Ch. 23 Flashcards
(6 cards)
Explain the difference between a magnetic field and a magnetic flux.
,
Magnetic flux - a measure of the “amount” of magnetic field that passes through a given area.
Figure shows a vertical iron rod with a wire coil of many turns wrapped around its base. A metal ring slides over the rod and rests on the wire coil. Initially, the switch connecting the coil to a battery is open, but when it is closed the ring flies into the air. Explain why this happens.
An induced emf will be developed in a conductor if it is moving in a magnetic field and hence some current flows through the conductor. In the case of a broken ring, the magnetic field does induce an emf between the ends of the broken ring, but the flow of the current will be prevented along the circumference because of the break in the ring.
A metal rod of resistance R can slide without friction on two zero resistance rails, as shown in figure. The rod and the rails are immersed in a region of constant magnetic field pointing out of the page. Describe the motion of the rod when the switch is closed. Your discussion should include the effects of motional emf.
Initially, the rod accelerates to the left, due to the downward current it carries. As it speeds up, however, the motional emf it generates will begin to counteract the emf of the battery. Eventually, the two emfs balance one another, and current stops flowing in the rod. From this point on, the rod continues to move with constant speed.
Explain what happens when the angular speed of the coil in an electric generator is increased.
When the angular speed of the coil in an electric generator is increased; the magnitude of the induced emf increases because induced emf is directly proportional to angular speed.
A bar magnet is inside a closed cubical box. Magnetic field lines pointing outward from the box produce positive flux, and those pointing inward produce negative magnetic flux. (a) Is the total magnetic flux through the surface of the box positive, negative, or zero? (b) Choose the best explanation from among the following:
I. There are more outward magnetic field lines because the magnet is inside the box.
II. The magnet draws field lines inward toward itself, producing more negative flux.
III. The same number of magnetic field lines leave the box as enter it.
A) According to gauss’s law, the outward magnetic flux through the surface is proportional to the number of field lines leaving the surface and is positive.
According to gauss’s law the inward magnetic flux through the surface is proportional to the number of field lines entering the surface and is negative.
Hence the total number of field lines entering and leaving the surface is equal.
Therefore the total magnetic flux through the surface of the box is the sum of the inward and outward flux through the surface.
Therefore the net magnetic flux is zero.
B) Statement III gives the best explanation for having the zero flux from the surface of the box.
The figure above shows the magnetic flux through a coil as a function of time. At what times shown in the plot does the magnetic flux have the greatest magnitude?
a) As shown in the graph, magnitude of magnetic flux is maximum at time 0s, 0.2 s, 0.4 s, and 0.6 s.
b) emf will maximum when rate of change of flux is maximum, i.e at times 0.1 s, 0.3 s, 0.5 s nd so on
