AC Generation & Back EMF - A-level Physics

[@dianamustaeva9756]

I am a physics teacher and your videos save my life! :) thanks a lot for them!

[@harrywilliams969]

I find a good way to think of it is that it's like with velocity and acceleration when throwing a ball up in the air. At 0 velocity the rate of it's change (acceleration) is greatest, yet at it's maximum/terminal the rate of change is zero. You can then just swap velocity with flux and acceleration with emf :)

[@partyDragon2000]

It’s funny, that’s the exact way I think of it.

[@MAyub-fq8pj]

My confusion is that at its max point its still gonna change in terms of decreasing if not increasing and thats what should be concerned of, the change with time whether increase or decrease so my point is that whether u have max flux or zero flux its still gonna change to decreasing or increasing the amount, am I right?

[@Nathan-p1j]

Except gravitational acceleration is a constant at g ms^-2, so no.

[@danielo7985]

@@Nathan-p1j well in a fluid, the acceleration of the Velocity will not be constant due to air resistance. Gravity is constant but not the acceleration of the ball

[@freerights6695]

@@danielo7985 yes the acceleration will be constant. The thing that air resistance affects is the velocity of the object

[@leelax_x6991]

Thank you so much! the only video that makes sense- even when it's 7 years old

[@spelunkerd]

I love the brilliant clear thinking. If you put a voltmeter across the DC motor windings above, would it show a voltage drop that is higher than source voltage? What brought me here is desire to understand how voltage drop across the start windings of an AC induction motor fed by a run capacitor is consistently higher than the incoming voltage after the motor gets up to speed. For example, if the motor is fed with 240 V single phase mains, once up to working speed the drop across the start windings is often 350V+! This is such a consistent finding that a device called a potential relay is used to pull a start capacitor out of service after the motor gets up to speed. Standard teaching is that this huge voltage drop is because of back EMF from generator effects of the motor. But I'm confused, wouldn't back EMF reduce voltage drop? I can explain a big drop by looking at RLC phasor equations, but I don't know how to add back EMF from a motor into the calculations....

[@cormacokane9466]

So V-ε is the Potential Difference across the motor?

[@mmaad2165]

What is the parameters that affect the back EMF and what is the formula of back EMF??

[@ethanlawrence2825]

ε = Φωcos(ωt) = ε₀cos(ωt) which is just Faraday’s law. :)

[@CesarAngeles28]

Hi there! Thank you very much for your video! Really good fun! Just a question! When inducing an EMF due to torque within the rotor, Wouldn't the back emf be more related to a magnetic field caused by the current flowing through a close loop of wire? Or why is the reason that when we load a motor, there is a massive spike of current within the feeding circuit? Thank you very much in advance!

[@georgestanciu5466]

Very good video. I want to ask if the back emf energy is released in the same amount as it was charged in the coil?

[@TheMrVerbal]

How would you go about sizing a diode to bleed out back EMF from a single phase 220vac motor that is activating other equipment tied to the same capacitor?

[@ScienceShorts]

...depends how big the back emf is?

[@TheMrVerbal]

@@ScienceShorts I'll have to update this when I'm back in the job site. I have a condenser fan motor giving just enough back EMF to kick the compressor and make it stutter once or twice then it's bled out. I'm trying to figure out if a isolation relay breaking the wires from the motor or a diode across the tabs on the run cap would be better. I was thinking bleed resister but I read somewhere they're not to great for AC, but I could be wrong.

[@hamidkhan-bg9fn]

sir what about the graph form when to link the current with the the induced emf in the case of self induction

[@niimarteimarkwei9502]

I think it would be good if you actually solve questions as well. It would really help

[@rayyan1183]

he does

[@niimarteimarkwei9502]

@@rayyan1183 yeah thanks

[@sarujanrupan4831]

When you say loaded/ unloaded for the back emf part, does that mean that the motor is physically lifting an item?

[@ScienceShorts]

That's an example of a 'load', yes. But it just means something that resists the motion of the motor.

[@sarujanrupan4831]

@@ScienceShorts Ah cheers!

[@adityamisra8869]

I got my exam in 40 mins please help me m!!!!!!!

[@trench01]

I kind of get it but not fully about back EMF. Why not use another metal than copper to reduce emf?

[@ScienceShorts]

But back emf is a good thing for motors! Means the faster it goes, the more efficient it is!

[@trench01]

@@ScienceShorts Thanks. In the way others explain it it sounds bad since the motors would go even faster. But you notice how the coil turns one way, and if one coil is next to another they rub against each other which causes friction it seems to cause heat. But what if their was another wire between that like how a gear is to take the rotating force. And instead of a thin cylinder wire a flat wire almost like a capacitor that is flat. And since copper has eddy current why not another metal that does not do eddy current like a ticker Iron or an alloy like copper/iron. Also most magnets are weak and the higher the magnet the less electricity needed to move an electromagnet so a circularneodymium halbach array (expensive)on the outside and inside of an electromagnet would go over 1 tesla. Just some thoughts. What do you think? Also another video called "How Electromotive Force Works" by " National MagLab" has a good animation about back emf but it still does not say why it happens.

[@fxphenix5162]

I'm working on something really neat... and i would love a sounding board, and a guy to ask questions about physics and Electromagnetism... Im trying something new... and need a couple 🧲 fundamental magnetism 🧲 questions 🤔 answered if it's not too much trouble...

[@BooleanDuck]

Is -cos(x) 3pi/2 out of phase with sin(x) or do you say that it is pi/2 out of phase with sin(x)?

[@ScienceShorts]

Saying they are pi/2 rads out of phase is fine.

[@BooleanDuck]

Ah right, thanks very much!

[@bellacarter8450]

why is flux linkage a maximum when perpendicular because cos90 is 0 ?

[@hakeemessippu8261]

you use the normal of the coil

[@dragoola69x]

Do you know why back happens and or if we cod work with the back Emf we could get much higher efficiencies in generators and motors

[@ibraheempatel4932]

Thats the tough thing about this topic. As with everything in science this is still a theory and with magnetism theres a lot of why it happens which nobody really knows. Best bet is, as intriguing as it is, to just learn what it says to learn for the exam which kind of sucks because you can get a much better understanding of topics if u knew why stuff happened but it is what it is.

[@hash-tag8065]

can you make a video on direct sensing! application of physics

[@hamzanawab8898]

When the coil is parallel to the field, the flux linkage is zero. But wouldn’t that mean the emf is also zero because of faraday law/formula?

[@ScienceShorts]

No, because it's the rate of change that determines emf. When a ball is thrown upwards and reaches its apex, its speed is zero, but its speed is still changing. Same deal.

[@MAyub-fq8pj]

@@ScienceShorts Can I ask why the rate of change of flux is minimum when the coil is perpendicular to the field lines because even though the flux linkage is max but its still gonna change when the coil starts moving and thats what we are concerned with, same happens when the flux is zero but it starts to change and when its max it also starts to change as the coil makes further turns.

[@anaghavarma751]

What did he say at 5:12 ? Three sets of status...?

[@ScienceShorts]

Stators.

[@anaghavarma751]

@@ScienceShorts Oh! Okay thanks! Also, thank you so much for doing this... I think I've learnt more in the last 9 minutes than in a whole school year

[@joebentley2990]

Does the back emf reduce the current through the motor so reduce the speed the motor turns

[@ibraheempatel4932]

since w= 2pi/t wouldnt cos(wt) just equal 2 pi??

[@tomogrady9828]

Ibraheem Patel w=2pi/T where T is the time period which isn’t a variable. t however is the variable (time) on the x axis which is changing

[@jacklane179]

Hello

[@mccourtneynambezi2446]

didnt understand

[@niksa28]

This applies to DC as well.

[@ScienceShorts]

😐

[@niksa28]

@@ScienceShorts What's the face for, this phase difference between flux and voltage applies to DC generators too.

[@magneat]

Why did you decide that you know actual way of the magnetic field move? You have no idea. Your only guess is that something is turning when it enters MF. What really happens with a magnetic field and which are it’s ways - is a mystery for today’s science.

[@ScienceShorts]

It's a model dude, but one that works. If you go down that route, everything's a mystery - gravity, light. Physics doesn't explain how things work, merely describes phenomena with maths.

[@magneat]

@@ScienceShorts that's right, it just tries to calculate =) and I do go that path and it's sooo cool =) there are much less mysteries there, but more explanation. for instance: make a realation of a wave travelling speed to the density of an environment. you'll see, that the less density, the less is speed of the wave. speed of the wave in air is roughly 300 m/s. speed of the same wave in water is 4 times more. speed in metal is roughly 5000 m/s. so the less is distance between particles, the more is speed. like a ball pendulum toy: there is no distance between hanging balls, so the force is travelling right away. now if we make some distance between the balls, then it will take time to transfer the weve, right? makes sence? =) NOW the question is: what is the density of an environment, that has it's wave travelling speed of 300.000 km/s? =))) the mystery suddenly gets some light shed on =)

[@magneat]

@@ScienceShorts i pointed to your idea of magnetic field so that you would hopefully next time point that out in your videos. because people that are unaware of the science dead ends, they would tend to think that the way of magnetic line is obvious. it's YOU who knows that they are not obvious. but please don't think that students that watch your videos all know that. if you point out the problem like "these are magnetic lines, but actually our science has no clue what they are", then your visitors will start THINKING. if you don't, then you don't help them grow mentally. you make them dumber in some certain way. unfortunately.

[@valarmorghulis209]

Bro if you are not benefiting from these videos just don’t watching them.

[@h.a2885]

U talk toooo much

[@kimjitchu3799]

U didnt even borther to explain how the change of angle affect how the graph is shape. U just draw without explaining at all.

[@valarmorghulis209]

( U )didn’t even appreciate the fact that he provides these for no reason except to help so stop acting like he owes you something.