Faraday’s Electromagnetic Induction

Introduction

How do motors work? Why does an electric current in a wire wrapped around an iron nail produce a magnet? These devices both use Faraday’s law of electromagnetic induction—a moving magnet can generate electric current and electric current can generate a magnetic field.

Concepts

• Faraday’s law of induction
• Induced current in a coil of wire

Background

During the nineteenth century, electricity and magnetism were unusual and not well understood phenomena and many scientists devoted their time experimenting on these wonders to determine their properties. In the course of their research, scientists such as Michael Faraday (1791–1867), Joseph Henry (1797–1878), Andre-Marie Ampere (1775–1836) and Heinrich (H.F.E.) Lenz (1804–1865) discovered that magnetic fields and electric currents were closely related. Through observation and experimentation, scientists determined that an electric current in a wire creates a magnetic field around the wire. Faraday wondered if the reverse was also true—can a current be generated by a magnetic field? Faraday answered this question by showing that a changing magnetic field generates (induces) a current in a wire.

Faraday determined that the current induced in a closed conducting wire loop, due to a change in the magnetic field’s strength or direction running through the closed loop, is proportional to the rate at which the magnetic field changes. This became known as Faraday’s law. The strength and direction of a magnetic field is called the magnetic flux. The induced current in the loop does not depend on how large the magnetic field is initially or how large the magnetic field is after the change. It only depends on how quickly the magnetic field changes in strength, direction or both.

Heinrich Lenz further investigated Faraday’s law and discovered that the induced current in the loop will travel in a particular direction to satisfy the conservation of energy principle. Like any electric current in a wire, the induced current produced by the changing magnetic field generates its own magnetic field. The induced current travels in the direction around the loop so as to maintain the original magnetic flux through the loop (see Figure 1). This is known as Lenz’s law. Thus, if the magnetic field through the closed loop decreases, the current induced in the loop will travel so that its magnetic field will increase the magnitude through the loop and therefore maintain the initial magnetic flux.

The direction of the magnetic field generated by the current traveling around a loop is determined by the Right-Hand Rule. By curling your fingers on your right hand around the loop so that you thumb points in the direction of the current around the loop, your fingers will point in the direction of the magnetic field produced by this current (see Figure 2). When the bar magnet is moved through a coil of wire, an electric current is produced in the copper coils as the magnetic field moves and changes. This type of device is known as a solenoid. The small current produced by the solenoid can be detected by a galvanometer. A galvanometer is simply an ammeter that detects very small currents. Each coil in the wire coil generates an induced electric current based on the movement of the magnetic field. Therefore, as the number of wire coils increases, the more the overall current increases. The small induced current in each loop adds together to increase the total current in the wire. This is observed when the 1000-turn coil generates more current than the 100-turn coil when the magnet is moved into and out of the coils at the same relative speed.

Materials

Safety Precautions

The materials in this demonstration are considered nonhazardous. Please follow all laboratory safety guidelines.

Disposal

The materials should be saved and stored for future use.

Prelab Preparation

1. Position the wire coils in the wire coil base as shown in Figure 3.
   {11876_Preparation_Figure_3}
2. Place the rubber band around the coils and base to secure the coils to the base (see Figure 3).
3. Connect one end of each connector cord with alligator clips to the prongs on the 100-turn wire coil, and the other ends onto the positive (+) and 50-mA (or equivalent low-current) plugs on the ammeter (see Figure 4).
   {11876_Preparation_Figure_4}
4. If using the projection ammeter, place it on an overhead projector.

Procedure

5. Starting with the bar magnet far away from the coil, slowly move the bar magnet closer to the 100-turn coil and insert it into the coil. Have students observe the needle of the ammeter. (The needle on the ammeter will be deflected—deflecting more as the magnet is just inserted into the coil.)
6. Stop moving the bar magnet when half of the bar magnet is inside the coil. What happens to the ammeter needle? (It is no longer deflected and moves back to zero.)
7. Slowly remove the bar magnet from the coil at the same speed as it was inserted. Which way does the ammeter needle deflect? (The needle will deflect in the opposite direction compared to how it deflected in step 5.)
8. Repeat steps 5–7 but increase the speed at which the bar magnet is inserted. How does the speed affect the ammeter needle deflection? (The needle will deflect further when the speed of the magnet moving through the coil increases.)
9. Disconnect the connector cords from the 100-turn wire coil and connect them to the 300-turn coil.
10. Repeat steps 5–8. Move the magnet with approximately the same slow and fast speeds as before.
11. Disconnect the connector cords from the 300-turn wire coil and connect them to the 1000-turn coil.
12. Repeat steps 5–8. Move the magnet with approximately the same slow and fast speeds as before.
13. Discuss the results with the students.

Teacher Tips

• This kit contains enough materials to perform the demonstration indefinitely.
• Any low-current ammeter will work for this demonstration. The ammeter range should be at most 0–50 mA. Electronic and benchtop ammeters can be used. An overhead projector-type ammeter is included with the demonstration kit (AP5636).
• The Alnico bar magnet is fragile. Do not drop the magnet or it may crack.
• A protective paper layer may be on two sides of the acrylic block. Peel this paper off the sides before use.
• Use a stronger magnet, such as a neodymium magnet (Flinn Catalog No. AP5666), to show that stronger magnets can generate more current. The initial strength and final strength of the magnet do not matter of course, but a fast-moving strong magnet produces a greater change in magnetic field in a given time compared to a weaker magnet traveling at the same speed in the same amount of time.

Further Extensions

1. Show students that the needle deflects more when the magnet end just enters the coil, compared to when the magnet is already inside the coil and moves back and forth without being removed from the coil.
2. Quickly move the magnet back and forth inside the coil to demonstrate how AC (alternating current) is produced. The ammeter needle will deflect in both directions as the magnet moves one direction and then another. If the magnet is moved at just the precise speed and time, resonance can also be demonstrated as the needle begins to move farther and farther away from zero with each swing. There is a limit to how far the needle will swing (i.e., how much current can be produced) and this is based on the internal resistance of the wire as well as the speed of the magnet.