When is emf positive

What is the difference between right hand rule and right hand thumb rule? Why do we use right hand thumb rule? Previous Article Who built the first high rise building? Next Article What businesses use cloud computing? Back To Top. The area of the loop is not changing in time, so it can be factored out of the time derivative, leaving the magnetic field as the only quantity varying in time.

Significance If the area of the loop were changing in time, we would not be able to pull it out of the time derivative. Since the loop is a closed path, the result of this current would be a small amount of heating of the wires until the magnetic field stops changing. This may increase the area of the loop slightly as the wires are heated. Check Your Understanding A closely wound coil has a radius of 4. At what rate must a magnetic field perpendicular to the face of the coil change in order to produce Joule heating in the coil at a rate of 2.

A stationary coil is in a magnetic field that is changing with time. Does the emf induced in the coil depend on the actual values of the magnetic field? A copper ring and a wooden ring of the same dimensions are placed in magnetic fields so that there is the same change in magnetic flux through them.

Compare the induced electric fields and currents in the rings. Both have the same induced electric fields; however, the copper ring has a much higher induced emf because it conducts electricity better than the wooden ring. Discuss the factors determining the induced emf in a closed loop of wire. How would changing the radius of loop D shown below affect its emf, assuming C and D are much closer together compared to their radii? Can there be an induced emf in a circuit at an instant when the magnetic flux through the circuit is zero?

As long as the magnetic flux is changing from positive to negative or negative to positive, there could be an induced emf. Does the induced emf always act to decrease the magnetic flux through a circuit? How would you position a flat loop of wire in a changing magnetic field so that there is no induced emf in the loop?

Position the loop so that the field lines run perpendicular to the area vector or parallel to the surface. The normal to the plane of a single-turn conducting loop is directed at an angle to a spatially uniform magnetic field It has a fixed area and orientation relative to the magnetic field. Show that the emf induced in the loop is given by where A is the area of the loop.

A turn coil has a diameter of 15 cm. You cannot have negative magnitude. Sign up or log in Sign up using Google.

Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown. Featured on Meta. Now live: A fully responsive profile. Related Hot Network Questions. Question feed. Physics Stack Exchange works best with JavaScript enabled. Accept all cookies Customize settings. The resistor could be anything from a light bulb to a voltmeter. Consider the area enclosed by the moving rod, rails, and resistor.

B is perpendicular to this area, and the area is increasing as the rod moves. Thus the magnetic flux enclosed by the rails, rod, and resistor is increasing. Figure 1. The magnetic field B is into the page, perpendicular to the moving rod and rails and, hence, to the area enclosed by them. Since the flux is increasing, the induced field is in the opposite direction, or out of the page. RHR-2 gives the current direction shown, and the polarity of the rod will drive such a current.

RHR-1 also indicates the same polarity for the rod. Note that the script E symbol used in the equivalent circuit at the bottom of part b represents emf. Entering these quantities into the expression for emf yields. Entering this into the last expression shows that. See Figure 1 b. Flux is increasing, since the area enclosed is increasing. Thus the induced field must oppose the existing one and be out of the page.

And so the RHR-2 requires that I be counterclockwise, which in turn means the top of the rod is positive as shown. Motional emf also occurs if the magnetic field moves and the rod or other object is stationary relative to the Earth or some observer. We have seen an example of this in the situation where a moving magnet induces an emf in a stationary coil. It is the relative motion that is important.