back emf explained

welcome to high school physics explained and today I want to briefly look at the concept of self inductance or what's often referred to as back EMF so if you're watching this video and you have no understanding of a motor or you have no understanding of the concept of electromagnetic induction I always suggest you review this but particularly if you're interested what back EMF is I'm going to go through this very quickly so the first thing is is what I have here in front is a model of how an electric motor works so you can see I have a wire here and these red arrows represent the direction of the current so can you determine the direction of the rotation of this coil take a moment to try working it out so let's have a look at it so first of all we have a current going into this side over here and out this side well using your right-hand palm rule since the magnetic field lines are moving from left to right and the current is going into the page or into the board or into the video depending on your perspective you can see that there will be a force applied on this side that is in the downward direction of course this section will receive no force whatsoever because the wire is actually parallel to the field so this section over here being in the opposite direction in terms of current it's pretty clear that the direction will be going upwards and so as a result this loop will turn in a counterclockwise or an anti-clockwise direction so so far so good that is what a motor is all about but now let's talk about induction you see the process of induction is that there needs to be a change in flux over time and the change in flux will always be is such that it opposes the motion that's what Lenz's law is all about so using your left hand you should be able to determine the direction of the EMF and by inference determine the direction of the kind if there was no other current present so let's deal with the EMF so using your left hand rule you can you determine the direction of the EMF based on the fact that this wire is moving upward and this wire is moving downwards so let's have a look at it using your left hand this time you will notice that if this wire is moving in a downward direction experiencing a changing flux and as a result it will produce an EMF in the direction like so now that is interesting because the EMF that generates this card is in the other direction similarly speaking on this side you're going to get an EMF such that it opposes the motion in this direction so what you in essence have is two EMF you have the input EMF or voltage applied which causes this current to flow but as the current flows that means we experience a force that means it starts to rotate that means that Y is experiencing a changing flux and as a result there will be in a oppositional EMF in the opposite direction it's as if you've got five people pushing something out the door but then we have a sixth person who basically says oh there is something moving towards me I'm going to oppose the motion but I'm only opposing the motion if those five people are moving the object out the door so in other words you might have five people pushing but only while the object is moving through our person post and so you have a net force I guess or a net EMF of five minus one equaling four that's really important because that's the part that contributes to the current so what you have is back EMF you have obviously this concept of an EMF in the opposite direction can I make it absolutely clear we do not have a back current that is wrong secondly what you need to understand is that the applied voltage is a little bit more than the actual voltage so if your input is 12 volts but you end up getting a negative 2 volt eeehm F because of the motion your net result will be 10 volts now obviously the faster the motor increases and that's increased by increasing the voltage because increasing voltage increases kind and increasing kind increases the motor effect and it goes faster well then the rate of change of flux changes and so that may increase as well so you may go to 14 but then you get minus 3 and that leads us to maybe an 11 volt overall net voltage so you yes make it go faster that is not as in great increase as what you put it in over here if the current will decrease as the motor as a result now what does that mean well the current is determined by this value over here that's really important so as a result the current decreases lastly I will just want to talk about this the fact that many devices that are designed are designed for this because clearly if you need an object that has really high stalk needs that you would into really high current so if you put in a certain input potential difference to get that high current then obviously you're going to get a decrease in talk because of this back EMF so the solution is to ramp up the initial kind by ramping up to the initial voltage but that must allow for the back EMF which means if you suddenly stop the motor then you no longer have this motion and the voltage spikes and as a result the current spikes so for example an electric drill has a particular high potential difference applied but it is allowing for the back EMF because it's spinning really fast so if suddenly the drill bit suddenly ceases we suddenly have a stop in the rotation of the motor as a result there's no more back EMF the voltage spikes the current spikes and the motor burns up as a result and often what those devices offer place in is a variable resistor that adjusts itself based on the rotation of the motion but that's in essence why a motor can burn out because it's allowing for back EMF anyway that gives you an understanding of back EMF I hope that helps you and thanks for watching bye for now I hope you found that video useful and remember like share and subscribe oh and if you have a comment or a question or your like a concept for me to explain to you please drop a comment down below I'm Paul from high school physics explained bye for now

Channel: PhysicsHigh

Views: 192,067

Rating: 4.6715159 out of 5

Keywords: back emf, electric motors, electrical generation, high school physics, emf, physics course, physics basics, physics high school, generators, physics help, generation, motor effect, induction, physics, induction back emf, back emf explained, back emf simplified, lenz law back emf

Id: 5mf4NmmLWnE

Length: 6min 56sec (416 seconds)

Published: Fri Oct 21 2016