Question

What is the meaning of negative power (in electricity) ?

Answer 1

@Vincent-Lyon.Fr ,can u help me out??

Answer 2

i know that it means that "ENERGY IS RETURNED TO THE SOURCE" ,but what does that actually mean?? it is the source who always provides the energy, right? how can something give energy back to the source?

Answer 3

its related to inductors in ac circuit...

Answer 4

In alternating current, there can be moments when the source is receiving current instead of providing it to the circuit.

During these moments, the source delivers to the circuit a negative power.

Answer 5

@LastDayWork

Answer 6

ok...but what is meant by "receiving current" in ac ? the ac source gives current from one way, and recieves it from the other way......next moment it recieves from one way (former) , and gives current from the other way(latter)....so what specifically is meant by "there can be moments when the source is RECIEVING current instead of providing it to the circuit."?? is it not true that the source is always recieving as well as providing current throughout??

Answer 7

since expert is here.. let him do the explanation :D

Answer 8

thats right...

Answer 9

It is just the matter when current and voltage are of the same sign, or when they are of opposite signs.

Answer 10

ok lemme try

Answer 11

i agree, thats used to find the sign of POWER,,,,and when power is negative it means that "ENERGY IS RETURNED TO THE SOURCE" ,,,what does that actually mean??

Answer 12

When current is leaving source by the higher-potential terminal, then the source acts as a generator (positive power provided to the circuit).
When current is leaving source by the lower-potential terminal, then the source acts as a receiver ("negative power" provided to the circuit).

Answer 13

ohhk, so both terminals of a ac source are not equivalent?

Answer 14

If we consider a dc source

like this

(a)...........|- .....

(b)..............-|....

now an oscillator like you said is changing its direction all the time.. so for one half cycle its like diagram (a), and the second half cycle its like diagram (b)

now in diagram (a), its supposed to give current towards the left side..
so if its in position (a) and gives current in the left direction we would say, that the source is giving current and power is positive
but if its in position (a) and the current is in the right direction we would say , that the source is RECIEVING current and power is negative

so its not just whether source is recieving or giving current, but its also about the polarity

Answer 15

does that make sense?

Answer 16

besides how did you build the iron man suit without understanding negative and positive power man? :D :D

Answer 17

haha

Answer 18

so did u get that?

Answer 19

yah.....but one question....in (b),,,isnt current flowing out of the source, towards the right direction ?

Answer 20

so i feel the source is giving current rather than recieving in (b)

Answer 21

@AnthonyStark
Why do you think current flows in the circuit in the first place ?

Answer 22

????

Answer 23

To rephrase my question -
" Why does current flow in a circuit ? "

Answer 24

because there is a potential difference.....

Answer 25

Agreed

Now, the flow of current leads to generation of heat (in resistors). Hence, someone has to do work to maintain the current, right?

Answer 26

yes..

Answer 27

the resistors are using up the power to dissipate energy

Answer 28

Name the agent that performs work to maintain a current in the circuit ?

Answer 29

CELL

Answer 30

or ac source

Answer 31

actually , all i wanted was ...to make sense of THIS

Answer 32

Now consider that someone does work on the battery; how will you describe the situation??

Answer 33

and the main circuit was,...

Answer 34

then the battery will act like a resistor

Answer 35

...same thing when 2 emfs are joined, one of them greater,,,,and the 2nd one's + towrds ist ones +ve

Answer 36

I was trying to relate it to negative power..

Answer 37

i could make out something of this ,, @Vincent-Lyon.Fr , @Mashy , @LastDayWork ,thanks a lot for helping.....

Answer 38

"yah.....but one question....in (b),,,isnt current flowing out of the source, towards the right direction ?"

we don't know what direction current is flowing.. that depends on the circuit..

so in (b) if the current did flow in the right direction, we would call it as cell giving out current and thus doing work and positive power..
but if current in (b) was towards left, then someone else is doing work.. and thus power is negative

Answer 39

ooooooohk,,,,,,,,i got it ..at last.....thanks a lot @Mashy ....u are the LORD :)

Answer 40

and last one......when the voltage and current are in opposite directions,,it means that there is some agency that is pushing current against the direction in which the actual source would have pushed.....this agency is the induced emf....so can induced emf be greater than the source itself?

Answer 41

yes precisely..

Answer 42

but never in case of DC right??

Answer 43

no cause u need changing magnetic fields (flux to be precise) to create EMF

Answer 44

in the beginning....when the current is building up...

Answer 45

yea.. that is possible.. thats what self induction is right? :D..
but once current becomes maximum, then no longer emf is generated..

Answer 46

ok...so in ac , when the source's voltage becomes less in magnitude,,the induced emf takes advantage of the situation and sends current against the voltage(source) right?

Answer 47

no no no..

Answer 48

induced emf is no so harsh.. it doesn't have such intension :D

Answer 49

what it does is, when the source voltage is DECREASING.. (regardless of its magnitude)
the current tries to decrease along with the source, but the Emf negates that..

Answer 50

for that reason, the current cannot go hand in hand with the source..

it would love to. but because of that emf, the current starts lagging

Answer 51

wow..beautifully explained,crystal clear,,,thanks again @Mashy

Answer 52

yea..no problem.. same holds when the source voltage is increasing, even the current wants to increase, but emf says sorry current.. i can't let you do that :D

Answer 53

and really if you have problems with EMF u are not alone..

induced fields are difficult as hell.. compared to static fields
static fields are conservative..
induced fields loop, and are NON conservative!

Answer 54

https://www.youtube.com/watch?v=eqjl-qRy71w

take this video.. you ll love it :D

Answer 55

when the source voltage is increasing, even the current wants to increase, but emf says sorry current.. i can't let you do that ,,,but finally the emf gives up ,and allows source to prevail ,,right?

Answer 56

no emf DOESn't give up..

emf is proportional to RATE OF CHANGE of current

so think of it this way

intially voltage is zer0.. and shoots to 5 V.. (lets say)
so the current also wants to shoot to its max .. but when it tries to do that, there is emf saying NOO.. and the MOMENT EMF is generated, the rate of change of current goes down

since that goes down, emf goes further down,
this keeps on happening until finally the rate of change of current is zero, and thus emf also becomes zero

so in this entire ordeal, the current keeps on increasing. but the rate of change of current (how fast current is increasing), that keeps becoming smaller and smaller and so does emf (this is the slope of i-t graph in yur self induction case, note how the slope is very high initially and then becomes smaller and smaller - draw tangents for this)

Answer 57

thats your self induction graph right?

see initially the current is trying to race upwards, but emf slows it down.. so initially rate of change of current is high.. point A.. look how steep the tangent is
and so emf is ALSO high

this DECREASES the rate of change of current, and as this decreases, emf also decreases..

see point B, the slope is so small now.. emf is also small

that is the meaning of faradays law

e = -d(phi)/dt

Answer 58

the video...was that the limitation of kirchoff law.?...well i actually dont know much about that,,as im still in 12th...

Answer 59

i thought of a stupid analogy for you

imagine you wanna run from point A to point B, and your coach hates it when u run fast, and so he would shout at you..
the shout is proportional to your speed.
and the moment u hear shouting you will decrease your speed..

now when u are at A, initially you ll run like crazy. (high di/dt).. your coach will shout at you like crazy (high emf), that will slow you down .. so your coach will also shout lower (lesser emf), so you ll keep on reducing your speed, and your coachs shouting will keep on reducing and reducing.. until you stop.. ( your speed is zero or di/dt) is zero

thats what emf does :P

Answer 60

it may take time for the concept to sink in.

and yea. the video demonstrates how different induced electric fields are compared to static electric fields

Answer 61

EPIC!!!!!......all your explanations were truly EPIC......are u a professor ?

Answer 62

i am a physics teacher :D.. but not a professor :P..

Answer 63

my teachers havent explained like this till date........hope they do

Answer 64

if you are really interested in these concepts.. not just for exam sake. but you truly wanna master them,

then i recommend you go through MIT 8.02 video series..

this is series of lectures on electricity and magnetism given by great teacher named Walter Lewin.
the clip i showed u was a small part of it..
he is an inspiration to all teachers .. he explains pure concepts..

almost all the concepts are included for your 12th standard CBSE syllabus!

Answer 65

Id surely do that,thanks

Answer 66

they are all available on youtube for free.. just type MIT 8.02.. and u ll see all the lectures :) :)