9. Electricity and magnetism
10. Electromagnetism

• Galvani’s experiments vs Frankenstein
• What is a battery, how does it work?
• Why are most batteries so heavy?
• Oerstad’s experiment
• How do you make an electromagnet?
• How does an electromagnetic switch work?
• How does a motor work?
• What is the origin of magnetism?
• How does a generator produce AC current?

• So we have Newton, who presented the laws of motion, and we have the the concept of a force, able to be recognized because it can cause a mass to accelerate. Newton formalized our understanding of the gravitational force. In addition, he developed the real important concept of applying force to move a mass a certain distance, with the result being energy, which can be transformed into many different forms. This is the essential concept in all this physics so far. If energy can be transformed into various forms then we may be able to find a way to use natural forces to make energy that we can transform into useful work for us.
• I can’t think off hand of very many useful applications of the gravitational force, except perhaps, hydroelectric power, which uses gravitation to move the water through turbines. We are lucky to have gravity, else how would we stay on the surface of our planet. However, most of the time we spend trying to overcome gravity.
• Anywise, it is the much stronger forces of electricity and magnetism that end up being useful. These took a while to understand since these forces are atomic in nature.
• Magnetism was studied first by Gilbert in the 1500’s, even before Newton, but only a cursory understanding was obtained. There are two poles, N, and S. Like poles repel each other, unlike poles attract. No magnetic poles are ever found that are isolated, they always occur in pairs. Magnetism was left as a curiosity.
• Electricity was also known since at least the Greek times, but formal study started with Ben Franklin in 1750’s when he demonstrated that lightning was electricity. Seeing the spark running down the kite string you could easily understand why he thought that electricity was a fluid. Imagine how difficult is can be to really understand something like electricity before you knew that electrons or even atoms existed. But this is how science is done.
• Coulomb determined the equations governing the magnitude of the electrical force, defining it in terms of a quantity called charge. But they had no idea what charge was. This is similar to the law of gravity, defined in terms of mass, but really, what is mass? These are still abstract concepts, to be better understood by future generations.
• A major scientific milestone was achieved when the connection between magnetism and electricity was found. The practical reason was that we could convert forces from magnets into forces applied through electricity. This is the subject of today’s lecture.

• An Italian named Galvani, late 1700s, was studying the “life force”. What makes a thing alive and so on. He showed that he could apply an electric spark to induce twitching of a dead frog’s leg. Later he demonstrated that he could also get the leg to twitch if he poked the leg at each end with a fork of iron and one of copper.
• He believed that the “life force” was “animal electricity”. Many people believed him.
• Other people carried this to extremes, getting human cadavers to sit up and kick their legs and so on. This led to the writing of Mary Shelley’s story: "Frankenstein".

• Volta did not believe Galvani, he thought that the twitching was a result of chemical reactions between the metals and the salty fluids of the frog’s leg.
• In investigating this for himself, he developed the battery. The battery is a device that converts stored chemical potential into kinetic energy of travelling electrons.

• This was the first time that scientists could also investigate electric charges when moving. Moving electric charges are called an electric current.
• The car battery is an example of such a device. It works by having two plates, Pb (lead) and PbO (lead oxide), connected together by wires that go to your starter. Both plates are immersed in sulfuric acid, H₂SO₄. When the battery is fully charged, and is then discharging, (ie making electricity) the Pb combines with the (SO₄)^-2 to become PbSO₄ (anglesite) and gives off two electrons, leaving H₂⁺² in solution. The PbO gets the 2 electrons, and also combines with the acid to form anglesite. However, it is easier for the electrons to travel along an outside wire then it is to travel through the solution. To recharge the battery you must push the electrons the other way.
• Batteries are one of the hot research areas these days. Pay attention to articles about battery development. Invest in the company that does it right. There are many uses for batteries.
• One of the most important are batteries that run a car, not just start it. This is because of the pollution created by burning gasoline, and using up a non-renewable resource. A major problem with batteries is their weight. This is because the way batteries work is to strip electrons off atoms. The weakest bonded electrons are around the large atoms, i.e. the heaviest ones, like Pb. There are several prototype electric cars driving around Tucson.

• In 1820 Oerstad (from Denmark) was giving a lecture on electricity using a battery. He noticed out of the corner of his eye, that a compass on a nearby table jumped whenever current was present. This set off the study of electromagnetism.
• Magnetic fields can be created by motions of electrical charges.
• The electromagnet is a device composed of a coil of wire that produces a magnetic field whenever a charge runs through the coil.

• Remember that Gilbert studied magnets in the 1500s. He found that if you stroke a piece of iron with lodestone then it will become magnetic. If you hammer iron then it becomes magnetic. Lightning hitting magnetite turns it into lodestone. Heating up a magnet causes it to lose its magnetism.
• Running a current through a coil was a new way to make a magnet. In addition, you could turn this magnet off and on at will. You could also vary its strength by varying the current.
• Many electric switches work from the electromagnet principle. Current is put into a coil of wire, it generates a magnetic field. This pulls a switch, which has a piece of steel, or something attached to it that is magnetic. The switch is under tension so once the electromagnet is turned off, it then springs back to its original position.
• The thermostat works this way. The thermometer signals when the temperature is too low and the electromagnet is turned on. This closes a switch, which allows current to flow, that turns the furnace on.

• Electric motors are another very important electromagnetic device. E.g. fans

• Electromagnets show us why magnets are dipoles. The magnetic field is a result of electrons in circular motion. Right hand rule.
• Lets look at permanent magnets at the atomic scale. Magnetism is related to spinning electrons. Most electrons are paired off in orbitals, we’ll get into that later. While in pairs they spin in opposite directions. So only lone electrons can spin without another electron offsetting the effect. And they cannot be bonded to other atoms, because then they get paired off again. So the main elements for which this works are Fe, Mn, Co because they have 4 or 5 unpaired electrons each.
• Next, many atoms must be oriented in the same way so that the cumulative effect is a magnetic field that can be felt at a distance.

• Explain how stroking a material causes it to go magnetic, hammering it, lightning, heating etc.
• Evidence for life on Mars comes from an interpretation of the shapes of magnetic minerals found in a meteorite that is believed to have come from Mars. The shapes, shown below, are consistent with those found on Earth that have been made by bacteria.

• Oerstad demonstrated that current through a coil produces a magnetic field in 1820.
• In 1831, Faraday demonstrated that a rotating magnetic field can produce an electric field. He put two coils of wire side by side. One of the coils was connected to a battery. He passed current through this one, thus making a magnetic field. He then observed that the other coil started developing a current, although it was not attached to a battery. We now call this electromagnetic induction.
• Faraday was asked by the Prime minister of England what good his electric motors were. He answered that “Someday, you’ll be able to tax it”.
• This also works simply by waving a permanent magnet in the vicinity of a coil of wires. It produces a current.
• A generator can be made by simply spinning a coil of wire through magnets.

• Because of the way the coil spins, the current goes in one way, and then in the other way. We thus end up with alternating current. An advantage of current that is obtained in this way is there is no real flow of electrons. They just oscillate back and forth along the wire.

• Most of our electricity comes from electric circuits that are connected to a generating plant. E.g. the light bulb, and how it works. Generating plant to your home. Through circuit breakers, which can melt if the current gets too high. Then through a switch into the light bulb. The moving electrons heat the wire, causing it to give off radiation, light.
• The flow of electrons is often made analogous to water flowing through a pipe.
• The amount of current is the flow rate. It is measured in amps. The pressure of water is like the voltage. The thickness of the pipe is like the resistance. The greater the resistance the harder it is to make the electrons move.
• The resistance is a result of electrons colliding with atoms.

• This lecture covered an interesting subject as far as its importance in today’s society. Yet, all the work was basic research, learning for learning’s sake. This shows how difficult it is to determine whether funding should be given to a scientific project. Would you have considered funding Galvani’s work on making frog legs jump? Yet, it led to the battery.
• As an aside, we are still looking for a connection between gravity and the other forces. If found, we may be able to tap into gravity to obtain energy.