Magnetism from Electricity http://www.bbc.co.uk/schools/gcsebitesize/science/triple_aqa/keeping_things_moving/the_motor_effect/revision/1/

Goal of the class Describe the magnetic field produced by current in a straight conductor and in a solenoid. Use the right-hand rule to determine the direction of the magnetic field in a current-carrying wire. Question of the Day: How does a CRT TV display a picture? Previous answer: The southern magnetic pole is near the geographic north pole. Previous question: Where is the magnetic field of the earth?

Electromagnets 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? Would such a magnet require the use of direct current or alternating current? 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. Students likely will know the answers to the first two questions but the last two questions ask for an understanding of WHY this occurs. Encourage them to think about how the particles inside the nail might behave differently than those in the magnet.

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 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 centre of the loop? C B A Students could investigate the model using electron spin to help understand why some materials are ferromagnetic. The domains each have their own magnetic fields. When they are somewhat aligned, the magnetic fields add to give a net magnetic field.

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 Students could investigate the model using electron spin to help understand why some materials are ferromagnetic. The domains each have their own magnetic fields. When they are somewhat aligned, the magnetic fields add to give a net magnetic field.

Electromagnets 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? Would such a magnet require the use of direct current or alternating current? 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.

Doorbell Circuit The electromagnetic doorbell is an example of a self-interrupting electrical circuit. Pushing the bell button closes the switch (S) and connects the electromagnetic coil (M) to the battery (B). The activated electromagnet then attracts the clapper (A), which hits the bell but also interrupts the electric circuit at the contact (C). The magnet is then cut off and the clapper is released, closing the circuit…. This process is repeated as long as the button is depressed.

Charged Particles in a Magnetic Field Magnetic fields exert a magnetic force on moving charged particles. Force is greatest when the movement is perpendicular to the magnetic field Force is zero when the particle moves along the field lines Force is in between these values for other directions When the movement is perpendicular, the magnetic force is: Fmagnetic = qvB where q is the charge, v is the velocity, and B is the magnetic field strength. This text limits discussion to situations in which charges move parallel or perpendicular to the magnetic field lines. When the movement is perpendicular, the equation shown on the slide applies. When the movement is parallel, the force is zero. F=qvxB

Practice An electron moving north at 4.5  104 m/s enters a 1.0 mT magnetic field pointed upward. What is the magnitude and direction of the force on the electron? 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 is a good application of the equation and the right-hand rule. Students will need to know the charge on an electron. Help them use the right-hand rule appropriately for a negative charge like the electron. You can also ask students what the force would be if the magnetic field was north or south instead of upward. (Zero, because the velocity and B field are parallel) Answer = 7.2  10-18 N west

Magnetic Force as Centripetal Force Determine the direction of the force. Which direction would the force be when the charge is at the top? the left side? the bottom? Always directed toward the center Because of this magnetic force, the charge moves in a circle. The force is centripetal. Charged particles entering Earth’s atmosphere get trapped into circular motion by the magnetic field and spiral toward the poles. This phenomenon causes the northern and southern lights.

Magnetic Force in a Current Carrying Wire Magnetic forces also exist on the moving charges in current-carrying wires. The right-hand rule to is used to determine the direction, as shown in the diagram. The magnitude of the force is as follows: F=IlxB

Applications - Cathode Ray Tube Old televisions and computer monitors use CRTs. A magnetic field deflects a beam of electrons back and forth across the screen to create an image.

Homework Please complete questions on page 681 Q 14, 22, 30, 38, 46