In an unmagnetized piece of iron, the atoms are arranged in domains. In each domain the atoms are aligned, but the domains themselves are random.

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1 4/7 Properties of the Magnetic Force 1. Perpendicular to the field and velocity. 2. If the velocity and field are parallel, the force is zero. 3. Roughly (field and vel perp), the force is the product of charge, vel, and field. 4. Force on negative charge is opposite the force on a positive charge. Electric Fields apply forces to electric charges. We know that electric charges produce electric fields. Magnetic Fields apply forces to electric currents. Do electric currents produce magnetic fields? Yes!!! Oersted connected the following circuit and placed a compass over the wire. North is toward the top of the diagram, which means that the compass points in that direction. When the switch is closed, a current I is produced as shown, and the compass needle swings from north to west. The current produces a magnetic field! The magnetic field around a long straight wire carrying current is in circles around the wire given by the right-hand rule. Point the thumb of your right hand in the direction of the current your fingers wrap around the wire in the direction of the magnetic field produced by the current. If magnetic monopoles don t exist, then all magnetic fields are produced by electric currents. What about a permanent magnet? Due to electron motion in atoms. Consider a loop --- like an electron orbiting a nucleus. In a ferromagnetic substance --- such as iron --- exchange forces between adjacent atoms tend to keep the magnetic fields of the atoms aligned. In an unmagnetized piece of iron, the atoms are arranged in domains. In each domain the atoms are aligned, but the domains themselves are random.

2 Place a strong magnetic field on the iron, and those domains with fields in the direction of the external field will grow at the expense of neighboring domains that aren t. Remove the external field, and there will still be more domains with fields in the direction of the external field --- get a permanent magnet. Note the similarity between the magnetic field of a bar magnet (on the left) and that due to solenoid coil (on the right). The Electric Motor Put a coil of wire carrying a current into a magnetic field. The current is into the page on top with the magnetic field directed upward. By the right-hand rule, the magnetic force is as shown. Since the current in the bottom part is opposite, so is the force. Torque on the coil produced by the magnetic forces causes the coil to rotate. The top shows the same configuration as the figure above but edge on. As the loop rotates, it becomes horizontal where the torque has become zero. As it rotates past horizontal, the torque reverses direction. Problem --- once the coil has passed horizontal, the torque reverses and the coil flips back --- oscillates. We need to get the current into the coil and we need to reverse the current --- use a commutator as shown below right. A voltage source such as a battery is connected to brushes that rub against the commutator, which is the small cylinder in the figure. The tan part of the commutator is an insulator with the grey parts being conductors. Note that there is a gap between the conductors. As shown, current travels in on the right and out on the left. The coil is connected to the conductors. As the coil rotates, the brushes slide over the commutator. Once the brushes pass a gap, the direction of the current is reversed in the coil. This maintains the same direction of torque on the coil, and it continues to rotate. Chapter 25 Electromagnetic Induction Almost all electrical power is produced by electromagnetic induction. Governed by Faraday s Law roughly a changing magnetic field will produce an electric field.

3 If a conductor is located where the electric field is located, a current will be produced. For power production need to consider magnetic flux. The flux of the magnetic field through an area is roughly how much magnetic field gets through the area. Roughly, the magnetic flux is the product of the magnetic field and the area. If we change the magnetic flux over the interior of a conducting loop, a current will be induced in the loop. Three ways to change the magnetic flux through the interior of the loop: 1. Change the magnetic field in the top figure, the magnetic field increases with the area staying the same, which means that the flux increases 2. Change the area in the middle figure, the area decreases with the magnetic field remaining the same, which means that the flux decreases. 3. Change the orientation of the loop relative to the direction of the magnetic field in the bottom figure, the loop rotates so that the plane of the area and the magnetic field are parallel, which means the flux decreases because the final flux is zero. No practical application of #2, there are for 1 and 3. Almost all of our electricity is generated using number 3. Number 3 is the basic principle behind the electric generator. Cause the loop to continuously rotate in a magnetic field and a current is continuously produced in the loop. We need to get the current out of the loop. Use a system similar to an electric motor. Here we attach the leads from the coil to slip rings that brushes slide over. Wires connect the brushes to a light bulb. Note that some agency is causing the coil to rotate. Number 1 is the reason we have alternating current coming out of the wall. Number 1 is the idea behind the electrical transformer. The electrical transformer is used to increase or decrease alternating current voltages. Two coils wound on top of each other or around a common iron core. Note that the picture at right is just representation of a transformer. In fact, the transformer has a third vertical bar extending from center

4 of the top part to the center of the bottom part, and both coils are wound around this third bar. If we feed ac into one coil the primary coil we will get ac from the other side secondary--- due to the varying magnetic flux produced by the varying magnetic field produced by the ac primary current. If the number of turns in the secondary is greater than the number of turns in the primary, the secondary voltage is greater than the primary voltage step-up transformer. If the number of turns in the secondary is less than the number in the primary, the secondary voltage is less than the primary voltage step-down transformer. We want to transmit electrical power at high voltage and low current to reduce loss of energy in transmission. We transmit power at high voltages tens of thousands up to 100,000 V but use a transformer in each neighborhood to lower the voltage to the more safe 110 V available from electrical receptacles in our homes. If the number of turns in both sides are the same, used for isolation. If the circuit on the primary side is grounded, there is no path to this ground from the secondary side. This is why the fat plug ac adapters are not polarized. We don t get something for nothing electric power current voltage in is the same as electric power out. When voltage is stepped up, current is stepped down by the same factor. And vice versa. Note, too, that every electronic device in our homes operates on direct current. Each device has a circuit called a rectifier that converts ac to dc. The heart of a rectifier is a diode, which only allows current in one direction. Putting ac through a diode produces pulsating dc. This pulsating dc is put through a filter that removes high frequencies and can produce smooth direct current. In the figure, the arrow objects are diodes that only allow current in the direction of the arrow. When a crest comes in, the top junction is positive relative to the right-hand junction, and a positive pulse is created in the output. When a trough comes in, the bottom junction is positive relative to the right-hand junction, and again, a positive pulse is created in the output. This type of rectifier is called a full-wave rectifier. Maxwell s Equations Made up of four equations: 1. Gauss Law electric charge causes electric fields. 2. Gauss Law for Magnetism magnetic monopoles don t exist. 3. Faraday s Law varying electric fields are produced by varying magnetic fields. 4. Ampere s Law magnetic fields are produced by electric currents and by time varying electric fields.

5 These equations predict the existence of electromagnetic waves. They predict the speed of these waves to be the speed of light. This means that light is a type of electromagnetic wave. Radio, Television, and Cell Phones Take an electromagnetic wave and encode it with information send it out it is received by a receiver which decodes the signal. Two methods for encoding the signal. 1. FM frequency modulation 2. AM amplitude modulation The sequence of events that produces a radio signal: 1. An electronic oscillator produces a carrier wave at the frequency of the radio station. 2. Some device microphone modulates the carrier wave with a signal that it produces. The wave third from the top shows amplitude modulation; the bottom one shows frequency modulation. 3. Goes to the transmitter amplifies the signal to the allowed power for the station. 4. The transmitter drives the antenna cause current to slosh up and down in the antenna producing a time varying magnetic field that produces a time varying electric field that produces a time varying a magnetic field an electromagnetic wave. 5. When the wave gets to the radio, it causes current in the antenna of the radio that replicates the current in the broadcast antenna but is, of course, much smaller. 6. In the radio, a circuit resonates at the frequency of the radio station and rejects frequencies from other stations. 7. In an AM radio, the bottom half of the signal is removed using a diode leaving the amplitude variation of the top half of the signal, which is amplified and sent to the speaker. 8. In an FM radio, a signal is generated that has the same frequency as the carrier. Combined with the signal from the radio station produces beats. These beats form the signal that is amplified and sent to the speaker. Chapter 26 Properties of light. The Electromagnetic Spectrum Goes from radio waves at one end to gamma rays at the other.

6 In terms of wavelength Radio Waves 10 cm and up Microwaves 10 cm to 0.1 mm Infrared 0.1 mm to 700 nm (nm is one-billionth of a meter) visible 700 nm to 400 nm Ultraviolet 400 nm to 10 nm X Rays 10 nm to 0.1 nm Gamma Rays 0.1 nm and down