* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Notes: Electricity and Magnetism
Survey
Document related concepts
State of matter wikipedia , lookup
Anti-gravity wikipedia , lookup
Magnetic field wikipedia , lookup
Maxwell's equations wikipedia , lookup
Field (physics) wikipedia , lookup
Magnetic monopole wikipedia , lookup
History of electromagnetic theory wikipedia , lookup
Aharonov–Bohm effect wikipedia , lookup
Superconductivity wikipedia , lookup
Electromagnetism wikipedia , lookup
Electrical resistance and conductance wikipedia , lookup
Electromagnet wikipedia , lookup
Lorentz force wikipedia , lookup
Electrical resistivity and conductivity wikipedia , lookup
Transcript
ISCI 2002 Notes – Electricity and Magnetism Electricity What exactly is electricity, and how is it produced? To understand electricity it is important to remember what you learned about atoms. An atom can be in its stable state or electrons equaling protons. This balances the atom or makes it electrically neutral. Also remember that metal atoms tend to lose electrons and become positive ions called cations. Nonmetal atoms tend to gain electrons and become positive ions or anions. Some materials, like metals, lose these electrons very easily (conductors). Plastics or wood tend to hold electrons very tightly (insulators). Believe it or not electrons are dislodged easily from your hair. Electrons are transferred to plastics like a comb. Your hair stands on end, and in the meantime the plastic comb picks up the electrons. The protons in the comb are attracting the electrons dislodged from your hair. So electricity can be described in one way as a transfer of electrons from one substance to another. This refers to the conservation of charge (see book). Electrical forces can be compared to gravitational forces also. Remember how the inverse square law applied to gravity? Remember Newton’s Law of Universal Gravitation? Many of the same rules apply to electrical force. The force of electricity is directly related to (q) charge of the particle and inversely related to the distance between them. This is called Coulomb’s law. See the formula on page 121. The force of the electrical current acts along a straight line from one charge to another. The unit of charge is the coulomb or (C). One coulomb equals 6.25 billion billion electrons. This is the amount of charge that flows through a 100 watt light bulb in a little more than a second. The charge on a single electron equals (after dividing the value of 1 C by the number of electrons) is 1.60 x 10-19. So the Universal law of Gravitation and Coulomb’s law are very similar except that gravitational forces are only attractive where as electrical forces can be both attractive and repulsive. If you take a balloon and rub it against your hair and then place it on the wall, it will usually stick to the wall. When the charged balloon touches the wall it effectively alters the charge distribution on the wall by altering their centers of charge. Positive (protons) are attracted to the balloon where as the electrons or negative charges move away or are repelled. The wall and its electrons have been polarized (charges move to opposite ends). What is an electrical field? What is Electrical Potential? Remember that a gravitational field is produced when two objects with specified masses are a specific distance apart. There is a space which surrounds each mass. This is called a gravitational field. The spaces around the masses are altered. So each mass interacts with the field, not directly with the other mass. How is this similar to an electrical force? Any space around an electric charge is filled with an electric field (extends through space similar to the gravitational force). This is a vector field (has magnitude and direction). The magnitude is the force per unit charge. If a charge (q) experiences a force (F) then the electric field becomes E = F/q. Direction is away from the positive charge and towards the negative charge. When a charged particle is placed in an electric field will experience a force. The direction of the force on a positive charge is the same direction as the field. This can be seen as field lines (figure 7.8). Field lines also show the intensity of the field. If the lines are tightly bunched together, this shows a very strong force. The field and field lines about an electron point towards the electron and about a proton points in the opposite direction or away from the proton. Any charged object has potential energy depending on its location in an electric field (similar to an objects potential in a gravitational field or GPE = mgh. Work is required to push a particle against the electrical field of a charged body. This changes the electric potential of the charged particle. If a positive particle is pushed closer to a positively charged sphere (such as a Van de Graff generator we will work with in class) increases the potential energy of the charged particle. The particle now has a certain amount of electric potential energy. If released, it travels away from the positive sphere with a specific amount of KE. If you push a particle with twice the charge you do twice the work. Twice the charge also possesses twice the potential energy. When working with electricity you consider the electric potential energy per charge. You do this by dividing the amount of energy by the amount of charge. This refers to electric potential: Electric Potential = electric potential / charge. The unit for Electric Potential is the volt and referred to as voltage. One volt (V) equals one joule (J) of energy per (C) of charge or: 1 Volt = 1 Joule / (C) - 1.5 volt battery give 1.5 joules of energy to every one coulomb of charge flowing through the battery. What is the significance of Electric Potential or Voltage? Definite values for it can be assigned to a location. For example in an electrical circuit, 12V higher charge is maintained at the positive terminal as compared to the negative terminal. This is important because when these terminals are used (connected) the charges will move between each location due to this difference (current). *Be sure to read about conductors and insulators on page 124! Voltage Sources and Electrical Current For charges to ‘flow’ between terminals, there must be a potential difference between them (as described above). Flow always moves from an area of higher potential to lower potential. To sustain a current flow requires a pumping device to maintain a proper difference in electric potential. Examples are batteries that do this by pulling negative charges apart from positive charges. Chemical batteries do this by disintegrating lead or zinc in acid. Generators separate charges by a process called electromagnetic induction. When this occurs the charged particles then flow in a specific direction. This is called an electrical current. In metals electrons move from the metal atoms move freely around the other metal atoms. In solids, the protons are fixed, but in fluids they flow as well (automobile batteries). So when you apply voltage to a metal wire you are basically setting them in motion in a certain direction and this is an electrical current. The rate of electrical flow is measured in amperes (rate of flow of 1 (C) of charge per second or 6.25 billion-billion electrons per second). The speed of the flow of electrons is slow. They do not move unobstructed. They bump into other atoms constantly. The drift speed is less than one centimeter per second. What is the difference between Direct and Alternating Currents? Electrical devices use both AC and DC devices. If you plug it into the wall you are using AC current. But if are using batteries you are using DC current. DC refers to direct current. Charges only flow in one direction. In batteries flow is from or away from the negative terminal to the positive terminal. It is also one way through a circuit (explain later). AC or alternating current works by charges moving in one direction and then moving in the opposite direction. Generators or alternators switch signs at terminals to achieve this movement. Most all AC currents in this country alternate at 60 cycles per second or 60-Hertz (Hz) current. Other countries may have 50, 25 cycles per second. We should mention circuits before we move on. Most circuits are either series or parallel. As you see in figure 7.21 a simple series circuit is a one-way operation with a switch at some point between the lights and battery. Current flows through each light or the same current exist in all three lights at the same time. Charges move from the negative terminal through the lights and eventually back to the positive battery terminals. The path again is one way. If the circuit is broken at any point the whole circuit is shut down! If one light burns out and breaks the circuit, then all of the lights in that circuit go out. Some older Christmas tree lights are series circuits. Lights in our house are not wired this way. Most lighting in homes are done via a parallel circuit. See figure 7.22. You see three lamps connected to the same two points A and B. When the electrons leave the negative terminal of the battery they only have to travel through one lamp before returning to the positive terminal. You have three separate pathways. If one pathway is broken it does not affect the other circuits. Electrical current does meet resistance in wires and other substances used to promote electrical current. Thin wires produced more resistance than thicker wires. Copper has a low level of resistance as compared to other metals. Rubber is used as an insulator because it has a very high level of resistance to electrical flow. Ever heard of a superconductor? This is any substance that when placed in low temperatures its resistance will equal zero! An Ohm is the unit for electrical resistance (Ω). Ohm’s Law explains how resistance is measured. It explains the relationship between current flow, voltage and resistance. Current is directly proportional to voltage and inversely proportional to resistance. Makes sense right! Current = (V)/ (Ω); or Amperes = volts / ohms; or I = V/R In lamp cords there is a resistance of about 1 ohm and the light bulb provides more than 100 ohms of resistance. Inside each of these are resistors which regulate the flow of current. Please read about how one receives an electrical shock and what produces the shock on pages 127-128. You should be able to work the following math applications: Ohm’s law problems on page 127, 129 Electrical Power on page 130 Magnetism What is a magnetic force? Very similar to electrical forces except the magnetic poles produce magnetic forces instead of electrical forces. Magnets have two distinct poles, north and south. Like poles attract each other and opposite poles attract. There is a big difference in electrical charges and magnetic poles. Electrical charges can be separated and are separate entities (electrons and protons) where as magnetic poles are like the heads and tails of a coin. If you break a magnet in the middle there will still be a north and south pole. See figure 7.25. The broken pieces will still act as a whole magnet. What are magnetic fields? Magnetic fields are produced by moving electric charges. The electrons that make up the magnet are in constant motion. The spin of the electrons and the revolution produce this movement. When an electron spins it produces a magnetic effect. If they spin in the same direction it makes a strong magnet. It is important to note that in most substances electrons spin in opposite direction cancelling the effect out. Only in iron, nickel and cobalt do we see electrons that spin in the same direction. So iron atoms are in effect a tiny magnet. Magnets that you buy are mostly made of iron, nickel, cobalt and aluminum (alloy). Spaces around a magnet produce a magnetic field. Place a piece of paper on top of a magnet and the spread iron filing on top of the paper. You see the lines of the magnetic field on the paper (figure 7.27). Distinct patterns are seen. Direction of the field is from the north to the south pole. What is a Magnetic Domain? When the atoms in iron spin in the same direction they produce a strong magnetic field. The atoms line up in the same direction producing magnetic domains. See figure 7.30. Again the direction is from north to the south pole. How are electric currents and magnetic fields related? When electric charges move or a current of charges is produced this produces a magnetic field. This field that surrounds a wire makes a pattern of concentric circles. The needle of a compass will be pointed in one direction. When the current is reversed, the needle will reverse its direction. If you place iron in a current-carrying coil of wire the alignment of the domains in the iron produces an electromagnet which is a very strong magnet. Ever seen a car or large piece of metal lifted up? This is the work of an electromagnet. Magnets and Electric Motors A simple motor works like this. Current in a motor is made to change direction of a coil which makes a half rotation. It produces a continuous rotation. See page 136. A magnet produces a magnetic field in an area of a loop of wire which is mounted to turn about an axis. Current passes through the loop it flows in the opposite direction in the upper and lower sides of the loop. One part is forced to the left and the other to the right. Current is then reversed or alternated during each half revolution. Rotation is continuous (so long as current is supplied). This would be example of a DC motor. In larger motors a simple magnet is replaced with an electromagnet. Important Terms 1. Diamagnetic materials – metals which have weak and negative susceptibility to magnetic fields. Repelled by magnetic fields. Solids with paired electrons that have opposite spins (most metals) 2. Paramagnetic – metals such as Mg, Li. Somewhat susceptible to a magnetic field. Slightly attracted to the field, but does not retain magnetic properties when the field is removed. In these type substances there are some unpaired electrons present in the metals. 3. Ferrromagnetic- metals which have a great amount of unpaired electrons with opposite spins. Strong attraction to the magnetic field and retains magnetic properties when the field is removed. Iron, cobalt and nickel are examples. When the field is applied domains are produced (atoms line up).