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A Brief History of Electricity and Magnetism Electromagnetism Electromagnetism is one of the fundamental forces in nature, and the the dominant force in a vast range of natural and technological phenomena The electromagnetic force is solely responsible for the structure of matter, organic, or inorganic Physics, chemistry, biology, materials science The operation of most technological devices is based on electromagnetic forces. From lights, motors, and batteries, to communication and broadcasting systems, as well as microelectronic devices. Electrostatics Electric Charge The Transfer of Charge SILK Glass Rod Some materials attract electrons more than others. Electric Charge The Transfer of Charge + - SILK Glass Rod As the glass rod is rubbed against silk, electrons are pulled off the glass onto the silk. Electric Charge The Transfer of Charge + + - SILK Glass Rod Usually matter is charge neutral, because the number of electrons and protons are equal. But here the silk has an excess of electrons and the rod a deficit. Electric Charge The Transfer of Charge + + - + + + SILK Glass Rod Glass and silk are insulators: charges stuck on them stay put. Electric Charge History 600 BC 1600 AD 1735 AD 1750 AD 1770 AD 1890 AD Greeks first discover attractive properties of amber when rubbed. Electric bodies repel as well as attract du Fay: Two distinct types of electricity Franklin: Positive and Negative Charge Coulomb: “Inverse Square Law” J.J. Thompson: Quantization of electric charge - “Electron” Electric Charge Summary of things we know: – There is a property of matter called electric charge. (In the SI system its units are Coulombs.) – Charges can be negative (like electrons) or positive (like protons). – In matter, the positive charges are stuck in place in the nuclei. Matter is negatively charged when extra electrons are added, and positively charged when electrons are removed. – Like charges repel, unlike charges attract. – Charges travel in conductors, not in insulators – Force of attraction or repulsion ~ 1 / r2 Charge is Quantized q = multiple of an elementary charge e: e = 1.6 x 10-19 Coulombs electron proton neutron Charge -e +e 0 Mass 1 1836 1839 Diameter 0 ~10-15m ~10-15m positron +e 1 0 Coulomb’s Law q1 q2 F12 r12 F 12 kq1q2 2 r12 Force on 2 due to 1 k = (4pe0)-1 = 9.0 x 109 Nm2/C2 Coulomb’s law describes the interaction between bodies due to their charges Gravitational and Electric Forces in the Hydrogen Atom M +e r12 -e m Gravitational force m = 9.1 10-31 kg M = 1.7 10-27 kg r12 = 5.3 10-11 m Electric Force Mm Fg G 2 r r12 1 Qq 2 Fe 4pe0 r12 Fg = 3.6 10-47 N Fe = 3.6 10-8N Conductors and Insulators • Conductors are materials in which the electric charges move freely – Copper, aluminum and silver are good conductors (more or less all metals!) – When a conductor is charged in a small region, the charge readily distributes itself over the entire surface of the material Insulators • Insulators are materials in which electric charges do not move freely – Glass and rubber are examples of insulators – When insulators are charged by rubbing, only the rubbed area becomes charged • There is no tendency for the charge to move into other regions of the material Semiconductors • The characteristics of semiconductors are between those of insulators and conductors • Silicon and germanium are examples of semiconductors Charging by Conduction • A charged object (the rod) is placed in contact with another object (the sphere) • Some electrons on the rod can move to the sphere • When the rod is removed, the sphere is left with a charge • The object being charged is always left with a charge having the same sign as the object doing the charging Metallic sphere Charging a Metal Object by Induction • When an object is connected to a conducting wire or pipe buried in the earth, it is said to be grounded • A negatively charged rubber rod is brought near an uncharged sphere • The charges in the sphere are redistributed – Some of the electrons in the sphere are repelled from the electrons in the rod Examples of Polarization • The charged object (on the left) induces charge on the surface of the insulator • A charged comb attracts bits of paper due to polarization of the paper Resistance in a Series Circuit • Each resistance in a series circuit adds to the total resistance of the circuit. Rtotal = R1 + R2 + R3... Total resistance (ohms) Individual resistances (W) • Each separate resistance creates a voltage drop as the current passes through. • As current flows along a series circuit, each type of resistor transforms some of the electrical energy into another form of energy • Ohm’s law is used to calculate the voltage drop across each resistor. Resistance in Parallel Circuit • In parallel circuits the current can take more than one path. • Because there are multiple branches, the current is not the same at all points in a parallel circuit. Short Circuit • A short circuit is a parallel path in a circuit with zero or very low resistance. • Short circuits can be made accidentally by connecting a wire between two other wires at different voltages. • Short circuits are dangerous because they can draw huge amounts of current. Circuit Analysis What is magnetism? Magnetism is the properties and interactions of magnets The earliest magnets were found naturally in the mineral magnetite which is abundant the rock-type lodestone. These magnets were used by the ancient peoples as compasses to guide sailing vessels. Magnets produce magnetic forces and have magnetic field lines Magnets have two ends or poles, called north and south poles. At the poles of a magnet, the magnetic field lines are closer together. Unlike poles of magnets attract each other and like poles of magnets repel. The earth is like a giant magnet! The nickel iron core of the earth gives the earth a magnetic field much like a bar magnet. What are magnetic domains? Magnetic substances like iron, cobalt, and nickel are composed of small areas where the groups of atoms are aligned like the poles of a magnet. These regions are called domains. All of the domains of a magnetic substance tend to align themselves in the same direction when placed in a magnetic field. These domains are typically composed of billions of atoms. Electricity and Magnetism – how are they related? When an electric current passes through a wire a magnetic field is formed. What is an electromagnet? When an electric current is passed through a coil of wire wrapped around a metal core, a very strong magnetic field is produced. This is called an electromagnet. What are electric motors? An electric motor is a device which changes electrical energy into mechanical energy. How does an electric motor work? Go to the next slide Simple as that!! We have seen how electricity can produce a magnetic field, but a magnetic field can also produce electricity! How? What is electromagnetic induction? Moving a loop of wire through a magnetic field produces an electric current. This is electromagnetic induction. A generator is used to convert mechanical energy into electrical energy by electromagnetic induction. Carefully study the next diagrams: