Hi. It's Mr. Andersen and this AP Physics essentials video 47. It is on measuring the
magnetic force. In this video a friend of mine is dropping a magnet through a copper
pipe. And watch how long it takes for the magnet to make it through. So then I took
a video of it from above and you can see that it is almost hovering as it moves through
the copper pipe. So what is going on? Well as the magnet falls through the copper pipe
it is inducing a current in the copper wire. And that copper wire is then producing a magnetic
force which is slowing the descent of the magnet. It seems magical but it is simply
a magnetic force which always occurs if we have an electric charge or a magnet interacting
with another electric charge or another magnet. And so what are magnets? Remember they are
essentially magnetic dipoles. They have a north and they have a south. And a way to
visualize that is like a vector that goes right through the middle. And so the north
is at the tip of the arrow. And the south is at the end of the arrow. And you cannot
break a magnet it half. What you will get is going to be just two new magnetic dipoles.
You can never separate the north and the south pole. Now let's say we put that magnet in
a magnetic field, what will it do? Well there will be a magnetic force that is going to
cause it to orient towards that magnetic field. But it is not just magnets that are influenced
by other magnets. If we have a moving charge moving through a magnetic field there will
be a magnetic force on that and we can use the right hand rule to figure out where that
force is going to be. And so a magnetic dipole remember has a north and a south. And an example
of one could be the earth. An example of one could be a compass needle. It is simply a
tiny little magnet. And so if we put these inside a magnetic field there will be a magnetic
force which causes them to orient towards that magnetic field. Now all magnetic fields
are not just linear like this. And so if we take a compass and put it all the way around
a magnet like this they will turn every which way, which seems confusing, until you start
to visualize where these magnetic field lines go. And a good way to do that is to use iron
filings on top of it and you will start to see how each of these magnetic dipoles is
orienting itself toward the magnetic field. And a good way to do that is to use iron filings
on top of it and you will start to see how each of these magnetic dipoles is orienting
itself towards the magnetic field. Now scientists knew for a long time that there was a relationship
between magnets and electric charge. But when they would hook a wire up and put a bunch
of compass needles around it nothing would happen, until they ran current through it.
And as you run current through that wire all those compass needles are going to line up
towards a magnetic field. And so a moving charge is producing these magnetic fields.
And so a way to figure out which was they are moving is with the right had rule. And
so if we look at this wire right here, the electric current is coming towards you. And
so what you do is you point your thumb in the direction of current flow and then your
fingers are going to wrap around in the direction of that magnetic field. And so if we have
a wire like this, so the current is moving from the bottom to the top, can you use your
right hand to figure out where that magnetic field is going to be? It is simply going to
wrap around like that. It is going to wrap around thumb in the direction of the current
and just wrap your fingers around. But it is not just right there. It is going to be
all through out the wire. And so an interesting question might be, well, that is a linear
wire. What happens if we take the wire itself and put that in a loop? What is the magnetic
field going to look like? Well it will look like this. And so now we have the current
coming in on this side. So the current is coming in on this side, and so you could just
kind of run your thumb around that loop. And what you will notice is as you move your thumb
around the loop the magnetic field is always going to go right through the middle on the
inside of that wire. And so by running current through a loop of wire, what you really create
is a magnetic dipole. You have that magnetic field produced right through the middle of
the loop. And so let me show you an example of how that works. Now we have a compass,
this is a PHET simulation. I am moving the compass around this magnet. So you can see
that it is starting to orient towards the magnetic field produced by the magnet. And
so we can turn on those magnetic field lines and you can see that it is orienting towards
them perfectly. It is lining right up. Now what we can do is run electricity through
a wire. I now have a compass, moving it around this wire, but it is not responding. And that
is because there is no current. When I turn it on then we can see how the compass is starting
to orient toward that magnetic field that is being produced by the current in the wire
itself. And so we could turn on those magnetic field lines and you can see it looks just
like a magnet. We can actually, if we are careful, move it right through the middle
of that loop. Now what would happen if we increase the number of wires to the electric
field or rather the magnetic field? You can see that it is getting greater and greater.
And that is why there is this link between electricity and magnetism. And so a neat experiment
you can do with this is using a solenoid. And so what we have is a wire that is just
twisted over and over and over again. Now in the lab a really good thing to use as a
solenoid would just be a slinky. You can stretch it out on a table. You can kind of tape it
down or clamp it down so it does not move. In this lab what I have here is a power supply,
so that is producing current. And then we have a rheostat so we can change the amount
of current that is actually making it through the wire. And so before we get to the actual
lab, where is the magnetic field going to be produced? Let's say the current is moving
from the right towards the left, where is the magnetic field going to be? It is going
to be right through the middle of the solenoid like that. And so we can use a magnetic probe.
There are lots of probes you can use where you can put the magnetic probe right in the
middle of the solenoid. And it is going to measure in teslas the strength of that magnetic
field. And so I can think of a number of different labs you could do here. So we could change
the current. What do you think is going to happen as we increase the current to the strength
of the magnetic field? What else could we do? We could have the wires clamping on at
different points. And so we could have different coils inside our solenoid and see how the
number of coils is influencing that magnetic field. Or we could take our slinky and bring
it really close to itself or stretch it way out. We could increase the distance of that
solenoid and we could measure that magnetic field as well. So this is a really cool inquiry
kind of an experiment you could do. And so did you learn to use the right hand rule to
figure out where the magnetic field lines are going to be? Again, point your thumb in
the direction of current and then just wrap your fingers around. And then finally, can
you see how we could plan the collection of date to measure how that movement of current
affects the magnetic field? I hope so. And I hope that was helpful.