the center and stopb) you go all the way through and

continue off into space c) you fall to the other side of

Earth and then returnd) you won’t fall at all

Follow-up:Follow-up: Where is your acceleration zero? Where is your acceleration zero?

A mass oscillates in simple harmonic motion with amplitude A. If the mass is doubled, but the amplitude is not changed, what will happen to the total energy of the system?

a) total energy will increaseb) total energy will not changec) total energy will decrease

ConcepTest 11.5aConcepTest 11.5a Energy in SHM I

A mass oscillates in simple harmonic motion with amplitude A. If the mass is doubled, but the amplitude is not changed, what will happen to the total energy of the system?

a) total energy will increaseb) total energy will not changec) total energy will decrease

The total energy is equal to the initial value of the elastic potential energy, which is PEs = 1/2 kA2. This does not depend on mass, so a change in mass will not affect the energy of the system.

ConcepTest 11.5aConcepTest 11.5a Energy in SHM I

Follow-up:Follow-up: What happens if you double the amplitude? What happens if you double the amplitude?

A mass oscillates on a vertical spring with period T. If the whole setup is taken to the Moon, how does the period change?

a) period will increaseb) period will not changec) period will decrease

ConcepTest 11.7cConcepTest 11.7c Spring on the Moon

A mass oscillates on a vertical spring with period T. If the whole setup is taken to the Moon, how does the period change?

a) period will increaseb) period will not changec) period will decrease

The period of simple harmonic motion only depends on the mass and the spring constant and does not depend on the acceleration due to gravity. By going to the Moon, the value of g has been reduced, but that does not affect the period of the oscillating mass-spring system.

ConcepTest 11.7cConcepTest 11.7c Spring on the Moon

Follow-up:Follow-up: Will the period be the same on any planet? Will the period be the same on any planet?

Two pendula have the same length, but different masses attached to the string. How do their periods compare?

a) period is greater for the greater massb) period is the same for both casesc) period is greater for the smaller mass

ConcepTest 11.8aConcepTest 11.8a Period of a Pendulum I

Two pendula have the same length, but different masses attached to the string. How do their periods compare?

a) period is greater for the greater massb) period is the same for both casesc) period is greater for the smaller mass

The period of a pendulum depends on the length and the acceleration due to gravity, but it does not depend on the mass of the bob.

TT = 2 = 2((LL//gg))

ConcepTest 11.8aConcepTest 11.8a Period of a Pendulum I

Follow-up:Follow-up: What happens if the amplitude is doubled? What happens if the amplitude is doubled?

A grandfather clock has a weight at the bottom of the pendulum that can be moved up or down. If the clock is running slow, what should you do to adjust the time properly?

a) move the weight upb) move the weight downc) moving the weight will not matterd) call the repair man

ConcepTest 11.9 ConcepTest 11.9 Grandfather Clock

A grandfather clock has a weight at the bottom of the pendulum that can be moved up or down. If the clock is running slow, what should you do to adjust the time properly?

a) move the weight upb) move the weight downc) moving the weight will not matterd) call the repair man

The period of the grandfather clock is too long, so we need to decrease the period (increase the frequency). To do this, the length must be decreased, so the adjustable weight should be moved up in order to shorten the pendulum length.

TT = 2 = 2((LL//gg))

ConcepTest 11.9 ConcepTest 11.9 Grandfather Clock

After a pendulum starts swinging, its amplitude gradually decreases with time because of friction.

What happens to the period of the pendulum during this time?

a) period increases

b) period does not change

c) period decreases

ConcepTest 11.11ConcepTest 11.11 Damped Pendulum

The period of a pendulum does not depend The period of a pendulum does not depend on its amplitudeon its amplitude, but only on its lengthlength and the acceleration due to gravityacceleration due to gravity.

gLT2

After a pendulum starts swinging, its amplitude gradually decreases with time because of friction.

What happens to the period of the pendulum during this time?

a) period increases

b) period does not change

c) period decreases

ConcepTest 11.11ConcepTest 11.11 Damped Pendulum

Follow-up:Follow-up: What is happening to the energy of the pendulum? What is happening to the energy of the pendulum?

a) yes

b) no

c) it depends on the medium the wave is in

ConcepTest 11.13ConcepTest 11.13 Sound It OutSound It Out

Does a longitudinal wave, such as a sound wave, have an amplitude ?

lowlow

highhigh

normalnormal

airairpressurepressure

xx

AA

a) yes

b) no

c) it depends on the medium the wave is in

All wave types—transverse,longitudinal, surface—have

all of these properties:wavelength, frequency,wavelength, frequency,

amplitude, velocity, periodamplitude, velocity, period

ConcepTest 11.13ConcepTest 11.13 Sound It OutSound It Out

Does a longitudinal wave, such as a sound wave, have an amplitude ?

lowlow

highhigh

normalnormal

airairpressurepressure

xx

AA

At a football game, the “wave” might circulate through the stands and move around the stadium. In this wave motion, people stand up and sit down as the wave passes. What type of wave would this be characterized as?

a) polarized waveb) longitudinal wavec) lateral waved) transverse wavee) soliton wave

ConcepTest 11.14ConcepTest 11.14 The WaveThe Wave

At a football game, the “wave” might circulate through the stands and move around the stadium. In this wave motion, people stand up and sit down as the wave passes. What type of wave would this be characterized as?

a) polarized waveb) longitudinal wavec) lateral waved) transverse wavee) soliton wave

The people are moving up and down, and the wave is traveling around the stadium. Thus, the motion of the wave is perpendicular to the oscillation direction of the people, and so this is a transverse wave.

ConcepTest 11.14ConcepTest 11.14 The WaveThe Wave

Follow-up:Follow-up: What type of wave occurs when you toss a pebble in a pond? What type of wave occurs when you toss a pebble in a pond?

a) water

b) ice

c) same speed in both

d) sound can only travel in a gas

Do sound waves travel faster in water or in ice?

ConcepTest 12.2aConcepTest 12.2a Speed of Sound I

a) water

b) ice

c) same speed in both

d) sound can only travel in a gas

Do sound waves travel faster in water or in ice?

ConcepTest 12.2aConcepTest 12.2a Speed of Sound I

Speed of sound depends on the inertiainertia of the medium and the restoring forcerestoring force. Since ice and water both consist of water molecules, the inertia is the same for both. However, However, the force holding the molecules together is greater in ice the force holding the molecules together is greater in ice (because it is a solid), so the restoring force is greater.(because it is a solid), so the restoring force is greater. Since vv = = (force / inertia),(force / inertia), the speed of sound must be greater in icegreater in ice !

Do you expect an echo to return to you more quickly or less quickly on a hot day, as compared to a cold day?

a) more quickly on a hot dayb) equal times on both daysc) more quickly on a cold day

ConcepTest 12.2bConcepTest 12.2b Speed of Sound II

Do you expect an echo to return to you more quickly or less quickly on a hot day, as compared to a cold day?

a) more quickly on a hot dayb) equal times on both daysc) more quickly on a cold day

The speed of sound in a gas increases with temperature. This is because the molecules are bumping into each other faster and more often, so it is easier to propagate the compression wave (sound wave).

ConcepTest 12.2bConcepTest 12.2b Speed of Sound II

If you fill your lungs with helium and then try talking, you sound like Donald Duck. What conclusion can you reach about the speed of sound in helium?

a) speed of sound is less in heliumb) speed of sound is the same in heliumc) speed of sound is greater in heliumd) this effect has nothing to do with the speed in helium

ConcepTest 12.2cConcepTest 12.2c Speed of Sound III

If you fill your lungs with helium and then try talking, you sound like Donald Duck. What conclusion can you reach about the speed of sound in helium?

a) speed of sound is less in heliumb) speed of sound is the same in heliumc) speed of sound is greater in heliumd) this effect has nothing to do with the speed in helium

The higher pitch implies a higher frequency. In turn, since v = f, this means that the speed of the wave has increased (as long as the wavelength, determined by the length of the vocal chords, remains constant).

ConcepTest 12.2cConcepTest 12.2c Speed of Sound III

Follow-up:Follow-up: Why is the speed of sound greater in helium than in air? Why is the speed of sound greater in helium than in air?

a) about the same distance

b) about 3 miles

c) about 10 miles

d) about 30 miles

e) about 100 miles

You hear a fire truck with a certain intensity, and you are about 1 mile away. Another person hears the same fire truck with an intensity that is about 10 times less. Roughly how far is the other person from the fire truck?

ConcepTest 12.4bConcepTest 12.4b Sound Intensity II

22

21

21

22

1

2

4

4

r

r

rP

rP

/

/I

I

a) about the same distance

b) about 3 miles

c) about 10 miles

d) about 30 miles

e) about 100 miles

You hear a fire truck with a certain intensity, and you are about 1 mile away. Another person hears the same fire truck with an intensity that is about 10 times less. Roughly how far is the other person from the fire truck?

ConcepTest 12.4bConcepTest 12.4b Sound Intensity II

Remember that intensity drops with the inverse inverse square of the distancesquare of the distance, so if intensity drops by a factor of 10, the other person must be 10 farther away, which is about a factor of 3.

a) the long pipeb) the short pipec) both have the same frequency

d) depends on the speed of sound in the pipe

You have a long pipe and a short pipe. Which one has the higher frequency?

ConcepTest 12.6aConcepTest 12.6a Pied Piper I

A shorter pipeshorter pipe means that the standing wave in

the pipe would have a shorter wavelengthshorter wavelength. Since

the wave speed remains the same, the frequency frequency

has to be higherhas to be higher in the short pipe.

a) the long pipeb) the short pipec) both have the same frequency

d) depends on the speed of sound in the pipe

You have a long pipe and a short pipe. Which one has the higher frequency?

ConcepTest 12.6aConcepTest 12.6a Pied Piper I

A wood whistle has a variable length. You just heard the tone from the whistle at maximum length. If the air column is made shorter by moving the end stop, what happens to the frequency?

a) frequency will increaseb) frequency will not changec) frequency will decrease

ConcepTest 12.6bConcepTest 12.6b Pied Piper II

A wood whistle has a variable length. You just heard the tone from the whistle at maximum length. If the air column is made shorter by moving the end stop, what happens to the frequency?

a) frequency will increaseb) frequency will not changec) frequency will decrease

A shorter pipeshorter pipe means that the standing wave in the pipe would have a shorter wavelengthshorter wavelength. Since the wave speed remains the same, and since we know that vv = = ff , then we see that the frequency has to increase frequency has to increase when the pipe is made shorter.

ConcepTest 12.6bConcepTest 12.6b Pied Piper II

If you blow across the opening of a partially filled soda bottle, you hear a tone. If you take a big sip of soda and then blow across the opening again, how will the frequency of the tone change?

a) frequency will increaseb) frequency will not changec) frequency will decrease

ConcepTest 12.6cConcepTest 12.6c Pied Piper III

If you blow across the opening of a partially filled soda bottle, you hear a tone. If you take a big sip of soda and then blow across the opening again, how will the frequency of the tone change?

a) frequency will increaseb) frequency will not changec) frequency will decrease

By drinking some of the soda, you have effectively increased the length of the air column in the bottle. A longer pipelonger pipe means that the standing wave in the bottle would have a longer wavelengthlonger wavelength. Since the wave speed remains the same, and since we know that vv = = ff , then we see that the frequency has to be lowerfrequency has to be lower.

ConcepTest 12.6cConcepTest 12.6c Pied Piper III

Follow-up:Follow-up: Why doesn’t the wave speed change? Why doesn’t the wave speed change?

a) depends on the speed of soundin the pipe

b) you hear the same frequency c) you hear a higher frequencyd) you hear a lower frequency

You blow into an open pipe and produce a tone. What happens to the frequency of the tone if you close the end of the pipe and blow into it again?

ConcepTest 12.7ConcepTest 12.7 Open and Closed Pipes

In the open pipeopen pipe, 1/2 of a wave1/2 of a wave “fits” into the pipe, while in the closed pipeclosed pipe, only 1/4 of a wave1/4 of a wave fits. Because the wavelength is larger in the closed wavelength is larger in the closed pipepipe, the frequency will be lowerfrequency will be lower.

a) depends on the speed of soundin the pipe

b) you hear the same frequency

c) you hear a higher frequency

d) you hear a lower frequency

You blow into an open pipe and produce a tone. What happens to the frequency of the tone if you close the end of the pipe and blow into it again?

ConcepTest 12.7ConcepTest 12.7 Open and Closed Pipes

Follow-up:Follow-up: What would you have to do What would you have to do to the pipe to increase the frequency?to the pipe to increase the frequency?

When you tune a guitar string, what physical characteristic of the string are you actually changing?

a) the tension in the stringb) the mass per unit length of the stringc) the composition of the stringd) the overall length of the stringe) the inertia of the string

ConcepTest 12.8ConcepTest 12.8 Out of Tune

When you tune a guitar string, what physical characteristic of the string are you actually changing?

a) the tension in the stringb) the mass per unit length of the stringc) the composition of the stringd) the overall length of the stringe) the inertia of the string

By tightening (or loosening) the knobs on the neck of the guitar, you are changing the tensionchanging the tension in the string. This alters the wave speed, and therefore alters the frequency of the fundamental standing wave because f = v/2L .

ConcepTest 12.8ConcepTest 12.8 Out of Tune

Follow-up:Follow-up: To increase frequency, do you tighten or loosen the strings? To increase frequency, do you tighten or loosen the strings?

Observers A, B, and C listen to a moving source of sound. The location of the wave fronts of the moving source with respect to the observers is shown below. Which of the following is true?

a) frequency is highest at A

b) frequency is highest at B

c) frequency is highest at C

d) frequency is the same at all three points

ConcepTest 12.11aConcepTest 12.11a Doppler Effect I

Observers A, B, and C listen to a moving source of sound. The location of the wave fronts of the moving source with respect to the observers is shown below. Which of the following is true?

a) frequency is highest at A

b) frequency is highest at B

c) frequency is highest at C

d) frequency is the same at all three points

The number of wave fronts hitting observer Cobserver C per unit time is greatest—thus the observed frequency is highest there.

ConcepTest 12.11aConcepTest 12.11a Doppler Effect I

Follow-up:Follow-up: Where is the frequency lowest? Where is the frequency lowest?

You are heading toward an island in a speedboat and you see your friend standing on the shore, at the base of a cliff. You sound the boat’s horn to alert your friend of your arrival. If the horn has a rest frequency of f0, what frequency does your friend hear?

a) lower than f0

b) equal to f0

c) higher than f0

ConcepTest 12.11bConcepTest 12.11b Doppler Effect II

You are heading toward an island in a speedboat and you see your friend standing on the shore, at the base of a cliff. You sound the boat’s horn to alert your friend of your arrival. If the horn has a rest frequency of f0, what frequency does your friend hear?

a) lower than f0

b) equal to f0

c) higher than f0

Due to the approach of the sourceapproach of the source toward the stationary observer, the frequency is shifted higherfrequency is shifted higher. This is the same situation as depicted in the previous question.

ConcepTest 12.11bConcepTest 12.11b Doppler Effect II

In the previous question, the horn had a rest frequency of f0, and we found that your friend heard a higher frequency f1 due to the Doppler shift. The sound from the boat hits the cliff behind your friend and returns to you as an echo. What is the frequency of the echo that you hear?

a) lower than f0

b) equal to f0

c) higher than f0 but lower than f1

d) equal to f1

e) higher than f1

ConcepTest 12.11cConcepTest 12.11c Doppler Effect III

In the previous question, the horn had a rest frequency of f0, and we found that your friend heard a higher frequency f1 due to the Doppler shift. The sound from the boat hits the cliff behind your friend and returns to you as an echo. What is the frequency of the echo that you hear?

a) lower than f0

b) equal to f0

c) higher than f0 but lower than f1

d) equal to f1

e) higher than f1

The sound wave bouncing off the cliff has the same frequency f1 as the one hitting the cliff (what your friend hears). For the echo, you are now a moving observer approaching the sound waveyou are now a moving observer approaching the sound wave of frequency f1 so you will hear an even higher frequencyeven higher frequency.

ConcepTest 12.11cConcepTest 12.11c Doppler Effect III

ConcepTest 16.1aConcepTest 16.1a Electric Charge IElectric Charge Ia) one is positive, the other is

negative

b) both are positive

c) both are negative

d) both are positive or both are negative

Two charged balls are Two charged balls are repelling each other as repelling each other as they hang from the ceiling. they hang from the ceiling. What can you say about What can you say about their charges?their charges?

ConcepTest 16.1aConcepTest 16.1a Electric Charge IElectric Charge I

The fact that the balls repel each other only can tell you that they have the same chargesame charge, but you do not know the sign. So they can be either both positive or both negative.

a) one is positive, the other is negative

b) both are positive

c) both are negative

d) both are positive or both are negative

Two charged balls are Two charged balls are repelling each other as repelling each other as they hang from the ceiling. they hang from the ceiling. What can you say about What can you say about their charges?their charges?

Follow-up:Follow-up: What does the picture look like if the two balls are oppositely What does the picture look like if the two balls are oppositely charged? What about if both balls are neutral?charged? What about if both balls are neutral?

a) have opposite charges

b) have the same charge

c) all have the same charge

d) one ball must be neutral (no charge)

From the picture, From the picture, what can you what can you conclude about conclude about the charges?the charges?

ConcepTest 16.1bConcepTest 16.1b Electric Charge IIElectric Charge II

a) have opposite charges

b) have the same charge

c) all have the same charge

d) one ball must be neutral (no charge)

From the picture, From the picture, what can you what can you conclude about conclude about the charges?the charges?

The GREEN and PINK balls must have the same charge, since they repel each other. The YELLOW ball also repels the GREEN, so it must also have the same charge as the GREEN (and the PINK).

ConcepTest 16.1bConcepTest 16.1b Electric Charge IIElectric Charge II

Two neutral conductors are connected by a wire and a charged rod is brought near, but does not touch. The wire is taken away, and then the charged rod is removed. What are the charges on the conductors?

ConcepTest 16.2bConcepTest 16.2b Conductors IIConductors IIa) 0 0

b) + –

c) – +

d) + +

e) – –

0 0

? ?

While the conductors are connected, positive positive

charge will flow from the blue to the green ball charge will flow from the blue to the green ball

due to polarizationdue to polarization. Once disconnected, the

charges will remain on the separate conductorscharges will remain on the separate conductors

even when the rod is removed.

Two neutral conductors are connected by a wire and a charged rod is brought near, but does not touch. The wire is taken away, and then the charged rod is removed. What are the charges on the conductors?

ConcepTest 16.2bConcepTest 16.2b Conductors IIConductors IIa) 0 0

b) + –

c) – +

d) + +

e) – –

0 0

? ?Follow-up:Follow-up: What will happen when the What will happen when the conductors are reconnected with a wire?conductors are reconnected with a wire?

QQ QQF1 = 3N F2 = ?

a) 1.0 N

b) 1.5 N

c) 2.0 N

d) 3.0 N

e) 6.0 N

What is the magnitude What is the magnitude

of the force of the force FF22??

ConcepTest 16.3aConcepTest 16.3a CoulombCoulomb’’s Law Is Law I

The force F2 must have the same magnitudesame magnitude as F1. This is due to the

fact that the form of Coulomb’s Law is totally symmetric with

respect to the two charges involved. The force of one on the other of force of one on the other of

a pair is the same as the reversea pair is the same as the reverse. Note that this sounds suspiciously Note that this sounds suspiciously

like Newtonlike Newton’’s 3rd Law!!s 3rd Law!!

QQ QQF1 = 3N F2 = ?

a) 1.0 N

b) 1.5 N

c) 2.0 N

d) 3.0 N

e) 6.0 N

What is the magnitude What is the magnitude

of the force of the force FF22??

ConcepTest 16.3aConcepTest 16.3a CoulombCoulomb’’s Law Is Law I

ConcepTest 16.3bConcepTest 16.3b CoulombCoulomb’’s Law IIs Law IIa) 3/4 N

b) 3.0 N

c) 12 N

d) 16 N

e) 48 N

If we increase one charge to If we increase one charge to 4Q4Q, , what is the magnitude ofwhat is the magnitude of F F11??

4Q4Q QQF1 = ? F2 = ?

QQ QQF1 = 3N F2 = ?

ConcepTest 16.3bConcepTest 16.3b CoulombCoulomb’’s Law IIs Law II

Originally we had:

F1 = k(Q)(Q)/r2 = 3 N

Now we have:

F1 = k(4Q)(Q)/r2

which is 4 times bigger4 times bigger than before.

a) 3/4 N

b) 3.0 N

c) 12 N

d) 16 N

e) 48 N

If we increase one charge to If we increase one charge to 4Q4Q, , what is the magnitude of what is the magnitude of FF11??

44QQ QQF1 = ? F2 = ?

QQ QQF1 = 3N F2 = ?

Follow-up:Follow-up: Now what is the magnitude of Now what is the magnitude of FF22??

a) 9 F

b) 3 F

c) F

d) 1/3 F

e) 1/9 F

The force between two charges separated by a distance d is F. If the charges are pulled apart to a distance 3d, what is the force on each charge?

QQFF

QFF

dd

QQ??

QQ??

33dd

ConcepTest 16.3cConcepTest 16.3c CoulombCoulomb’’s Law IIIs Law III

Originally we had:

Fbefore = k(Q)(Q)/d2 = F

Now we have:

Fafter = k(Q)(Q)/(3d)2 = 1/9 F

a) 9 F

b) 3 F

c) F

d) 1/3 F

e) 1/9 F

The force between two charges separated by a distance d is F. If the charges are pulled apart to a distance 3d, what is the force on each charge?

QQFF

QFF

dd

QQ??

QQ??

33dd

ConcepTest 16.3cConcepTest 16.3c CoulombCoulomb’’s Law IIIs Law III

Follow-up:Follow-up: What is the force if the original distance is halved? What is the force if the original distance is halved?

Two balls with charges Two balls with charges ++QQ and and +4+4QQ are separated by are separated by 33RR. Where . Where should you place another charged ball should you place another charged ball QQ00 on the line between the on the line between the two charges such that the net force on two charges such that the net force on QQ00 will be zero? will be zero?

3R

+Q +4Q

R 2R

a) b) c) d) e)

ConcepTest 16.4bConcepTest 16.4b Electric Force IIElectric Force II

A proton and an electron are held apart a distance of 1 m and then released. As they approach each other, what happens to the force between them?

a) it gets bigger

b) it gets smaller

c) it stays the same

p e

ConcepTest 16.5aConcepTest 16.5a Proton and Electron IProton and Electron I

By Coulomb’s Law, the force between the two force between the two charges is inversely proportional to the charges is inversely proportional to the distance squareddistance squared. So, the closer they get to each other, the bigger the electric force between them gets!

A proton and an electron are held apart a distance of 1 m and then released. As they approach each other, what happens to the force between them?

a) it gets bigger

b) it gets smaller

c) it stays the same

p e

ConcepTest 16.5aConcepTest 16.5a Proton and Electron IProton and Electron I

Follow-up:Follow-up: Which particle feels the larger force at any one moment? Which particle feels the larger force at any one moment?

a) proton

b) electron

c) both the same

p e

ConcepTest 16.5bConcepTest 16.5b Proton and Electron IIProton and Electron II

A proton and an electron are held apart a distance of 1 m and then released. Which particle has the larger acceleration at any one moment?

The two particles feel the same forcesame force. Since F = ma, the particle with the smaller masssmaller mass will have the larger accelerationlarger acceleration. This would be This would be the electron.the electron.

a) proton

b) electron

c) both the same

p e

ConcepTest 16.5bConcepTest 16.5b Proton and Electron IIProton and Electron II

A proton and an electron are held apart a distance of 1 m and then released. Which particle has the larger acceleration at any one moment?

A proton and an electron are held apart a distance of 1 m and then let go. Where would they meet?

a) in the middle

b) closer to the electron’s side

c) closer to the proton’s side

p e

ConcepTest 16.5cConcepTest 16.5c Proton and Electron IIIProton and Electron III

By Newton’s 3rd Law, the electron and proton feel the same forcesame force. But, since F = ma, and since the protonproton’’s mass is much greaters mass is much greater, the protonproton’’s s acceleration will be much smalleracceleration will be much smaller!

Thus, they will meet closer to the protoncloser to the proton’’s original s original positionposition.

A proton and an electron are held apart a distance of 1 m and then let go. Where would they meet?

a) in the middle

b) closer to the electron’s side

c) closer to the proton’s side

p e

ConcepTest 16.5cConcepTest 16.5c Proton and Electron IIIProton and Electron III

Follow-up:Follow-up: Which particle will be moving faster when they meet? Which particle will be moving faster when they meet?

ConcepTest 19.1aConcepTest 19.1a Series Resistors ISeries Resistors I

9 V

Assume that the voltage of the battery Assume that the voltage of the battery is is 9 V9 V and that the three resistors are and that the three resistors are identicalidentical. What is the potential . What is the potential difference across each resistor?difference across each resistor?

a) 12 V

b) zero

c) 3 V

d) 4 V

e) you need to know the actual value of R

Since the resistors are all equalequal, the

voltage will drop evenlyevenly across the 3

resistors, with 1/3 of 9 V across each

one. So we get a 3 V drop3 V drop across

each.

ConcepTest 19.1aConcepTest 19.1a Series Resistors ISeries Resistors I

9 V

Assume that the voltage of the battery Assume that the voltage of the battery is is 9 V9 V and that the three resistors are and that the three resistors are identicalidentical. What is the potential . What is the potential difference across each resistor?difference across each resistor?

a) 12 V

b) zero

c) 3 V

d) 4 V

e) you need to know the actual value of R

Follow-up:Follow-up: What would be the potential difference if What would be the potential difference if RR= = 1

ConcepTest 19.2aConcepTest 19.2a Parallel Resistors IParallel Resistors I

In the circuit below, what is the In the circuit below, what is the

current through current through R1??

10 V

R1= 5

R2= 2

a) 10 A

b) zero

c) 5 A

d) 2 A

e) 7 A

The voltagevoltage is the samesame (10 V) across each resistor

because they are in parallel. Thus, we can use

Ohm’s Law, VV11 = I = I11 R R11 to find the current II11 = 2 = 2

AA.

ConcepTest 19.2aConcepTest 19.2a Parallel Resistors IParallel Resistors I

In the circuit below, what is the In the circuit below, what is the

current through current through R1??

10 V

R1= 5

R2= 2

a) 10 A

b) zero

c) 5 A

d) 2 A

e) 7 A

Follow-up:Follow-up: What is the total current through the battery? What is the total current through the battery?

ConcepTest 19.2bConcepTest 19.2b Parallel Resistors IIParallel Resistors IIa) increases

b) remains the same

c) decreases

d) drops to zero

Points P and Q are connected to a Points P and Q are connected to a battery of fixed voltage. As more battery of fixed voltage. As more resistors resistors RR are added to the parallel are added to the parallel circuit, what happens to the circuit, what happens to the total total currentcurrent in the circuit? in the circuit?

ConcepTest 19.2bConcepTest 19.2b Parallel Resistors IIParallel Resistors IIa) increases

b) remains the same

c) decreases

d) drops to zero

As we add parallel resistors, the overall resistance of the circuit dropsresistance of the circuit drops. Since V = IR, and V is held constant by the battery, when resistance decreasesresistance decreases, the current must current must increaseincrease.

Points P and Q are connected to a Points P and Q are connected to a battery of fixed voltage. As more battery of fixed voltage. As more resistors resistors RR are added to the parallel are added to the parallel circuit, what happens to the circuit, what happens to the total total currentcurrent in the circuit? in the circuit?

Follow-up:Follow-up: What happens to the current through each resistor? What happens to the current through each resistor?

ConcepTest 19.3aConcepTest 19.3a Short CircuitShort Circuit

Current flows through a Current flows through a lightbulb. If a wire is now lightbulb. If a wire is now connected across the connected across the bulb, what happens?bulb, what happens?

a) all the current continues to flow through the all the current continues to flow through the bulbbulb

b) half the current flows through the wire, the half the current flows through the wire, the other half continues through the bulbother half continues through the bulb

c) all the current flows through the wireall the current flows through the wire

d) none of the abovenone of the above

The current divides based on the ratio of the resistances. If one of the resistances is zerozero, then ALLALL of the current will flow through that path.

ConcepTest 19.3aConcepTest 19.3a Short CircuitShort Circuit

Current flows through a Current flows through a lightbulb. If a wire is now lightbulb. If a wire is now connected across the connected across the bulb, what happens?bulb, what happens?

a) all the current continues to flow through the all the current continues to flow through the bulbbulb

b) half the current flows through the wire, the half the current flows through the wire, the other half continues through the bulbother half continues through the bulb

c) all the current flows through the wireall the current flows through the wire

d) none of the abovenone of the above

Follow-up:Follow-up: Doesn’t the wire have SOME resistance? Doesn’t the wire have SOME resistance?

ConcepTest 19.3bConcepTest 19.3b Short Circuit IIShort Circuit II

Two lightbulbs A and B are Two lightbulbs A and B are connected in series to a connected in series to a constant voltage source. constant voltage source. When a wire is connected When a wire is connected across B, bulb A will:across B, bulb A will:

a) glow brighter than beforeglow brighter than before

b) glow just the same as beforeglow just the same as before

c) glow dimmer than beforeglow dimmer than before

d) go out completelyd) go out completely

e) explodee) explode

Since bulb B is bypassed by the wire, the total resistance of the circuit decreasestotal resistance of the circuit decreases. This means that the current through bulb A current through bulb A

increasesincreases.

ConcepTest 19.3bConcepTest 19.3b Short Circuit IIShort Circuit II

Two lightbulbs A and B are Two lightbulbs A and B are connected in series to a connected in series to a constant voltage source. constant voltage source. When a wire is connected When a wire is connected across B, bulb A will:across B, bulb A will:

a) glow brighter than beforeglow brighter than before

b) glow just the same as beforeglow just the same as before

c) glow dimmer than beforeglow dimmer than before

d) go out completelyd) go out completely

e) explodee) explode

Follow-up:Follow-up: What happens to bulb B? What happens to bulb B?