Practice Final Exam

(Answers keys)

1. A group of students arrange two level tracks side-by-side so they can have a race between two carts. They mount identical fan units (each with two real batteries) on two identical carts. When the two carts, with fans turned on, are released

simultaneously from the end of the tracks they speed up at the same rate, traveling side-by-side, and so the race ends in a tie (Experiment 1).

The students then add extra mass to one of the carts and repeat the experiment (using the same fan units), and record speed-time data for both carts (Experiment 2).

a. The next week, when they look at the speed-time data they graphed (shown

below), there are two lines (labeled A and B), and they are not sure which is

which. Can you help by identifying which line represents the motion of the

cart with the added mass? Justify your choice.

0

10

20

30

40

50

60

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

Time (s)

Sp

ee

d (

cm

/s)

A

B

Line B represents the motion of the cart with added mass. Since the fan units were

identical they would have provided the same strength force on each cart. However,

the cart with the added mass would have sped-up more slowly than the one without

the extra mass. The line that represents a lower rate of speeding up is B.

However, one of the students remembers that they also attempted to break the

tie in Experiment 1 by adding a additional battery to one of the fan units without

adding extra mass to either cart (Experiment 3). Could the speed-time data

shown in the graph on the previous page be from Experiment 3? Yes or No.

Justify your answer.

Yes, the data could be from Experiment 3. In this experiment two different

strength forces were applied to identical carts. This would mean that the cart with

the stronger fan force acting on it would speed up at a higher rate than the other

cart – this would correspond to line A, with line B being the cart with the weaker fan

force acting on it.

2. In one of your experiments you used a friction cart with a fan mounted on it. You carefully adjusted the friction pad so that, with the fan running, the cart still did not move. However, you then made it move by giving it a push with your hand and found that the cart moved at a constant speed after the push. Using ideas about forces, write your own explanation for why the speed of the cart was constant after the push.

Explanation: Why does the cart move at a constant speed after it is given a quick push?

Draw the Force Diagram

Force exerted on

cart by fan unit Frictional force exerted

on cart by track

Write the explanation:

When a balanced combination of forces acts on an object at rest it will remain at rest. With the fan running the cart did not move. This tells us the force that the fan unit exerted on the cart and the frictional force opposing motion were balanced. After being given a quick shove to get it started the force of the push was gone and so the only remaining forces acting on the cart were that of the fan unit and the frictional force opposing its motion, and we know

from before we know that these two forces are balanced. When balanced forces act on a moving object it will continue to move at a constant speed, so with these balanced forces acting on it the cart moves at a constant speed.

3. A child playing with a toy car gives it a quick shove on a smooth level floor. (The car does not have any type of motor inside it.) After his push, the car very gradually slows down and stops.

Four students are discussing why the car very gradually slows down and

stops, after the shove.

Kristen

Daryl

Samantha

Victor

Which student do you agree with (if any)? Please explain your reasoning.

The car slows down because

the force pushing it forward is

getting weaker and weaker.

It slows because, after the shove,

there is no more force to keep it

moving.

It slows down because the forces

acting on it are balanced, and balanced

forces make a moving object come to

rest.

The car slows down because there is

a force acting on it in the opposite

direction to its motion.

I agree with Victor. He has an idea that is consistent with those we developed in class;

that objects slow and stop because a force (or unbalanced combination of forces) on

them in a direction opposite to their motion.

Kristen’s idea is not consistent with our class ideas. She seems to be saying that there

is still a force pushing the car forward after the initial shove, but our idea was that the

force of the shove is gone as soon as the hand loses contact. Her idea that the car slows

down because the forward force gets weaker is also inconsistent with our class ideas.

We said objects slow down because stop because a force (or unbalanced combination of

forces) on them in a direction opposite to their motion.

Daryl’s idea might be OK, but without more information it is impossible to tell. He

implies that for the car to keep moving a force is needed in the direction of motion. If

he is accounting for the effects of the friction force that slows the car down, he is

correct, but he doesn’t say that. On the other hand his thinking could be something like

Kristen’s, in that he thinks that for any motion to continue a continuous forward force is

needed, even in the absence of friction, and without such a forward force it is just

natural for the car to stop. This is not consistent with our class ideas.

Samantha thinks that if balanced forces act on a moving object, then it will slow and

stop. However, in class we found out that when balanced forces act on an object its

speed will remain constant. So if balanced forces acted on the car after the initial shove

then it would not slow down, but continue at a constant speed.

Energy Description of Gravitational Interactions

4. An archer uses a bow to shoot an arrow straight up into the air on a still, calm day. The arrow rises to a certain height and then falls straight back down, sticking in the ground. The speed time graph for the arrow shown below is for the period starting just after it leaves the bow to just before it sticks in the ground.

a) At what time does the arrow reach its highest point? Briefly justify how you know.

The arrow reaches its highest point at 5 seconds. We can tell this from the graph

because it is at this time that it stops slowing down as it rises, stops for an instant

at its highest point, and then begins to speed up again as it falls.

b) Indicate on the graph the entire region during which the arrow was an energy

receiver. Briefly justify how you know.

The arrow was an energy receiver from 5 seconds to 10 seconds. You can tell this

because it was during this period that it was speeding up, hence its kinetic energy

was increasing, indicating it must be receiving mechanical energy in an interaction.

c) During the entire time that the arrow was an energy receiver, what was the energy

source?

During this time the energy source was the gravitational field of the Earth and the

arrow.

5. Imagine holding a small ball at arms length and then releasing it, so that it falls to the ground. (Assume air resistance is negligible.)

a) Is the ball involved in an interaction as it falls?

What evidence supports your answer?

Yes, the ball is involved in an interaction as it falls, because it speeds up as it does

so.

b) Is the falling ball an energy source or an energy receiver? How do you know?

The falling ball is an energy receiver. Since its kinetic energy is increasing as it falls

it must be receiving mechanical energy in an interaction.

c) What other object does the ball interact with while it is falling? Why does the

interaction have only an imperceptible effect on this other object?

The ball is interacting with the Earth as it falls. This interaction has only an

imperceptible effect on the Earth because its mass is very much larger than the

ball.

d) Draw an energy diagram for the interaction that makes the ball fall.

Gravitational Field of Earth

and Ball

Earth

And Ball Mechanical

Energy

Decrease in stored

gravitational field energy

Increase in kinetic

energy of ball

Energy Source Energy Receiver

Gravitational Interaction

(There is also an imperceptible increase in the kinetic energy of the Earth that may

be omitted from the energy diagram.)

6. A small child attempts to push a box full of toys across the floor in his playroom. However, despite pushing as hard as he can, the box does not move. Which one of the following statements best describes the reason the box does not move while he is pushing it.

a. The force resisting moving the box is greater in strength than the child’s push.

The child weighs less than the box does.

b. The force resisting moving the box is equal in strength to the child’s push.

c. The strength of the child’s push is greater than the strength of the force

resisting moving the box, but not great enough to move it.

Please explain your reasoning

If an object is at rest and remains at rest, then the forces acting on it must be

balanced. This means the force of the boy and the force resisting him must be equal.

7. The child in the previous question calls for help and his mother comes and helps him push the box, in the same direction. Which one of the following statements best describes why the box now begins to move.

a. The combined push of the mother and child is equal in strength to the force

resisting the movement of the box.

b. The mother weighs more than the box does.

c. The combined push of the mother and child is greater in strength than the

force resisting the movement of the box.

d. The strength of the mother’s push alone is greater in strength than the force

resisting the movement of the box.

Please explain your reasoning

8. Consider the following three arrangements of battery, bulb and wire(s). Circle the arrangement(s) where you predict the bulb would glow. In the space below the pictures, explain why you think so. If you do not think any of the bulbs would glow, explain why not.

A.

The tip of the bulb touches the positive end of the battery, on the knob. A wire touches the negative end of the battery and the flat part of the positive end of the battery.

B.

The screwy side of the bulb touches the negative end of the battery. A wire touches the bottom tip of the bulb and the flat part of the positive end of the battery.

C.

The bottom tip of the bulb touches the negative end of the battery. There are two wires. One wire touches the screwy side of the bulb and the negative end of the battery. The other wire touches the negative end of the battery and the knob on the positive end of the battery.

In order for the bulb to light both its metal tip and the metal side need to be

connected, one to each end of the battery. When this is done a complete

circuit is established from one end of the battery, through the bulb, and then

to the other end of the battery. Then electric current can flow round the

circuit, through the bulb, thus making it light.

A WILL NOT LIGHT. The tip of the bulb is connected to the + end of the

battery, but the side of bulb is not connected to anything. Electric current

cannot flow through the bulb.

.

B WILL LIGHT. The metal tip of the bulb is connected to the + end of the

battery by the wire (Note: the wire does not need to touch the ‘knob’ on the

end.) and the side of the bulb is connected to the – end of the battery by

touching it directly. Thus current can flow through the bulb and it will light.

C WILL NOT LIGHT. Both the tip and the side of the bulb are connected to the

– end of the battery, the tip by touching it directly and the side by the wire.

Neither part of the bulb is connected to the + end of the battery. Thus

current cannot flow through the bulb and it will not light. (Note: the extra wire

is actually a short circuit that connects the + and – ends of the battery

directly – a high current will flow through this wire and it will get hot.)

9. Students in a class were trying to decide which was a better model for how electric current flowed in a circuit: the two flow model, in which current flows out of both the positive and negative ends of the battery and meets at the bulb; or the one flow model, in which current flows out of one end of the battery and travels all around the circuit, through the bulb, and into the other end of the battery. Below are representations of the two models.

TWO FLOW MODEL

ONE FLOW MODEL

a) Describe one good example of evidence from an experiment you did in class that would support either the two flow or the one flow model, but not both.

In a regular circuit connected like

this, with the two sides of the

bulb holder connected to the + and

– ends of the same battery, the

bulb lights

We did an experiment in class

where we connected the two sides

of a bulb holder to the + and –

ends of two separate batteries,

like this:

In this set-up the bulb did not

light!

b) State which model, the two flow or the one flow, is supported by your evidence.

This evidence supports the one flow model

c) Carefully explain why your evidence supports the model you chose and not the other model. (In so doing it would be best to also describe what would have happened if the other model were a better model.)

This evidence supports the one flow model because in the regular circuit the electric charges can

flow out of one end of the battery, through the bulb, and back to the battery, through battery, and

then back out into the circuit again. This could not happen in the second circuit since the charges

flowing into the battery could not flow through it and back out into the circuit.

The evidence does not support the two-flow model because it says electricity flow out from both

ends of the battery and meets in the bulb, making it light. If this were the case the second circuit

should still work since there are connections that would still allow this to happen from the two

ends of the different batteries.

10. How did the “blowing-through-straws” analogy help you understand the idea that thinner bulb filaments have more resistance to the flow of electricity than thicker bulb filaments (of the same length)? In answering this question, make sure you refer specifically to what was done in the experiments you did in class and how that helped you change or enhance your own model or idea.

Note: Acceptable student responses to this question will vary depending on what

model or ideas they had before being introduced to the blowing-through-straws

analogy, and how they interpreted the analogy itself. Given below is an example of

students using the analogy to move from an idea of the independence of resistance

to variations in thickness, to a definite dependence.

When thinking about resistance of bulb filaments we first though that they would

all be the same, since we thought that the battery supplied the same current to all

bulbs and so the resistance to current flow in all circuits would be the same.

We then blew through a thin straw and a thicker straw of the same length and

made two observations:

i. It was easier to blow through the thicker straw

ii. For the same strength blow, more air flowed through the thicker

straw than the thinner straw

This meant the thinner straw had more resistance to air flow through it than the

thicker straw.

This made us think that perhaps in a bulb it is easier to push the charges through

a thicker filament than a thinner filament. Therefore, for the same push from a

battery, there would be more current flowing through a thicker filament than a

thinner filament. This would then also mean that the thicker filament must have

less resistance to the flow of electricity than the thinner filament. Since more

current also means a brighter bulb, this would mean that when connected to the

same battery (or other power source) a bulb with a thicker filament would glow

brighter than a bulb with a thinner filament of the same length.

We confirmed these ideas in two ways. Firstly we used the simulator and saw that

as we increased the resistance of the bulb in a circuit the current in then circuit

decreased and the brightness of the bulb decreased also.

For an object at rest to start moving, the forces acting on it must be unbalanced. This

means the combined forces of the boy and the mother must be greater than the force

resisting them.

11. Below is a circuit with three batteries, an ammeter and three bulbs. The ammeter reads 337.3 mA, and all three bulbs glow equally bright.

Imagine that the bulb on the left is removed from its socket; everything else remains the same—nothing else is changed. (In the picture below, the ammeter and bulbs are covered so you cannot tell what happens.)

a) Would the ammeter reading be higher, lower or remain at 337.3 mA? Briefly explain in terms of electric circuit ideas.

This is a PARALLEL circuit. In a parallel circuit each loop is an independent circuit

and the current flowing through each loop is determined only by the bulb(s) in it.

The battery will supply whatever total amount of current is needed. In the circuit

above the ammeter is measuring the total current flowing out of the battery,

before it splits and goes into the separate loops.

Removing the left bulb means that loop is now open and no current will flow through

it. However, this has no affect whatsoever on the current flowing through the

other two loops. Since the battery only now needs to supply current to two loops

instead of three, the total current it has to supply will decrease. Therefore the

reading on the ammeter will DECREASE.

b) After the left bulb is removed, would the bulb on the far right get brighter, get dimmer or remain just as it is now? Briefly explain in terms of electric circuit ideas.

As stated above, in a parallel circuit all of the loops are independent of each

other. Therefore, removing the left bulb will not affect the current flowing

through the other loops and the bulb on the right will REMAIN JUST AS BRIGHT

IT IS.

12. To the right is a circuit with one battery and one bulb. The bulb glows with a certain brightness.

In the space below, draw two different circuits.

One must be a parallel circuit and one must be

a series circuit. Each circuit must include two bulbs and any number of batteries that you need. However, all the bulbs in the two circuits must glow with the same brightness as the bulb to the right. Make sure you draw your

circuit diagrams carefully so it is clear how bulbs and batteries are connected to each other. For each circuit, briefly explain (in terms of electric circuit ideas) why all the bulbs glow with the same brightness as the one above.

Series Circuit

In this series circuit there are two identical

bulbs in a single loop. This means there is more

resistance in the loop than if only one bulb were

present and so, with only a single battery a

smaller current would flow, making the bulbs

dimmer. However, by adding a second battery the

push of the batteries is doubled and so the

current flowing will be doubled also. This will

mean the current flowing will now be the same as

in a one-battery, one-bulb circuit, so the two

bulbs will glow with the same brightness as the

single bulb in the circuit above.

Parallel Circuit

In this parallel circuit there are two identical

bulbs connected to a single battery, each in their

own loop. In a parallel circuit each loop behaves

independently, as if it were the only loop

connected to the battery. This mean the current

flowing in each loop in this parallel circuit will be

the same as in a one-battery, one-bulb circuit.

Therefore each bulb will glow with the same

brightness as the single bulb in the circuit above.

13. Students in a class were trying to decide which was a better model for how electric current flowed in a circuit: the two flow model, in which current flows out of both the positive and negative ends of the battery and meets at the bulb; or the one flow model, in which current flows out of one end of the battery and travels all around the circuit, through the bulb, and into the other end of the battery. Below are representations of the two models.

TWO FLOW MODEL

ONE FLOW MODEL

d) Describe one good example of evidence from an experiment you did in class that would support either the two flow or the one flow model, but not both.

In a regular circuit connected like

this, with the two sides of the

bulb holder connected to the + and

– ends of the same battery, the

bulb lights

We did an experiment in class

where we connected the two sides

of a bulb holder to the + and –

ends of two separate batteries,

like this:

In this set-up the bulb did not

light!

e) State which model, the two flow or the one flow, is supported by your evidence.

This evidence supports the one flow model

f) Carefully explain why your evidence supports the model you chose and not the other model. (In so doing it would be best to also describe what would have happened if the other model were a better model.)

This evidence supports the one flow model because in the regular circuit the electric charges can

flow out of one end of the battery, through the bulb, and back to the battery, through battery, and

then back out into the circuit again. This could not happen in the second circuit since the charges

flowing into the battery could not flow through it and back out into the circuit.

The evidence does not support the two-flow model because it says electricity flow out from both

ends of the battery and meets in the bulb, making it light. If this were the case the second circuit

should still work since there are connections that would still allow this to happen from the two

ends of the different batteries.

14. Below are four pictures from the Electricity and Magnetism Field Patterns Simulator, showing different arrangements of magnets with a nearby Magnetic Field Meter. Identify the two arrangements that would produce the highest and lowest readings on the Field Meter.

A.

B.

C.

D.

Briefly justify your answers.

We learned from the simulator homework that strong magnetic fields are produced when many

small magnets are aligned in the same direction and weak fields are produced when equal numbers

are aligned in opposite directions. In D there are three small magnets, all in the same direction, so

this will produce a strong field. In C the four magnets are aligned in opposite directions, so they

cancel each other out to a large degree.

15. A student did an experiment to determine whether the size of a nail makes a difference in how strong it can be magnetically. She used two nails, one large, one small.

Highest Lowest

S N S N S N S NS N S N S N S N

S N S N S N S NS N S N S NS N S N S N

She rubbed the two nails identically with a bar magnet (rubbing the south pole of the magnet from tip to head of each nail). To determine the strength of the magnetic field produced by each rubbed nail, she placed a magnetic field meter (similar to the one used in the simulator) equal distances from the tip of each nail.

Which nail do you think produced the higher reading on the magnetic field meter?

Explain your thinking in terms of the domain model. (Write some sentences and draw some pictures. You do not need to identify the interacting objects, nor draw force or energy diagrams.)

The larger nail will produce the higher reading on the magnetic field meter.

There are more magnetic domains in a large nail than in a small nail. We know from the

simulator homework that when a large number of domains is aligned in the same

direction it produces a stronger magnetic field than when a smaller number is aligned.

Thus when the two nails are rubbed, the larger one has more aligned domains, making it

a stronger magnet.

16. Imagine taking an elevator ride from the1st floor to the 10th floor of a building. While moving between the 1st and 2nd floors the elevator speeds up, but then moves at a constant speed between the 2nd and 9th floors. Which one of the following statements about the elevator best describes the forces acting on it, as it moves upward at a constant speed? (Assume any frictional forces can be neglected.) Circle your choice.

The upward pull of the cable is stronger than the downward gravitational pull of the Earth.

g) The upward pull of the cable is equal in strength to the downward gravitational pull of the Earth.

h) The upward pull of the cable is weaker than the downward gravitational pull of the Earth.

Explain your reasoning

When an object moves at a constant speed the forces acting on it must be balanced. In

this case the only two forces involved are the downward gravitational pull of the Earth

and the upward pull of the cable. For the forces to be balanced these two must be equal

in strength. (If one of the forces was stronger than the other, the forces would be

unbalanced and the speed of the elevator would be changing.)

17. You have observed that mercury rises in a thermometer when placed in hot water.

Carefully draw two particle-level (microscopic) models of the mercury—one at 32 °C and 87 °C.

Describe how your drawings account for the observed thermal expansion of mercury in the closed system of the thermometer.

As the mercury warms up the mercury particles move further away from each other,

so there are fewer in the microscopic window as shown above. Since each of the

particles moves further away, the mercury liquid takes up more volume, that is, it

expands. (Note: it would be wrong to show each particle growing in size when

heated.)

The density of the mercury changes during thermal expansion? Does it increase or decrease? Describe how your drawings account for this change in density.

The density of the mercury decreases as it expands. We know this because the same

amount of mercury is taking up more volume. Since density is mass/volume this ratio

decreases. The drawings show this because fewer particles are visible in the

ultrascope window, meaning there is less mass in the same volume.

18. A scientist collected the following data for ethanol:

i. The melting point of ethanol is -114 °C; the boiling point of ethanol is 78 °C. ii. The specific heat of solid and gaseous ethanol are ~ 0.3 cal/g°C; the specific heat of liquid ethanol is ~0.6 cal/g°C. iii. Boiling liquid ethanol requires twice as much heat energy than melting the same mass of solid ethanol.

Select the graph that best represents the heating curve for ethanol given the data above.

In the space below, explain why you chose the graph that you did. For maximum credit, be sure that your explanation describes how the graph addresses data i, ii, and iii.

Graph A is the correct graph. (i) the melting point is the short horizontal line at -114 °C

and the boiling point is the longer horizontal line at 78 °C.

(ii) the specific heat of solid and gaseous ethanol is twice that as liquid ethanol; specific

heat is the amount of heat energy (related to the amount of time required at constant

heating) required to raise the temperature of one gram by 1°C. On the heating curve, the

greater the slope, the lower the specific heat. On graph A, the slope for the warming

liquid is less than the slope for the warming solid or gas, implying the specific heat for

the liquid is greater than the specific heat for the solid or gas.

(iii) On the heating curve, the length of the horizontal segments are related to the

amount of heat energy required for all the material to change phase. Since the

horizontal line for boiling is about twice that for melting, it would take twice as much

heat energy to boil the liquid ethanol than to melt an equal mass of solid ethanol.

At room temperature both water and ethanol are liquids, but at 90 °C, water is still liquid and ethanol is a gas. What does this suggest about the attractive forces between particles of ethanol in comparison to those of water?

19. Rocky grabs a cold can of soda from the refrigerator and sits down on his soft chair to watch some TV. He falls asleep almost immediately, while still holding the can. After about ten minutes he wakes up and, to his disappointment, the can of soda is no longer cold. Explain why the can warms up by filling in the following steps.

What is the type of interaction that causes the soda can to become warmer and

what are the interacting objects?

The main interaction that causes the soda can to become warmer is the heat conduction

interaction between Rocky’s hand and the can.

a) Draw a Source/Receiver energy diagram to describe the

interaction. Include the interacting objects, and label the interaction

type, energy transfer and the energy changes within the interacting

objects.

Can

Heat Conduction Interaction

Increase in

thermal

energy

Heat Energy Hand

Decrease in

thermal

energy

b) Write a few sentences to explain why the can becomes warmer.

While Rocky is holding the can there is a Heat Conduction Interaction between his warm

hand and the cold soda can, during which there is a transfer of heat energy from the

hand to the can. This input of heat energy to the soda can means that its thermal

energy increases. This increase in thermal energy means of the soda can means that its

temperature rises and so it becomes warmer.

20 (5 points) A boy has a ball in his hand. He then tosses the ball straight upward. The ball rises to a certain height and then falls back down, and is caught by the boy again.

Below, sketch a speed-time graph for the entire history of the ball’s motion. Use the following labels to show on the graph each separate part of the ball’s motion.

TOSSES RISES FALLS CAUGHT

Tosses

Rises

Falls Caught

i) Draw a force diagram for the ball for the part of the motion where it is

being tossed upwards. Make sure you label the force(s), pay attention to the length(s), and also include a motion arrow.

j) Draw an energy diagram for the part of the motion where the ball is falling back down, but before it is caught. Consider the ball and earth as a single system.

Earth

And Ball

Increase in

kinetic energy Decrease in

gravitational potential energy

Force exerted

on ball by the

hand

Gravitational

force exerted on

ball by the

Earth

21) Imagine you are standing waist deep in water in a lake at night. You have a flashlight in your hand. A friend who is under the water wants you to shine your light on a non-shiny white object at the bottom of the lake, so he can see it. Below is a side view diagram of your eye, your friend’s eye and your flashlight. Also shown is the surface of the lake, the bottom of the lake, and the object. Carefully draw a light ray diagram, showing how light goes from your flashlight to the object, and how your friend can see it. (You may re-aim the flashlight in any direction you think is necessary.) You do not need to write an explanation.

22) After performing some experiments with magnet-rubbed and un-rubbed nails, students were asked to propose a model to describe the difference. Some students came up with the following model:

Before Rubbing After Rubbing

a. What magnetism behavior can this model account for quite well? There are a few behaviors that this model can account for quite well. For example: the

observation that when a nail is rubbed with a magnet its ends behave differently, with

like ends attracting and unlike ends repelling. It would account for this by saying that N

and S entities attract each other, but N and N, or S and S, entities repel each other.

(It could also account for the observation that when a rubbed nail is hammered it

becomes un-rubbed, by saying that hammering the nail mixes up the N and S entities

again.)

b. What magnetism behavior can this model not account for at all? This model cannot account for what is observed when a rubbed nail is cut in half.

According to this model when a rubbed nail is cut, the tip half will have all the S entities

at the tip end (making a South pole) and nothing at the cut end (making No pole).

Meanwhile the head half will have all N entities at the head end (making a North pole)

and nothing at the cut end (making No pole). However, we know from observation that

when a rubbed nail is cut in half, each half will have both a North and South pole.

N NoP SNoP

Model Prediction

N S SN

Observation

23) If you hold the head of an unrubbed nail near the North Pole of a magnet, the nail becomes magnetized with its head acting like a South Pole.

Suppose you then turned the magnet around, as shown to the right. What do you predict will happen to the nail? (Circle one of the following choices.)

c. the nail is magnetized, with its head a north pole d. the nail is magnetized, with its head a south pole e. the nail becomes un-magnetized

Justify your prediction in terms of the domain model of magnetism.

When the head of the un-rubbed nail is brought close to the North Pole of the magnet

the south end of all the domains in the nail are attracted to the magnet and the north

ends of the domains are repelled. This means that the domains will line themselves up

with all the south ends pointing toward the magnet, making the head end a South Pole.

N S N S N SN S N S N S

N S

Now, when the head of the un-rubbed nail is brought close to the South Pole of the magnet the north

end of all the domains in the nail will be attracted to the magnet and the south ends of the domains will

repelled. This means that the domains will flip over and line themselves up with all the north ends

pointing toward the magnet, making the head end North Pole.

S N S N S NS N S N S N

S N