Transcript Part II
EMF Induced in a Moving Conductor Copyright © 2009 Pearson Education, Inc. This figure shows another way the magnetic flux can change. It can change if a conducting loop is moved in a static magnetic field. Copyright © 2009 Pearson Education, Inc. The induced current in the figure is in a direction that tends to slow the moving bar. That is, It takes an external force to keep it moving. Copyright © 2009 Pearson Education, Inc. The induced emf has magnitude This equation holds only if B, l, & v are mutually perpendicular. If they are not, then it is true for their perpendicular components. Example Does a moving plane develop a large emf? An airplane travels at speed v = 1000 km/h in a region where the Earth’s magnetic field is about B = 5 10-5 T & is nearly vertical. Calculate the potential difference induced between the wing tips that are l = 70 m apart. Copyright © 2009 Pearson Education, Inc. Example Electromagnetic blood-flow measurement The rate of blood flow in our body’s vessels can be measured using the apparatus shown, since blood contains charged ions. Suppose that the blood vessel is 2.0 mm in diameter, the magnetic field is 0.080 T, & the measured emf is 0.10 mV. Calculate the flow velocity v of the blood. Copyright © 2009 Pearson Education, Inc. Example: Force on a rod. To make the rod move to the right at speed v, you need to apply an external force on the rod to the right. Calculate (a) The magnitude of the required force. (b) The external power needed to move the rod. Copyright © 2009 Pearson Education, Inc. Electric Generators A generator is the opposite of a motor. It transforms mechanical energy into electrical energy. The figure shows an ac generator: The axle is rotated by an external force such as falling water or steam. The brushes are in constant electrical contact with the slip rings. Copyright © 2009 Pearson Education, Inc. If the loop is rotating with constant angular velocity ω, the induced emf is sinusoidal: For a coil of N loops, Copyright © 2009 Pearson Education, Inc. Example: AC generator. The armature of a 60-Hz ac generator rotates in a 0.15 T magnetic field. The area of the coil is 2.0 10-2 m2, Calculate the number of loops needed for the peak output to be E = 170 V. A dc generator is similar to an ac generator, except that it has a split-ring commutator instead of slip rings. AC DC Copyright © 2009 Pearson Education, Inc. Automobiles now use alternators rather than dc generators, to reduce wear. Copyright © 2009 Pearson Education, Inc.