Electrostatics (1958)

this is a metal surface which can be electrified we'll explain exactly what this means in a moment and also how it's done right now let's see what happens I've got to operate the machine but you watch the balloons [Music] [Applause] here obviously is a force of some kind and it's not gravity we've never been able to turn gravity on and off nor is it magnetism who ever saw a magnet attracting something made of rubber yet it's an extremely important kind of force for the pool which held these balloons together is in fact identical in character to the poles which hold atoms together for instance it's the same pole which stops this drop of water from falling apart it may seem a big step from the attraction this comb has for this paper - the strength in this enormous cable or for the still greater strength weight for weight in this tiny thread of a spider but it's this connection which is going to occupy us for much of the rest of the course but electricity means not only force it also means light indeed electrified particles are the original cause not only of light but in every form of radiant energy here's an example but we will need the lights off to see another case is the spark and of course lightning this film is really about electrostatic charges what types there are how they behave how they're produced and so on now let's see if we can use these electrical forces to find out something more about charge now here's a coat hanger made of plastic I'm gonna charge this by rubbing with a wool scarf friction is a very general method of giving materials and electric charge think of when you walk across a carpeted floor and get a shock when you touch the doorknob now I've suspended this and I'm gonna take another material this time of brush made of a different plastic and I'm going to charge this with a different material of Dacron sock now we find that the brush attracts the hanger what about a comb that charges and this time we get a repulsion quite strong now this is something quite new we've never met a repulsive force before and it proves once again that it can't be anything to do with gravity gravity never repels it also proves that there must be at least two ways in which a body may be charged these two charges are called positive and negative now let's see what the reaction is between these for this I'll have to suspend the brush which is not quite so easy there we are now I'm recharging this because things don't hold their charge indefinitely and this time we get an attraction apparently differently charged bodies attract each other how about similarly charged bodies well we can find out this by taking an identical coathanger charging it and the answer is they repel this turns out to be a general rule all sorts of materials can be given a static charge sometimes positive sometimes negative but always oppositely charged bodies attract and similarly charged bodies repel but what are these charges what is their mechanism now a warning we haven't as yet given any evidence for this but later in the course we shall find that the mechanisms as follows the two kinds of charge which we've just discovered are mirrored at the atomic level where again there are two kinds of minut charged particles positive and negative now in any ordinary chunk of stuff this is meant to be a scarf their numbers balance their charges cancel out and the result is electrical neutrality this is why neither comb nor scarf normally show any reaction now the negative particles or electrons are fairly easily detached according to this fuel charging is the transfer of electrons from one body to another we rob Peter to pay Paul here the combs receiving a surplus and so becomes negative whereas the wall is left for the deficit and is therefore positive now the next thing to find out is whether these surplus electrons can move about or migrate within a charged body suppose we take a charged comb and touch this metal wire if the surplus electrons are free to move along the wire down this metal rod to those two leaves both the leaves should receive a similar charge and so repel each other now watch and charging the comb and I touch the wire the charge on the sphere is instantly shared with the leaves all metals are good electrical conductors now here we have a string instead of the wire and this time we get no repulsion now suppose we wet the string this by the way is why electrostatic experiments work better in dry weather now we get a slow repulsion because water helps to move charges about the difference between insulators and conductors is only one of degree for example in this respect the human body is about halfway between a good conductor and a good insulator they're good enough conductors for us to be able to discharge objects by touching them here the charge has leaked away through my body to earth this is called grounding cam charges be transferred from one body to another well we've already assumed they can be by simple contact as when we grounded the leaves but here's a more ingenious demonstration now we're going to give this plate a negative charge which means that it'll always have a supply of surplus electrons to do this we use a machine similar to the one you saw at the beginning of the film and this ping-pong ball has been painted with aluminum to make it conducting and when it hits the charge plate it'll pick up some electrons become similarly charged and therefore be repelled when it hits the uncharged plate it'll give up some electrons and so again be repelled therefore the ball ought to bounce to and fro like a pendulum well we switch on and stop the ball now why is the ball going slower the Machine still going let's stop and think each time the ball hit this plate it transferred some more electrons so that the charge on the plate built up progressively and this made it harder and harder for the ball to approach if this explanation is right we should be able to remove the charge on this plate by grounding and so restore the situation we had at the beginning right grounded grounded again and each time the ball goes faster so much for transfer of charge by content and now induction of charge I'll have to give the hanger another rub for this we take an ordinary table knife uncharged I'm grounding it now watch now this really is extraordinary this was an uncharged knife and yet it must have become charged when it came near the hanger but how this isn't charging by contact there is no contact the explanation is that the surplus electrons on the hanger push the movable electrons along the conducting metal knife toward the handle thus creating a shortage on the blade this leaves the handle negative and the blade positive but since the blade is nearer to the hanger than the handle its attraction will overpower the handles repulsion such a separation of charge is called induction now this is not easy to demonstrate we need a method of labeling charge surfaces and since this particular methods rather messy I put on a lab coat on gloves here is a yellow and a purple powder and here is a mixture of the two when shaken together friction between the grains gives each powder an opposite charge will check that the mixture works first using these two plates both of which have also been given an opposite charge now watch the grains separate the yellow go to the negative surface and the purple to the positive don't of course think that charges are colored this is just a method of labeling them next we'll use this globe to represent the table knife because the shape of the table knife raises problems we will try and induce a charge on it using the usual generator the generator will be negative so the induced charges should be positive purple negative yellow let's try as predicted positive negative now let's get all this mess cleared after you go back to the hangar but charged bodies attract not only uncharged conductors like the knife but insulators too here's a glass bottle or a wooden handle brush or a lever shoe now in these cases we know there cannot be no simple migration of electrons over the insulating surface it must be that the negative part of each insulator molecule is pushed away slightly and the positive part pulled nearer such a distortion is in detectable small in the case of a single molecule but when totaled up over billions it does result in a net separation of charge of course this now comes as no surprise here's the machine for generating static charge which we used at the very start it's called a Van de Graaff after its inventor and we're now in a position to understand in principle how it works indeed there's no reason why you shouldn't construct your own here as a junior model which has the advantage that it can be taken apart the vandegraaff consists essentially our hollow metal sphere separated from the rest of the machine by an insulating column the aim is to accumulate a charge in the form of surplus electrons on the sphere and the electrons are carried to the sphere on an endless insulating belt which travels between two pulleys here and here and is driven by a motor the electrons are sprayed onto the lower end of the belt from the teeth of a charged metal comb the comb is charged either by induction or from an outside source the electrons are removed by a second comb after being conducted to the sphere they push each other away and so spread out all over its surface and here is the biggest generator of this type ever built now a museum piece we ourselves are now inside the sphere which is so large that it was originally designed as a small laboratory in this machine three belts are used and here are the combs here's the machine in action [Applause] [Applause] don't think however that the scientific purpose of Van de Graaff s-- is to produce sparks many are used to produce x-rays our machines are used for atom-smashing this is the target area at the bottom of the world's most powerful vandegraaff now we need a convenient and sensitive way of detecting charge the powder method was altogether too clumsy for this purpose a good instrument is the electroscope it's essential feature is a pair of metal leaves whenever this rod becomes charged the leaves fly apart now I'm going to charge the electroscope by simple contact more usually charging is done by induction and the steps involved in this will serve as a useful review we saw that a charged body can induce a charge in nearby objects which were previously uncharged both insulators like the bottle and conductors like the rotten leaves charges can also be transferred by direct contact a particular case is grounding here it's the negatives which are grounded because the positives are still strongly attracted by the comb materials in which charges are free to migrate are called conductors those in which they are not free to migrate are called insulators we can now use the charged electroscope for instance to detect the sine of unknown charges but here's a different interesting application of the instrument if I were a perfect insulator a charge once placed on the leaf system could never leak away it would stay constant with time in practice depending on conditions the leaves do slowly collapse the rate of their collapse is of course a measure of the rate of charge leakage here they're collapsing faster things which are particularly effective in destroying the insulating properties of our ah x-rays now watch this happens quickly also certain kinds of atomic radiation such as that which is given off by this radium source now this isn't the sort of thing you'll fool around with an electroscope is therefore a means of detecting such agents and in various forms is carried by thousands of workers in atomic industries to check their exposure to dangerous radiation finally an intriguing problem even when an electroscope is carefully designed to prevent leakage of charge by conduction and even if it's also shielded against external radiation there is still a slight loss of charge strangely this gets less if we go down into the earth as in a coal mine conversely the rate gets greater in the stratosphere why now go home and make your own electroscope from a milk bottle a nail and some metal leaf and your own vandegraaff from anything you can find around the house and try and explore some of these questions for yourselves