Try these "busters" to exercise your brain ... they should help you grasp the concepts underlying the properties of magnetic induction, Faraday's and Lenz's Law, etc. To gain the maximum effect you should attempt to answer them before looking at the answers!
[1] Why does an iron core increase the magnetic induction of a coil of wire?
[2] When you place a metal ring in a region where a magnetic field is rapidly alternating, the ring may become hot. Why?
[3] A length of wire is bent into a closed loop and a magnet is passed through it, inducing an emf and, consequently, a current in the wire. A second length of wire, twice as long, is bent into two loops of wire and a magnet is similarly passed through it. Twice the emf is induced, but the current is the same as that produced in the single loop. Why?
[4] A friend says that according to Ohm's Law, high voltage produces high current. Then your friend asks ... how can power be transmitted at high voltage and low current in a power line? How would you respond?
[5] When a magnet is dropped through a vertical length of copper pipe it falls noticeably more slowly than when dropped through an identical piece of plastic pipe. If the copper pipe is long enough the magnet will reach a terminal falling speed. Why?
[6] A sphere is placed near a long, straight wire that carries a steady current. What can you say about the total magnetic flux passing through the sphere?
[7] A long straight wire carries a steady current. A rectangular loop lies in the same plane as the wire, with two sides parallel to the wire and two sides perpendicular to the wire, as shown below.
Suppose the loop is suddenly pushed towards the wire ... what happens?
[8] The primary of a transformer is connected in series with a switch, a battery and a resistor. The secondary is connected to an ammeter (a meter that measures current).
Explain what happens when the switch is closed and then, after a short while, re-opened.
[9] When the switch is closed in the circuit below the current increases exponentially, approaching a maximum value of I = E/R.
If the number of turns per unit length of the inductor is increased, what can you say about the time it takes for the current to reach a value of I/2, i.e., would it take a longer time, a shorter time or the same time? (Try to answer this conceptually, without using fomula's.)
[10] A magnetic field has to be present if there is magnetic flux passing through a coil. If there is no magnetic flux passing through a coil does it mean there is no magnetic field?
[11] You'll learn a lot from this question! A conducting rod is free to slide along a pair of conducting rails, in a region where a uniform and steady magnetic field is directed into the page, as shown below.
Initially, the rod is at rest. Explain fully what happens when the switch is closed, assuming the contact between the rod and rails is frictionless.
[12] Nice one, this! When there is lightning storm near a radio you can often hear "electrical noise" or if near a TV you can see the "noise" on the screen; this is due to an extraneous current induced in the appliance. How come a bolt of lightning can produce an extraneous current even though it does not actually "strike" the appliance?
[13] In the figure below, a metal ring fits loosely over an iron core that has a coil wrapped around it.
When the switch is closed a current begins to flow in the coil. What happens to the ring, if anything? What would happen if the battery was turned the other way round so the current flows in the opposite direction?
[14] One transformer is a step-up device while another is a step-down device. If they both have the same voltages across and the same currents through their primary coil, does either one produce more power to a circuit connected to their secondary coils? Explain your answer. (You can ignore any losses through heat, vibration, etc.)
[15] A copper ring is dropped through a region where there is a magnetic field - shown shaded in the figure below - directed into the page.
Describe what happens at the points A through E.
[16] "Eddy currents" are electric currents that can arise in a piece of metal when it moves through a region where the magnetic field is changing. In the picture below, a piece of metal is being withdrawn from a region where the magnetic field is directed into the page.
At the instant the picture was made only half of the sheet is in the field and since the field is changing, there is an induced emf (Faraday's Law) that produces "eddy currents" as shown. It turns out that the existence of eddy currents causes the sheet to slow down ... why?
[17] The picture shows a bar magnets falling through metal rings. On the left, it is a complete ring, on the right the ring has a cut in it.
In both cases, explain what happens when the magnets fall through; for instance, do they fall through each ring at the same rate?
[18] Picture (a) shows a simple pendulum consisting of a metal sheet attached to a fine suspension. The sheet swings quite freely just like a regular pendulum. However, when it passes through a magnetic field, for example, between the poles of a magnet, as shown in (b), it stops swinging almost immediately! However, if the sheet has slots cut in it, as shown in (c), it will swing through the magnetic field for a much longer time than (b).
Can you explain all these observations?
[19] In the arrangement below, the coil on the left is carrying an alternating current (proportional to sin(ωt)). The changing flux is detected by the coil on the right.
Is the current induced in the detector coil, in-phase with or out-of-phase with the current in the transmitting coil?
If, now, a sheet of metal is placed between the two coils, is the current in the detector coil increased, decreased or does it stay the same?
[20] Why is it that an electric fan will operate quite happily for a long time while the blades are rotating ...
... but when you stop the blades, the chances are the windings of the motor will "burn out"?
[21] How is the self-inductance of solenoid affected if