Emf Definition and 902 Threads

Does the back emf in a dc motor start to appear after a slight delay(say 1 or 2 seconds) after starting up the motor? I have had this simple doubt for a long time.According to the "generator principle" both applied emf and back emf will always exist simultaneously.Also it would be in violation...

We just went over Faraday's law in class and our teacher stressed how a changing flux is needed to induce an emf in some coil/loop/wire. I was just wondering then, how is an emf induced in a straight conducting wire moving at a constant velocity in a constant magnetic field? It seems as if...

The flux rule for motional emf can be stated as: d\phi/dt = -\epsilon I have some questions regarding this. 1) Should I find this rule intuitive? And secondly a proof of this rule is given in my book, which I have attached. I don't really understand the idea of the proof - is the idea to...

Hi all! I'm studying electrical generators, and while I was trying to come up to the expression of EMF induced in a conductor I got two different results following two different ways, and I can't find the error. With sinusoidal B, with e=bvl I get an RMS value E=2K_{f}f\phi, while with...

1. A rectangular wire loop (length 60 cm, width 40 cm) lies completely within a perpendicular and uniform magnetic field of magnitude of 0.5 T. If the length of the loop starts increasing at a rate of 20 mm/s at time t = 0, while the width is decreasing at the same rate, what is the magnitude...

I've come across one thing about energy conversions: "To make an energy conversion possible, it is important that the supplied form of energy must be opposed by another force, otherwise this energy cannot be converted." Agreed! Once the motor starts generating torque, a back emf is produced...

Hi, So, I am learning about induced emf. I am having a hard time grasping the idea that we are interested in the angle between the normal to the plane of the coil and the magnetic field. In the standard illustration I actually understand it, but the following question confuses me. We are...

Homework Statement A square loop with side-length a is positioned at the centre of a long thin solenoid, which has radius r (with r>a), length l and N turns. The plane of the loop is perpendicular to the axis of the solenoid, find V_{emf} induced in the solenoid Homework Equations...

Okay so I have conducted an experiment to find the internal resistance of a battery pack by gradient of a graph of voltage against current. All so far okay. I went on to find the EMF also, again all so far okay. Now I need the short circuit current, and I can't find any information anywhere...

I'm actually a teacher and I can't explain this problem from the 2007 HSC. I've attached a copy of the question and the two graphs. The question shows a graph of change in magnetic flux vs time and asks you to choose which graph of induced emf best fits it. I don't understand why the induced emf...

Hello everyone. I'm trying to understand how an 'induced' emf behaves on a current carrying conductor. Basically the example that's giving me problems is in the case of a motor, if a current starts to pass through a coil it will get an induced 'emf' to oppose the motion. And hence it...

Homework Statement A student is experimenting with a bicycle wheel. He turns the bicycle upside down and spins the wheel in a vertical plane at a constant rate. The diagram shows the wheel. At the place where the experiment is performed, the Earth‟s magnetic field is in a horizontal direction...

Homework Statement I have two questions. Both questions are in the picture below. Dont mind my workings lol http://img163.imageshack.us/img163/5680/scanpic0001qe.jpg Ans to the 1st qn: C The potential difference across the 3ohm resistor is zero. Thus, it can be considered that the 4ohm and...

1.what is the main cause for the emf produced in a thermocouple when their two junctions are at different temperatures? 2.why is it necessary that the emf produced causes the current to pass from the metal (which is at the top of thermoelectric series) to the other metal (which is at lower...

Homework Statement An aluminum ring of radius r1 = 5.00 cm and a resistance of 2.65 x 10^-4 Ω is placed around one end of a long air-core solenoid with 970 turns per meter and radius r2 = 3.00 cm as shown in the figure. Assume the axial component of the field produced by the solenoid is...

]Homework Statement A closed loop of wire 7.2 x10^{}-3 m^{}2 is placed so that it is at an angle of 60degrees to a uniform magnetic field. The flux density is changing at 0.1 T/s. The emf, in V, induced in the loop of wire is A) 3.6x10^{}-4 b)3.6x10^{}-2 6.9 Homework Equations emf...

Hi I am new to the forums. I am sure I have found the right place for help with an idea I have been working on for a while. First off please forgive my lack of physics knowledge. What amount of magnetic force would I need to stop or dramaticallyjust deccelarate a standard size 5000lbs car going...

hello... potential difference between two points in a circuit is the work done by the electric field to get a charge from point to another and for the emf it's also the work done but my question is for a potential difference the work is negative( the electrons lose energy) and for the emf the...

Homework Statement In the circuit shown in the link, the 6.0 \ohm resistor is consuming energy at a rate of 24.0 J/s when the current through it flows as shown. A) Find the current through the ammeter A. B) What is the polarity and emf of the battery \epsilon assuming it has negligible...

Okay so are EMF and Potential Difference the same thing ? I know they are now but I am not able to understand how ? I came through this Derivation today if someone could explain it to me it would be great The figure shows a battery connected to a resistor V(A) - V(B) = V(b) = ε (emf)...

Hi! Does electromagnetic waves induced emf or current in the coil or inductor due to faraday law as the magnetic field is continously changing in em wave?

for a rotating coil in a uniform magnetic field, why is the EMF maximum when the coil is PARALLEL to the field (i.e: flux linkage = 0)

Just a quick question, if the maximum induced EMF of a generator is say 200v, would the minimum EMF be -200v or 0v? No equations necessary I say.

Homework Statement The magnetic field perpendicular to a circular wire loop 6.8cm in diameter is changed from +0.39 T to -0.62 T in 160 m/s , where + means the field points away from an observer and - toward the observer. Homework Equations e=Blv The Attempt at a Solution e=...

I am a bit confuse on the back EMF waveform shape of the brushless permanent magnet machine. As I know brushless DC permanent magnet motor has a trapezoidal shape back EMF waveform. If I use this motor as a generator, do I still get the same waveform shape or I will get a sinusoidal waveform?

I guess it does not produce any field in 2nd coil because there's no change in current in coil right?So there won't be any emf .. but coil 2 has magnetic field passing through it which results in current in coil(by forcing movement of electrons of coil 2 SINCE ELECTRONS ARE ALREADY IN MOTION...

Well I have been researching a lot about inductors and motors and so on and I just had a question that I can't seem to answer. Is it possible to get rid of or reduce the back emf generated in say a inductor or motor. I know you can decrease the inductance of a inductor in many ways, but is there...

Homework Statement A conducting rod of length l moves with a constant speed v perpendicular to an infinately long, straight wire carrying a current I. What is the emf generated between the ends of the rod when it is a perpendicular distance r from the wire? Homework Equations no idea...

Homework Statement I'm currently reviewing some of the old E&M material that I haven't seen in a while and have gotten stuck on this problem. I'm sure I'm just making a simple mistake but I can't seem to make any headway. Here's the problem: Figure 35-32 shows a copper rod moving with...

Homework Statement electromagnetic field Homework Equations E=hv The Attempt at a Solution I know that t is the variable and h1 and h2 I and I2 are considered given all we are doing is doing the algebra set up but not sure how to show this exactly.?

Homework Statement Imagine a pliable round metal loop that can expand or contract. In a region with a constant magnetic field B0 that is oriented perpendicular to the plane of the loop, suppose that the loop expands, with its radius growing with time as r = r0(1+at2). As the loop expands and...

Homework Statement A coil with zero resistance has its ends labeled a and b. The potential at a is higher than at b. Explain why the two following statements are consistent/correct with the situation. a) The current is increasing and is directed from a to b. b) The current is decreasing...

Homework Statement Two bars, each 30cm long, and each having a resistance of 2-ohms, are connected to a 1,500 volt battery. The bars are attached to each other with 3 insulating springs, each having a spring constant of 9N/m. The two bars are initially at rest, 4cm apart. The switch is...

If a straight length of wire moves in a suitable direction through the magnetic field of a magnet, there will be a motional emf in the wire. If the magnet moves toward the wire, is there an emf in the wire, motional or otherwise?

Homework Statement The Attempt at a Solution I now know the units are nWb. I'm just getting a little confused by answer. ((6.1mH)(7.3mA))/224turnes=.198795) (mH*mA)/turnes So would that equal= 198.795 nWb?

Homework Statement http://www.xtremepapers.com/CIE/International%20A%20And%20AS%20Level/9702%20-%20Physics/9702_s02_qp_4.pdf <------ CLICK ON THE LINK The Attempt at a Solution Is the above diagram the correct answer ? Please note that the diagram in the question paper is for COIL...

Homework Statement The resistance of the armature windings of a DC motor is 5Ω. The motor is connected to a 120V line. When the motor is spinning at full speed, it draws a current of 4A. The back EMF at full speed is: Homework Equations Pav=\frac{1}{2}I2peakR Pav=\frac{1}{2}εpeakIpeak...

A 30mH solenoid inductor is wound on a form 0.4 m in length and 0.1 m in diameter. A coil is tightly wound around the solenoid at its center. the coil resistance is 5.0 ohms. The mutual inductance of the coil and solenoid is 60μH. At a given instant, the current in the solenoid is 0.3A and is...

NOTE : My Current question and source of debate is in post number 10. Lets say I have a square coil. I accelerate it in a direction such that it is perpendicular to a magnetic field directed into the page. As it enters the field and before it is completely in the field an...

This question about the variation of the aforementioned quantities in a transformer. Now according to me and my knowledge of electricity this is how I feel it should play out : If the input current were a sine curve that varies with time then : Input voltage = cosine curve i.e. it is...

Hi, I would appreciate any help with the following: The coil in a loudspeaker has an inductance of L = 112 microH. To produce a sound of frequency 40kHz, the current must oscillate between peak values of + and - 4.4 A in half a period. What average back emf is induced in the coil during this...

Hello experts! Higher the emf of the battery implies that the higher the drift speed of the electrons or increase the flow time of electrons from the conductor? Which one is true and how??

is the a direction in emf ?? Does electromotive force have a direction? In some circuits it is showed as in the image above.. Please tell me that it having a direction ?

Faraday's Law of Induction: EMF=-\frac{\partial \Phi}{\partial t} If two identical coil with same turns of wire and same dimension ( same cross section area in the middle), one has only air and the other has a magnet core. If the two placed in an uniform varying magnetic field, is there...

If i am given a graph of B(magnetic field) vs Time,how can i find the induce emf from this graph?? I know that the emf=-Nd∅/dt and ∅=BAcosθ but somehow i don't quiete arive to the right answer

What is the equation I should be using for the following question: At 298K a cell reaction has a standard emf of +0.17V. The equilibrium constant for the cell reaction is 5.5 X 10^5. What is the value of n for the cell reaction. Grateful for anyone's help!

I just need some clarification on the induced EMF Formula.. I know that the negative sign in front of N * dFlux/dt is just an indicator of Lenz's law, but if we were given the magnetic flux as a function of time, would there be situations where -d/dt(F(t)) would be different than just...

Hi, I am doing coursework on Faraday's law of induction. My assignment was to carry out experiments which confirm Faraday's law and also to explain the physics of how faraday's law works... My experiments all worked perfectly, producing straight line graphs showing that: \epsilon\propto N...

A metal wire slides on a horseshoe-shaped metal loop of width 0.25 m. The loop has negligible resistance, but there is a 1Ω resistor in the circuit as well as a 6 volt battery. There is a uniform magnetic field directed into the plane of the page of magnitude 0.5 T. The slide wire is pushed...

Actually my sir asked me the definition of EMF so I just tell him that "Suppose a resistance(R) is connected across the terminals of a battery.A potential difference is developed across its ends.Current(or positive charge) flows from higher potential to lower potential across the resistance by...