Chapter 20 Section 1 Section 1 Electricity from Magnetism

What do you think? The loop of wire is rotating in a counterclockwise direction. Electrons in metal are free to move. The magnetic field is horizontal and to the left. See the next slide for questions. When asking students to express their ideas, you might try one of the following methods. (1) You could ask them to write their answers in their notebook and then discuss them. (2) You could ask them to first write their ideas and then share them with a small group of 3 or 4 students. At that time you can have each group present their consensus idea. This can be facilitated with the use of whiteboards for the groups. The most important aspect of eliciting students’ ideas is the acceptance of all ideas as valid. Do not correct or judge them. You might want to ask questions to help clarify their answers. You do not want to discourage students from thinking about these questions and just waiting for the correct answer from the teacher. Thank them for sharing their ideas. Misconceptions are common and can be dealt with if they are first expressed in writing and orally. The next slide has a larger view of the diagram, along with questions for students to answer.

What do you think? Will there be a force on the electrons in the left and right segments of the loop? If so, in what direction is that force? In which direction will the electrons flow if the two wires from the ends are connected? Ask students to apply the right-hand rule learned in the last chapter to the right and left sides of the loop. (The back side is difficult to consider because half of it is moving upward and the other half downward, and the force is not along the wire but across the wire.) Hopefully they will find that the force on the electrons is toward the back of the loop segment on the right and toward the front of the loop segment on the left. Therefore, the net motion of electrons is in a counterclockwise direction (viewed from above). In other words, moving the loop through the magnetic field creates a current in the loop. Try to get students to think about applications of this concept. What use could be made of moving a wire though a magnetic field? Does it matter which way it moves through the magnetic field (up, down, sideways)?

Electromagnetic Induction Imagine a wire moving to the right as shown. In what direction is the force on the negative charge? Upward This force will separate the charges. As negative charges move upward, the wire will develop a potential difference. Batteries use chemical energy to separate the charges and create an emf. A moving wire in a magnetic field produces a similar effect. If the wire was connected to an external circuit such as a light bulb, the electrons would flow out of the top, through the bulb, and back into the bottom of the wire.

Electromagnetic Induction Electromagnetic induction is the process of creating a current in a circuit loop by changing the magnetic flux in the loop. This can be accomplished by moving the loop, moving the field, or changing the strength of the field. If the magnetic flux does not change, no current is induced. The current is increased if the loop size or magnetic field strength are increased.

Ways of Inducing a Current in a Circuit Click below to watch the Visual Concept. Visual Concept

Lenz’s Law As the magnet enters the coil, a force pushes the electrons around the loop, inducing a current. The induced current creates a magnetic field that opposes the motion of the magnet. It would be helpful to have a large coil of wire and a magnet to show students how this current and opposing magnetic field are produced. Use the right- hand rule to find the direction of current. It is best to imagine the coil moving to the right instead of the magnet to the left. Consider the top of the coil: it moves to the right and the B field is upward at that point, so the electrons would move toward the back (or the current toward the front as shown). Then have students use the other right-hand rule to find the direction of the magnetic field produced by this current. The magnetic field will come out of the coil as shown, with the north pole on the right. The PhET website may be useful at this time. http://phet.colorado.edu/new/index.php Choose “Simulations,” then choose “Electricity, Magnets and Circuits,” and then choose “Faraday’s Electromagnetic Lab.” At this time, it would be useful to show them the “Pickup Coil” option. You can then demonstrate the moving magnet inducing a current in the coil and see how it reverses directions as the magnet leaves the coil.

Lenz’s Law Now the magnet is being removed from the coil as it moves to the right. This induces a current in the opposite direction. Once again, the magnetic field in the coil opposes the motion of the magnet.

Lenz’s Law The magnetic field of the induced current is in a direction to produce a field that opposes the change causing it. This rule can be used to find the direction of the current in the loop. Point out that the induced current creates a magnetic field that opposes a change. If the north pole is moving away, it creates a south pole. If the north pole is approaching, it creates a north pole. If the north pole is not moving, no current is induced.

Lenz's Law for Determining the Direction of the Induced Current Click below to watch the Visual Concept. Visual Concept

Faraday’s Law The magnitude of the induced emf depends on the number of loops (N), the magnetic flux (M), and the rate of change. M = AB cos

Classroom Practice Problem A coil with 25 turns of wire is moving in a uniform magnetic field of 1.5 T. The magnetic field is perpendicular to the plane of the coil. The cross-sectional area of the coil is 0.80 m2. The coil exits the field in 1.0 s. Find the induced emf. Determine the induced current in the coil if the coil’s resistance is 1.5 . Answers: 3.0  101 V, 2.0  101 A For problems, it is a good idea to go through the steps on the overhead projector or board so students can see the process instead of just seeing the solution. Allow students some time to work on problems and then show them the proper solutions. Do not rush through the solutions. Discuss the importance of units at every step. Problem solving is a developed skill and good examples are very helpful. This problem provides a good opportunity to review the units for tesla (T), which can be expressed as (V•s)/m2 .

Now what do you think? The loop of wire is rotating in a counterclockwise direction. Electrons in metal are free to move. The magnetic field is horizontal and to the left. The ends of the coil are connected to a load such as a light bulb (not shown). See the next slide for questions. The next slide has a larger view of the diagram, along with questions for students to answer.

Now what do you think? In which direction will the electrons flow around the loop? What is the direction of current in the loop? Use the right-hand rule to find the magnetic field created by the current in the loop. Does this magnetic field oppose the motion of the loop? Electrons flow counterclockwise as seen from above. They exit the connection on the lower left side of the loop. The current is in the opposite direction, clockwise (because conventional current is opposite the direction of electron flow). This induced current creates a magnetic field that comes goes down through the center of the loop (out of the bottom of the loop and into the top). This makes the region above the loop a south pole and the region below the loop a north pole. This magnetic field of the loops opposes the motion of the coil because the south pole of the loop is being rotated into the south pole of the permanent magnet. Lenz’s law is confirmed by the two right-hand rules, one to find the direction of current and the other to find the direction of the magnetic field caused by the current.