Conceptual Physics

of a circuit, the emf causes an induced current to flow. Remember that the units of emf are volts, the same as for potential difference. We will

often measure the amount of induced emf by measuring the potential difference it causes across a component like a light bulb.

Why does current flow? It flows because the motion of the wire causes the electrons in it to be moving in a magnetic field, and the magnetic

field exerts a force on the moving electrons that is directed along the wire.

In the illustration on the right, you see how the motion of the sliding wire causes the bulb to glow. You may wonder: Where does the energy to

illuminate the bulb come from? The answer is that it comes from whatever is doing the work of pushing the wire through the field.

29.2 - Interactive problem: motional induction

In this simulation, you can drag a wire left and right, perpendicular to a magnetic

field that is pointing directly into the screen. This is the same configuration that was

used to explain motional induction in the previous section. If your efforts induce an

emf, current will flow and the light bulb will light.

Conduct some experiments: Drag the wire slowly, and then drag it very fast. Does

the speed of the wire through the magnetic field affect the amount of potential

difference across the light bulb? How do the two relate? How does changing the

direction you move the wire change the current? How does it change the potential

difference?

You can also use the magnetic field strength control to make the magnetic field in

the simulation stronger (or weaker, or oppositely directed). Does changing the field

strength change the results of your efforts?

To help you to answer these questions, the simulation has an oscilloscope that

measures the potential difference across the light bulb. The oscilloscope in this

simulation is functionally similar to real-world ones. You can change the output

scale by clicking on its dial. Initially, it is set so that one box of the display grid equals 0.5 volts, but you can change that so one box equals 0.1

volts, 10 volts, and other values shown on the dial as well. (We chose not to show the oscilloscope’s connection to the circuit in this simulation

for the sake of visual simplicity.) An output gauge shows the amount of current; we show its value as positive or negative to indicate direction.

There is also a slider control that lets you change the viewing angle of the simulation. This may allow you to better see the orientation of the

magnetic field, and the wire moving through it.

29.3 - Induction: a coil and a magnet

At the right, you see an apparatus often used to

demonstrate induction. A bar magnet passes through

a coil of wire loops and the bulb lights up.

A current flows when the magnet passes by the wire,

or when the wire moves past the magnet. It does not

matter which one is described as moving: The change

in the magnetic field inside the coil as one moves past

the other induces an emf that causes a current.

The emf induced in this demonstration will vary,

based on several factors:

1. The strength of the magnetic field. The stronger
   the field, the greater the change in field
   strength as the loops move by, and the greater the induced emf.


2. The speed of the wire relative to the magnetic field. The faster one passes by the
   other, the greater the emf.


3. The area of the loops. The greater the area enclosed by each loop, the greater
   the emf.


4. The number of loops of wire. Increasing the number of loops increases the total
   area through which the field passes. This, too, increases the induced emf.

The list of factors above includes field strength and surface area, two of the factors that

determine a quantity called magnetic field flux. We will discuss magnetic field flux

shortly; it is analogous to electric field flux. Later, we will discuss how it is the rate of

change of this flux that determines the amount of induced emf.

Although this is a classic way of illustrating motional induction, it is difficult to calculate

the actual emf induced in this configuration. The strength and orientation of the

magnetic field that intersects the coil both change as the bar magnet approaches the

wire loops. It is easier to calculate the induced emf for other configurations, like a

straight segment of wire moving in a uniform magnetic field.

Motional induction demonstration: The magnet is dropped through the coil

of wire, and the resulting induced emf is displayed on the oscilloscope.

Induced emf depends on:

Strength of magnetic field

Speed of magnetic field past wire

Area of loop

Number of loops

·Or wire past field

(^540) Copyright 2007 Kinetic Books Co. Chapter 29