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The Discovery of the Electron
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About the lecture
In this mini-lecture, we discuss the discovery of the electron. In particular, we consider: (i) a cathode ray tube demonstration; (ii) a demonstration of crossed fields where perpendicular magnetic and electric fields are set up, and an electron beam in the system can be bent upon manipulation of the magnetic field; (iii) the left hand rule, involving force, magnetic field, and current; (iv) J.J. Thomson’s cathode ray tube experiment used to determine the specific charge of an electron; and (v) Millikan’s oil drop experiment that was used to determine the charge of an electron.
About the lecturer
Janet Lovett is a Lecturer in the School of Physics and Astronomy at the University of St Andrews. Her primary research interest is in measuring nanometre distances for structural biology using Electron Paramagnetic Resonance (EPR) spectroscopy. Dr Lovett teaches a first-year ‘Properties of Matter’ course at the University in addition to other courses such as magnetic resonance.
last time we looked at the makeup of atoms,
00:00:06and we looked at some historical experiments that
00:00:10were used to create useful early models.
00:00:12This time we're going to look at some early experiments
00:00:17that looked at exploring the existence and properties of the electron
00:00:19and specifically in this lecture, its charge.
00:00:22There were many important experiments and discoveries,
00:00:29but what I want to show you now is a cathode ray tube.
00:00:32Here we have an evacuated glass tube. At one end. There is a cathode
00:00:37and at another
00:00:42and an ode
00:00:44to be precise.
00:00:45This is a cold cathode discharge tube,
00:00:46which means that there is a residual amount of air left inside,
00:00:48and these were invented before hot cathode devices,
00:00:52which would have been the ones you'd find in old TVs and computer monitors.
00:00:55They're often called CRTs.
00:00:59If I turn on the electrical potential,
00:01:02we see the paddlewheel begin to spin and move away from the catheter.
00:01:04This seems to be evidence that the cathode
00:01:08was producing a stream of negatively charged particles.
00:01:10Indeed, it turns out that this is correct,
00:01:14but actually later calculations showed that the
00:01:16electrons are not directly moving the wheel,
00:01:19but instead causing it to heat.
00:01:21This in turn, heats the gas particles remaining in the glass tube,
00:01:24and this causes an expansion of the volume of the gas.
00:01:27And this is what pushed the wheel around
00:01:31the next type of discharge tube I want to show
00:01:34you is one that allows for so called crossed fields.
00:01:36This means an electric field can be set up perpendicular to a magnetic field.
00:01:40But here I just have a bar magnet to produce the
00:01:45electric field so it can be moved around for this demonstration.
00:01:48Here we have the electron beam showing up on this special paper as a blue line.
00:01:53We have an electric potential between the plates
00:02:01and the electron beam is, of course, attracted to the positive plate.
00:02:04Now I have a magnet, and this is the north face.
00:02:08So remember the field lines will come out of the
00:02:12north face and round and into the south face.
00:02:13So if I pull this magnet so that it's next to the electron beam,
00:02:15we see that the beam is deflected downwards.
00:02:21If I bring the magnet right round, we see the beam is deflected further up,
00:02:25and this is the same as taking the south face of the magnet
00:02:31and putting on this side.
00:02:34Oh,
00:02:37how to understand what's happening?
00:02:38Well, we use the left hand rule
00:02:40and,
00:02:42um,
00:02:44we put our
00:02:45thumb
00:02:47for the direction of the Force
00:02:48F
00:02:50our four finger for the magnetic field B
00:02:51and our middle finger for the current i
00:02:57f B I.
00:03:01So remember the current
00:03:03by prevention runs from positive to negative,
00:03:06so in the opposite way to the electron beam.
00:03:09So we put our
00:03:13middle finger that way,
00:03:15our four finger pointing in the direction of the magnetic field.
00:03:17So the field lines coming out of the north face
00:03:22and our thumb shows us that the force will be downwards, which is
00:03:24exactly what we see.
00:03:30J. J.
00:03:33Thompson used this type of experiment to measure the deflection
00:03:34of an electron being by electric and magnetic fields.
00:03:36He found it to be independent of the properties of
00:03:39the cathode ray tube or the type of gas inside,
00:03:42and he was able to quantify the specific charge of the particles in the beam.
00:03:45Specific charge just means the ratio of the charge of the particles to their mass.
00:03:50The results showed that the specific charge
00:03:55of the electron was about 1000 times higher
00:03:57than Faraday had earlier found for the specific
00:04:00charge of hydrogen Qatar Iron I a proton
00:04:04was the electron therefore really light or really highly charged?
00:04:09Other experiments showed that the charge on the
00:04:15electron was the same as for the proton,
00:04:17but opposite inside.
00:04:19And so the electron must be much lighter.
00:04:21But what is the value of this quantized charge
00:04:24the value we now call E
00:04:27through knowing this,
00:04:29we can find the accurate mass of the electron
00:04:31and therefore the proton.
00:04:34Finding E was the goal of a very famous experiment.
00:04:37Millikan Oil Drop.
00:04:41A spray of tiny oil droplets is placed in this chamber,
00:04:46and you can see them fall due to the force from gravity,
00:04:50although you'll notice they actually appear to rise.
00:04:54And that's because of the way we're viewing it through a lens,
00:04:56and that makes it appear upside down.
00:04:59The mass of the oil droplets can be calculated
00:05:02by measuring the rate at which they fall.
00:05:04Now the chamber here actually has metal plates at its top and bottom,
00:05:07so that an electrical potential difference can be applied
00:05:11and therefore an electric field force applied.
00:05:14Since the oil droplets have acquired a negative charge,
00:05:16the positively charged plate is at the top of the setup, which remember,
00:05:21looks like the bottom now through the lens.
00:05:24And so when the voltage is applied, which is happening now,
00:05:27the negatively charged oil drops are attracted to the positive plate.
00:05:30The electric force is modified in the
00:05:36experiment until an oil drop appear stationary.
00:05:38This is when the electric and gravitational forces are balanced.
00:05:41Since the mass of the oil droplet has already been found,
00:05:45it's now possible to find the charge on the droplet.
00:05:49It was found that the charge on the oil drops was always
00:05:52an integer multiple of 16 times 10 to the minus 19 Cullen,
00:05:55which is an incredible result,
00:06:00given the fairly basic nature of the equipment and the
00:06:02assumptions needed to calculate the mass of the droplets.
00:06:04But it's very close to what we now know to be the true value,
00:06:08which is 1.602176634 times 10 to the minus 19 cologne.
00:06:11In this section, we looked at the electron and specifically the discovery of the,
00:06:19uh, property of the charge of the electron in the next section,
00:06:24we're going to look at the nucleus and some nuclear physics.
00:06:29
Cite this Lecture
APA style
Lovett, J. (2022, January 12). 18. Electric Fields - The Discovery of the Electron [Video]. MASSOLIT. https://massolit.io/options/18-electric-fields?auth=0&lesson=4478&option=14780&type=lesson
MLA style
Lovett, J. "18. Electric Fields – The Discovery of the Electron." MASSOLIT, uploaded by MASSOLIT, 12 Jan 2022, https://massolit.io/options/18-electric-fields?auth=0&lesson=4478&option=14780&type=lesson