Electromagnetism - Part 1 - A Level Physics

hello today we're going to continue in our a level physics revision series by looking at the subject of electromagnetism we've already considered electricity so now let's think about magnetism what is a magnet a magnet has a North Pole and a South Pole and the magnetic field flows from the North Pole to the South Pole now an important thing about a magnet and what distinguishes it from an electric charge is that whilst you can have an electric charge that is positive or negative for example a proton is positively charged an electron is negatively charged and you can have those entirely separately you can never have a North Pole on its own or a South Pole on its own theoretically they could exist but they've never been found in other words you can never have a single Pole they always come together and the magnetic field flows from north to south the earth is in fact a magnet it's tipped to one side but it's North Pole and it's South Pole are here and the magnetic field lines flow from north to south and that's the reason the particles get trapped by the Earth's magnetic field magnetic field is sometimes called flux density and it's units are Tesla you'll remember them in relation to charges we say that like charges repel unlike charges attract and it's the same with magnetism like poles repel bring to north poles of a magnet together and they will repel and the reason is that their fields interact they push against one another whereas if you bring a North Pole and a South Pole together then the field lines readily go from north to south and create an attractive force so like poles repel unlike poles attract now a current flowing in a wire here's a current I flowing in a wire will create a magnetic field around that wire and which way does that magnetic field go well we have to imply what's called the corkscrew rule think about a corkscrew if you twist it clockwise you drive the corkscrew into the bottle if you twist it anti-clockwise you pull the corkscrew out of the bottle it's the same as a screwdriver if you turn the screwdriver clockwise you drive the screw in if you turn the screwdriver anti-clockwise you pull the screw out and that's the principle we apply here if the current is for example going into the paper then just as the screw or the corkscrew goes in that means if you want it to go in you have to turn it clockwise and so the magnetic field will go clockwise if on the other hand the current is coming out of the paper then this is coming out then the field will be anti-clockwise now if you put a wire carrying a current in a magnetic field here is a magnetic field north to south and here is a wire which you have to imagine is actually coming out of the paper and the current is flowing towards me what you will find is that there will be a force on that wire and it will move and the reason is that there is a field between north and south but there's also a magnetic field caused by the going through the wire and that magnetic field caused by the current in the wire interacts with the magnetic field that you've got on either side and that will cause the wire to move but in what direction and here we have to apply Fleming's left hand rule if you take your left hand and get your thumb first finger and second finger in this shape that enables you to tell what is going to happen to the wire the rule is very simple your thumb MA is the motion of the wire mud motion your first finger for the fur is the field the magnetic field your second finger kakuka ii cooker for second is the cont so thumb motion for first field field first finger first finger field and second finger current and now if you line those up you will find out in which direction the motion will be so in this case the field is north to south the current is coming that's this finger is coming out of the paper and that means the motion which is the thumb is upwards this tells us that the wire will be pushed up the wire will be pushed in an upwards direction in other words there's a force acting on the wire what is the value of that force well the force is equal to the strength of the magnetic field which we use the letter B to represent multiplied by the current flowing in the wire which is I multiplied by the length of the wire which is L and is important to notice that that L is not the entire length of the wire but the length of the wire that is in the magnetic field and you'll notice that in in this example the magnetic field B and the current I are right angles because the field is essentially going left to right and the current is coming out of the paper so it's actually coming straight up at me and that's at right angles what happens if B and I are not at right angles let's suppose for example that B is in this direction and I is in this direction what then is the value of the force well the angle between the two is Theta and the force will then be V il the same as it was before but now multiplied by the sine of the angle between them and you can see that when theta is 90 degrees the sine of 90 is 1 but when theta is 0 the force is zero in other words if the magnetic field and the current are both in the same direction there will be no force if we look at our left hand rule again this is with all of the fingers at right angles to one another but if we take the field and the current we can actually narrow the angle but you'll notice that my thumb is still perpendicular so where you have the field and the current in a particular plane the force will always be perpendicular to that plane what is the value of a field close to a wire carrying a current in other words here's the magnetic field around the wire carrying the current by experiment it has been found that the magnetic field is equal to MU naught I divided by 2 pi R where mu naught is what is called the permeability of free space I is the value of the current in the wire 2 pi is just what we we know PI to be and r is the distance of the magnetic field from the wire a solenoid is a coil of wire through which current passes and the magnetic field inside the solenoid is again given by experimental observation to be mu naught times n times I where mu naught is once again the permeability of free space n is the number of turns in the wire and I is the current there are ways of measuring the magnitude of the force we could for example use a simple balance here is a balance on the one side we can put weights so that we can measure the weight on the other side we perhaps put a coil of wire around which a current is flowing and we put that between a magnetic field north to south and there will be a force on that cable in which direction will that force operate well the current is going into the paper the field is going in this direction and so the thumb motion is down you see the current is going into the paper the field is in this direction and therefore the motion is down let's apply the left hand rule to this side this part of the current well the field is in this direction north to south the current is going downwards and so the motion will be my thumb will be up so here the force will be up but I think you can see that on the other side where the current is going in the other direction the force will be down and that will cause this coil to flip it will rotate of course it won't rotate very far but if we arrange it so that we've got what are called commutator 's that is to say fix gadgets so that as this flips around this wire will attach to that commutator and this wire will attach to that commutator then you'll always have a current flowing up that line and flowing down that line and so that will mean that this will continue to flip and if we put a little driver on here that will drive something that will turn and from that we've got a motor now let's think about the difference between charged particles in an electric field and in a magnetic field if we take a charged particle let's take for example an electron in a electric field so here's two plates one positively charged one negatively charged there is an electric field between the two if you place an electron in that field it's negatively charged it will immediately move towards the positively charged plate by contrast if you take a magnetic field here is north here south and there is a field between the north and the south and you place an electron stationary in that magnetic field it won't move it will stay there but if you have exactly the same arrangement the north and south poles with the fields if now you have a moving electron then that moving electron is the equivalent of a current and you've got a current flowing in a magnetic field and that will create a force on the electron and it will Bend round in a curve in this case if we apply the rule of the lead Fleming left-hand rule you will find that what the electrons will do is they will curve into the paper so to recap a charged particle that stationery in a magnetic field won't move but if it's already moving then it will be curved by the force of the magnetic field now let's go back to our formula that said that the force was equal to the magnetic field times the current times the length of the wire current we know is the flow of charge per second Q over T where Q is the charge and T is the time in seconds and the velocity of the charge is the length traveled divided by time distance divided by time and that means that T is equal to length divided by velocity which means that we can now put in this formula here we can say that I equals Q times T well T is L over V sorry I is Q over T so I is Q divided by T and T is L over V so we now have that I is Q V over L but F is B i L so we can now substitute this for I and that gives us that F equals V times I which is Q V over L times L and of course the l's cancel and so now that you've got the force is B QV which is the value of the magnetic field times the charge on the particle times its velocity now we've just shown that a moving charge particle in a magnetic field will curve so that now let's have a look at here's a particle which is moving but being curved in a magnetic field and let's say it goes round in a circle of radius R well we can say that the force on that particle is b QV we just derived that but we can also say that if anything moves in a circle then there is a centripetal force acting on it to cause it to do so and the value of the centripetal force is MV squared over R when M is the mass of the particle V is its velocity and R is the radius of the circle and those two have to be equal and that means that b QV is equal to MV squared over r and if you rearrange that you'll find that r is equal to MV divided by b q and this now gives us the capacity to separate out different velocities of charged particles and the equipment we need to do that is an electric field so we have two plates perhaps connect connected to a battery such that this is positively charged and this is negatively charged and there will of course be an electric field between them and we have a magnetic field which goes into the paper and the way we describe that is we use crosses inside a circle the idea is think about an arrow if an arrow were going away from you what would you see you would see the feathers of the arrow as it went away from you so that's it that symbol simply means that the magnetic field is going away from you what would you see if the arrow is coming straight toward you you'd see the point of the a lobe and that symbol means the field is coming towards you out of the paper that means it's going away from you into the paper so here we've got a magnetic field going into the paper and an electric field coming this way and here we've got a stream of particles let's say that they're positively charged particles what will happen to that stream of particles well they're positively charged so they will want to veer towards the negatively charged plate but if you do the left-hand rule you will find that the magnetic field which is going into the paper will cause them to bend in this direction and if you balance the magnetic field with the electric field then the part of particles will just go straight ahead and what is that balance well the force that applies to the particles from the electric field is e the value of the field times Q what is the force as a result of the magnetic field well we've worked that out it's be QV and when those two are equal eq equals b QV which means if you rearrange that v is equal to e divided by v so when the velocity of those particles is equal to the strength of the electric field divided by the strength of the magnetic field then the particles will just go straight through because the force of the electric field which is trying to cause them to go that way is exactly balanced by the force of the magnetic field that's trying to cause them to go that way and they end up going straight ahead and so you've got a velocity selector

Channel: DrPhysicsA

Views: 734,964

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Keywords: Level, Physics, revision, Electromagnetism, magnetic, field, electromagnetic, induction

Id: Dc8Vay7mRss

Length: 18min 37sec (1117 seconds)

Published: Sat Mar 10 2012