工程师 - What is EMF?
What is EMF???
Youtube : Electrician U
What is EMF?
When we say EMF in electrical circuits we're talking about the electromotive force and a lot of people confuse voltage and EMF and they're not the same thing.
So EMF is energy that's provided by a voltage source, so a generator or battery how much energy is stored inside of that thing to be able to be used to pass current through a circuit, that's EMF.
So when you think of EMF you're thinking about just a power source. When you think about voltage you can test voltage anywhere in a circuit but not all voltages are EMF. EMF are only for a power supply and when we use calculations we always put E is voltage we just use E instead of V. But really E is talking about EMF from a source but when we go into a circuit to test we're still using voltages.
So just remember not all voltages are EMFs but all EMFs are voltages.
Potential difference VS Electromotive Force - A Level Physics
Youtube : vt.physics
the category of voltage can be split into two sub categories potential difference and EMF.
EMF is acronym for electromotive force although it isn't actually a force. So what is the difference between EMF and potential difference?
here's a simple circuit with a cell and a light bulb. let's imagine this to be an electron. electrons in a circuit go from the negative terminal to the positive terminal. the conventional current goes from the positive side to the negative side. so in this video we will use the conventional currents when we consider the direction of currents.
a cell or a battery is a store of chemical energy. as an electron travels through the cell, the cell gives up a tiny bit of this chemical energy. I'm going to represent this energy using this bit of Lego. you can also think of this electron transferring the chemical energy from the cell along the circuit. the energy transfer from the cell to the electron is from chemical energy to electrical energy. when the electron passes through the light bulb the electron gives up its energy to the bulb.
this energy transfer is from electrical to light and thermal energy .
so when we talk about EMF, that is the energy supplied to a unit of charge by a source, and in this process the energy transfer is from other types of energy into electrical energy. but when we refer to potential difference that's the energy dissipated by units of charge when it goes through a components. so the process of energy transfer here is from electrical energy to other types of energy. so you can think of cells batteries and any other power sources as having EMF. the components like resistors light bulbs LEDs and speakers, they will have a potential difference across them.
back emf explained
Youtube : PhysicsHigh
High School Physics Explained
Welcome to high school physic 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 would 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（左边和磁力线垂直的导线的受力方向是向下的）.
of course this section(和磁力线平行的导线) will receive no Force whatsoever, because the Y 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 anticlockwise direction (从屏幕这边看过去逆时针转动).
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 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 current 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, it's 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 current 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 emfs, 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 the wire is experiencing a changing flux and as a result there will be an 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 the other person post and so you have a net force I guess or a net EMF of 5 - 1 equaling 4. 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 -2v EMF 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 current and increasing current 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 three and that leads us to maybe an 11 volt overall net voltage. so you yes make it go faster but is not as in great increase as what you put in over here.
if the current will decrease as a result. now what does that mean? well the current is determined by this value over here (10V和11V). that's really important. so as a result the current decreases.
lastly I would just want to talk about is the fact that many devices that are designed for this, because clearly if you need an object that has a really high torque needs then you want it into really high current. so if you in a certain input potential difference to get that high current then obviously you're going to get a decrease in torque because of this B EMF. so the solution is to ramp up the initial current by ramping up 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 out as a result, and often what those devices often 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 you 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.