Chapter 19 Magnetism Section 2 Magnetism from Electricity

What do you think? Electromagnets are used every day to operate doorbells and to lift heavy objects in scrap yards. Why is the prefix electro- used to describe these magnets? Is electricity involved in their operation or do they create electricity? Would such a magnet require the use of direct current or alternating current? Why? 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. Students may be familiar with electromagnets but unclear about their operation. Help them clarify their ideas about how electromagnets operate.

Magnetism from Electricity A compass needle held near a current carrying wire will be deflected. Electric current must produce a magnetic field. Discovered by Hans Christian Oersted Many compasses placed around a vertical current carrying wire align in a circle around the wire.

Right-Hand Rule To find the direction of the magnetic field (B) produced by a current (I): Point your right thumb in the direction of the current Curl your fingers and they will show the direction of the circular field around the wire. Remind students that current (I) is opposite the flow of electrons.

Magnetic Fields C B A Use the right hand rule to decide what direction the magnetic field would be at points A, B, and C. Since magnetic fields are vectors, how would the net field appear in the center of the loop? The fingers of the right hand should always curl around the wire and upward through the middle. This creates a magnetic field that comes out of the top of the loop and into the bottom of the loop. This is demonstrated in the Visual Concepts clip on the next slide.

Magnetic Field of a Current Loop Click below to watch the Visual Concept. Visual Concept

Magnetic Field Around a Current Loop Magnets and loops of wire have magnetic fields that are similar. Solenoids are coils of wire similar to the single loop. More loops strengthens the field Placing an iron rod in the center strengthens the field as well Called an electromagnet For solenoids, all loops must go in the same direction to make the B fields additive. An iron core will become magnetized by the field and thus adds its own B field to that of the solenoid. Do the quick lab in this section of the Student Edition, and hold a compass near the end of the coil with the nail and without the nail to demonstrate the enhancing effect of the iron core. The PhET website may be useful at this time. http://phet.colorado.edu/new/index.php Choose “Simulations,” then choose “Electricity, Magnets and Circuits,” then choose “Faraday’s Electromagnetic Lab.” At this time, it would be useful to show students the “Electromagnet” option. You can show a compass and the field. You can reverse the electron flow (note that it is the opposite the direction of current) by adjusting the potential difference. You can also change the number of loops in the solenoid. There is an AC option as well. Ask students what will happen with each case before running the simulation.

Now what do you think? Electromagnets are used every day to operate doorbells and to lift heavy objects in scrap yards. Why is the prefix electro- used to describe these magnets? Is electricity involved in their operation or do they create electricity? Would such a magnet require the use of direct current or alternating current? Why? An electric current produces a magnetic field. With an iron core, a coil of current-carrying wire can behave just as a magnet would. The advantage in doorbells and scrap yards is the fact that, when the electric current is turned off, the magnetism is dramatically reduced in the iron core, and the device returns to a nearly unmagnetized state. As a result, the doorbell chime springs back to its starting position, and the crane drops the scrap metal. DC current is necessary to align the domains within the iron core. AC would keep switching them back and forth. High-frequency AC current is used as a demagnetizer.