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Hi, this is a quick rundown of the e/m setup and apparatus
because e/m is a very important experiment, there are lots of
interesting physics going on, but there are a few connections in there
so we want to give you this ahead of time, so when you are in the lab, you can get a good sense of the actual physics and appreciate it.
ok, so the e/m apparatus here consist of these 4 things:
the black box, the red box and a couple multimeters
The black box houses the bulb, as well as the electron gun, and the Helmholtz coil which produce the magnetic field
The power supply supplies the power
and we are using the multimeters to get better readings of the voltage and current
So, let's take a closer look at what's inside the black box
So the black box here looks quick dark because that the only way we will see the beam of electrons coming through, but let's open it up...
inside *grunt*
So inside here, there is the Helmholtz coil which has current going around and around.
That's going to produce a uniform magnetic field that goes in and out
you can figure out the specific direction if you wish.
This glass thing here is the bulb. Inside, it's filled with a certain gas that as the electron moves through it,
it excites the gas and we can see the path of the electrons.
While we don't see the electrons themselves, we are definitely seeing where the electrons are going
and this metal part here, that's actually
a tiny filament
which we heat up so it bleeds out electrons, that's an electron gun.
Then there are a couple parallel plates in there that accelerates the electron to go at a
nice fast speed so that it will be affected by the magnetic field
ok, let's start hooking this thing up. We need to connect the red box to the black box
to supply a few things. We talked about the heater current which is for the filament that
you shoot electrons out of, so it needs heating. That's coming out of the AC source here.
The accelerating voltage to speed up the electron, that's the 0-500 V supply
and then also the Helmholtz coil needs quite a bit of current, up to 2 A, so we will use the 5 A current source.
so let's get going...
and that's good. You will notice that we haven't actually hooked in the multimeters yet
because there are a few things we have to go through there. But let's get thing fired up and see how it goes.
So the two adjustments you will do here is: through this ***, you will adjust the ...
accelerating voltage
and through here you control the current. Usually you have these other ones on max.
This is the current control here for the magnetic field
and take a look
So it's just coming through fairly faintly on the camera, I do apologize, but in the lab it's going to be a lot better
so knowing that the electrons are shooting out downwards, and knowing which way the force is acting on it
we can actually figure out the direction of the magnetic field as well, but I will leave it up to you to do.
Some qualitative things we can do around here is we can think about...
if I decrease the coil current, is that going to make my circle bigger or smaller? So here we go
I am going to decrease my coil current
so hopefully you can see that it is getting a little bigger, and if I increase the coil current again
of course that's going to increase my magnetic field, and increasing my magnetic force
Also, I can also adjustment my accelerating voltage.
If I decrease my accelerating voltage, it's going to make my electrons go slower
and that's also going make my circle smaller and smaller
Of course, in this lab were trying to maintain a certain radius
while changing the voltage and current. so we'll go back and talk about how to get some good numbers.
So as I was saying, in order to get good values for your
e/m, we will need
good numbers for voltage and current. Unfortunately, while these are shown here
they are not down to that much precision. Escpecially the one for the current.
so we will use these digital multimeters. Hopefully, you know how to use these already.
If not, there will be a detailed video about these
also posted
In any case, for the digital voltmeter, you always use the "COM" anyways. For the voltage, you would put the other lead on the "V"
and you switch it over to Vdc
The voltage and current are fairly high here, so make sure you turn it off before you make any connection.
The voltage is the easy one. You don't have to disconnect anything
You just go parallel to it
now that is no connection here, so I will just
swap this through
This is for my accelerating voltage
then the more important one
is the current
for the ammeter, of course once again, you would use "COM"
and switch it over to Adc
The important thing here, you want to use the "10 A" scale and not the "300 mA", becasue we are expecting current of up to 2 A.
If we have as high as 2 A goes through this one (300 mA), we will blow the fuse.
This, though, you connect in series, so you have to disconnect the circuit somewhere
probably here
The positive goes to the positive side and the negative goes back
double-check nothing is touching or shorting out
Adc and Vdc
Turn it on again
so while this is reading 243 V, this is reading 245.0 V
Both of these are giving me extra digits and from that you can good graphs and hopefully find a good value for e/m
Thanks, we will see you in the lab