MICHELLE STRAND: Hi, I'm Michelle Strand.
Welcome back to the AP Daily practice
sessions for AP Physics C, Electricity and Magnetism.
Today we're going to practice multiple choice questions, also
known as MCQs.
So just some general strategies for multiple choice questions.
One is to read through the multiple choice question.
If you feel you can answer it quickly, do so,
and then move on.
If you feel like you're not sure,
but can eliminate some obviously incorrect answers,
then do that and move on, coming back to the question
after you've gone through all 35.
If you have no idea, skip the question.
Move on, come back to the question
after you've gone through all the
35, because you have 45 minutes to answer 35 multiple choice
questions.
So you have just over a minute per question.
So you have to be strategic about it,
and you don't want to spend too much time on early questions
when there might be easy questions near the end.
So let's take a look at a few examples.
Here we have question 1.
This is a set question.
So the first three questions are actually from the same prompt.
And we have two wires, wire X and wire Y. You are given--
in this particular example, you are given the information.
This is called the stem of the problem.
So it's giving you the verbal information
to match the picture.
And then it asks the question.
So we have five possible graphs here.
It's asking us how to--
which graph correctly would indicate the magnetic force
versus the voltage in the wire?
So in order to figure out what the graph would look like,
we need to understand the relationships.
So I do know that magnetic force--
one of the equations is ILB.
So the current times the length times the magnetic field.
And I know from my circuits, right?
The V equals I times R.
So if I substitute the I into the magnetic force equation,
then you can see that the force--
the magnetic force and the voltage are directly related.
And so we can now go through.
And it's not going to be a curve.
It's not constant, meaning that there's no effect.
And we could cross out C and E the same way
we crossed out A and B because it can't be a curve.
It has to be a direct relationship.
So moving on to the second part of this question.
Again, it's a three part set.
You notice that the same diagram and same information
is given to you here.
But now we have a new question, right?
It wants to know which of the following
could be true of Y, wire Y. And this
is what we call a Roman numeral question.
So it gives us three statements about the wire Y,
and it wants us to decide which of those are correct.
It could be none of them.
It could be one of them.
It could be two of them.
It could be all of them.
We have to decide.
Sometimes the easiest way is to cross out the ones that
make no sense at all.
So we know that if we apply a voltage to wire X,
it creates a current, and the wire experiences
a magnetic force toward wire Y.
Well, we know from Newton's third law,
that if there is a force this way,
then wire Y has to be a force in the opposite direction, right?
Equal and opposite forces.
So two can't--
Roman numeral II can't be correct.
So that crosses out really any choices that
have Roman numeral II in them.
And then it says it experiences a force of different magnitude
than the force on wire X. And that
doesn't match with what I know about Newton's third law
either.
So Roman numeral II and III are incorrect,
which means I have to look at Roman numeral I.
And decide whether it's correct, because, obviously,
at this point, I've crossed out the option for II.
I've crossed out the option for III.
I'm left with none or 1 only.
And this is about when you have attractive force,
it means the current is in the same direction.
And so Roman numeral I is correct.
Here is the third part to that three part question.
They are asking here, if the distance between the two wires
is tripled, what's the magnitude of the new magnetic force
exerted on wire X?
So this one, again, it sometimes helps to cross out
answers that make no sense.
So if I make the distance bigger between the wires, right?
Thinking about should the magnetic force
be bigger or smaller?
Well, if they're separated by a bigger distance,
I would guess that they're smaller.
And so that way, I can eliminate either equal or the bigger
options.
So I'm left with two options.
And one is essentially meaning that as I make it
like three times bigger, I'm going
to cut the force by three.
In other words, it's an inverse relationship.
Or I cut it by nine, which would be an inverse squared.
And the equation for the force divided by length
is mu naught, which is the constant, I1 I2 over 2 pi r.
And so the r relationship is the distance between the wires.
And obviously that is an inverse.
It's not squared.
So that means that B is the correct answer.
So this example of a set uses the same prompt,
but you're asked three separate questions
about that same prompt.
Moving on to another question.
This one does not have a diagram with it.
This one is simply asking--
it tells us an expression for the electric potential,
and it wants to know where the electric field is zero.
So this is a question where you have to figure out
what the relationship is between the electric field
and the electric potential.
And if you remember, the electric field
is the negative derivative of the electric potential.
So I need to take the derivative of this expression.
And so if I take the d-- and this is going to be ax minus bx
squared dx, what I end up with is minus--
and I have a minus 2bx.
And we want the electric field to be zero.
And so this is going to be 0 on the left-hand side,
and that's going to equal minus a plus 2bx.
And if I move the a to the other side
by adding it to both sides, then I
end up with x equals a over 2b.
Zero is not going to make any sense
because it's just a derivative.
At no point is not going to make any sense.
And we've just shown that it's equal to a over 2b.
So now we have what I call a what if question, where
it's asking us-- it's giving us a scenario,
and now they say we're going to change something,
and they want to know what happens to the power.
And so for this one, right?
We're talking about two resistors connected in series,
and now they're going to be connected in parallel.
They are connected to the same battery.
So remember that power is I times V.
But with V being I times R, we want--
if I plug--
I want to solve this for I. So I have V over R.
And so I'm going to plug this in where I is,
and I end up with power equals V squared over R.
So we know V is the same.
But the power is an inverse relation to the resistance.
When I'm in series, if I have two 5 ohm resistors in series,
I have to add them together to get 10 ohms.
When I have two in parallel, we do the 1 over.
So we have the 1 over 5 plus the 1 over 5
is going to give us 2.5.
So comparing 2.5 to the 10 when I had them in series, right?
There is a times four, a multiple of four that's there.
And again, it's thinking about is the power dissipated
going to be bigger or smaller?
Well, if I have less resistance, I'm
going to have more power, right?
According to this relationship here.
And so I can eliminate those three options.
And now it's a matter of how is it linear, or is it--
it's linear here.
And we know that it's a times four difference, right?
I cut the resistance by four when
I'm in parallel, which means I'm going
to have four times the power.
And so number 6 is another set.
This one has three parts again.
So this one has a diagram of equipotential lines.
And they show us some different points that
are in the plane of the page.
And it wants to know for this first part,
which two points can a negatively charged particle
be moved from rest at one point to rest at the other
with no work being done by the electric field?
So remember that the work done is
equal to the change in energy.
In particular, the change in potential energy
is q times delta V.
So if the change in the delta V, the change
in the volts or potential is zero, then the work done
or the energy would have to be zero.
And so the only option that happens
is A and C are on the same equipotential lines.
And our final question with this one
is we want to know the electric potential.
How can the electric potential be created by which
of the following?
So again, it's thinking about if I'm
going to get a flat line of zero that I'm showing in the picture
here, I have to have--
I can cross out wrong answers, right?
I know there's a negative.
I know there's a positive.
So we can't have a ring of positive charge.
We can't have-- right, we can't just
have all negative or all positive.
And I need to have--
in order to have it--
have that zero flat line, it has to be symmetric.
So that's why it has to be two lines of charge
as opposed to a line and a negative point charge.
So a number gives us a diagram.
It talks about a solid conducting sphere.
The surface-- all of the charges uniform on the surface.
And it wants to know the potential
at the center of this sphere.
So this is one of those that's asking us
about the electrostatic equilibrium rules.
And since this is a conductor, again, all the charge
is on the surface.
And so there's no electric field inside this particular sphere.
But there is a potential inside this particular sphere.
It's a positive charge.
So it can't be the negative answers here.
It's not zero because there's a charge.
And it is not undefined.
One of the rules in [? equistatic ?] equilibrium
is that your potential is the same everywhere,
from the surface all the way in.
And so A is the answer.
And that's-- remember that this 1 over 4 pi epsilon 0 is also
sometimes known as just K. So this is the K Q over R
for potential.
And then we have our last one.
We've got some magnetic field.
Five particles are moving, and they
pass through a magnetic field, and they
want to know which one's a positive charge.
So when a charged particle moves in a magnetic field,
then it's going to curve.
That's the number one thing.
It has to curve.
So it can't be choice E.
A, C, and D are all curving in the same direction,
if you look at them closely.
So if they're all in the same direction,
they can't be what we're looking for.
And so that leaves us with B. So E is first to cross out.
And then A, C, and D are all in the same direction.
They would be the same answer.
We can only have one answer.
So it has to be B.
So I want to thank you for your time.
Good luck.