Fundamentals of Electricity Circuits 1 Fall 2005 Harding University Jonathan White

Outline  Benjamin Franklin / History of Electronics  Atoms/Electrons –Electron shells and orbits –Valence electrons  Charge  Current  Voltage  Electrical Ground  Resistance  Electric circuits

History of Electronics  Began with Benjamin Franklin in 1747 –He determined that electricity was a single force, with positive and negative aspects –Coined over 25 new terms, including armature, battery, and conductor –Famous kite-flying experiment in a thunderstorm was performed in 1752, near the end of his work in the field  Ben Franklin arbitrarily assumed that the actual carriers of electrical current had a positive electrical charge. –While this assumption was wrong, all his experiments still worked, and this assumption is still often used today.  However, the theories of electricity go back a lot farther than this.

History 1  600 B.C. – Thales of Miletus writes about amber becoming charged by rubbing.  1600 – English scientist William Gilbert coins the term electricity from the Greek word for amber. Experiments with magnets, coining the terms electric force, magnetic pole, and electric attraction.  1745 – Dutch physicist Pieter van Musschenbroek invented the “Leyden Jar”, a device that stored static electricity. This was the first capacitor.  1747 – William Watson discharged a Leyden Jar through a circuit. This begins the comprehension of current and circuits.  1800 – Alessandro Volta invents the first electric battery. He also proved that electricity could travel over wires.

History 2  1820 – Oersted and Ampere observe that a coil of wires acts like a magnet when a current is passed through it  1821 – Faraday invents the first electric motor  1826 – Ohm states a relationship between potential, current, and circuit resistance  1873 – Maxwell writes equations that described the electromagnetic field  1876 – Edison Electric Light Co. founded  1879 – First commercial power station opens in San Francisco, uses a Brush generator and arc lights  1883 – Transformer invented  1886 – Alternating current electric system developed by William Stanley  1897 – Electron discovered by J.J. Thompson

Atoms  Atom – Smallest particle of an element that retains all the characteristics of that element. –Consists of positively charge protons and uncharged neutrons in the nucleus, and several negatively charge electrons that surround the nucleus.  Electrons occupy specific energy levels, or “shells” around the nucleus

Electrons  Each shell is discrete and electrons will try and occupy the lowest energy level available to them. –Negative electrons that are close to the nucleus are attracted to the positive nucleus and are tightly bound. –Electrons farther out are loosely bound, and hence have a higher potential energy  However, there is a limit to the number of electrons that each shell can hold. –Number of electrons possible = 2N 2, where N is the shell number.  The outermost shell is known as the valence shell, and the electrons in it are called valence electrons.  The factor that becomes important is that those elements with only 1 or 2 electrons in their outermost shells don’t hold on the them very strongly. Therefore it requires little energy to pull these electrons from their parent atoms and move them someplace else.

The Copper Atom  29 electrons that orbit the nucleus in 4 shells.  When a copper atom gains sufficient thermal energy, it can break away from the parent and become a free electron. –This happens at room temperature for copper.

Categories of Materials  Conductors: readily allow current, large number of free electrons, 1 – 3 valence electrons. What’s the best conductor? Exp: silver, copper, gold, aluminum, iron.  Semiconductors: 4 valence electrons in their structures. Exp: silicon, germanium.  Insulators: poor conductors of electric current, more than 4 valence electrons.

Electrical Charge  2 types, positive and negative. Charge of a proton is equal in magnitude to the charge of an electron. –Symbolized by letter Q.  Static electricity is the presence of a net positive or negative charge.  Like charges repel, unlike charges attract. –This attraction or repulsion is a force called an electromagnetic field. –This force and gravity are the only forces that we humans can experience directly.  Charge is measured in coulumbs –One coulumb is the total charge possesed by 6.25 X electrons  Like matter, charges follow an inverse square law. –F 1,2 = q1 q2/(4πE 0 r2)  Where q1, q2 are measured in coulumbs, r is the distance between the charges measured in meters, and E 0 is a fundamental constant of nature, = ~ x Farads/meter  However, charge is different than matter. Does anti-matter have a negative mass? Also, two masses always seem to attract each other.

Charge 2  The charge on an electron is always the same for every electron in the universe.  If you have one electron and one proton, then the two charges cancel each other out and the atom is said to have no charge. Also, the electron and proton will cancel out each other exactly.  The charge on an electron is a fundamental quantity - a constant of nature.  Where is charge used? –When you plug in a light bulb, charge flows from the socket, through the connecting wire, and then through the bulb filament, heating it up and giving off light. The electrons aren’t destroyed, but they do lose energy. –Your car battery stores energy by storing charge on the battery plates. When you start your car, charge flows from your battery to the engine, providing enough energy for the vehicle to run.

Current  Current – charge in motion. –The “flowing” of charges through something.  We typically think of charge flowing through a wire, but it can also flow through water, air and even vacuum.  You can think of current as water flowing through the interior of a pipe, though current actually flows though the empty spaces between atoms in a wire.  Current is represented by the mathematical symbol i. –i = Q/t, or, current is equal to the number of electrons that flow past a point in a given amount of time. –Current is measured in amperes, which is equal to coulombs/sec. Amperes is abbreviated with the letter A.  Current is a “through” variable, meaning that in order to measure it, you need the current to go through something.

Current 2  Charge comes in discrete packets, but it is useful to assume that it can take continuous values. Then, we can imagine making the time frame very small and find the current at an instant. Then, i(t) = dQ(t)/dt  It takes energy to make charges flow through something. The energy that makes current is called voltage.

Voltage  Can be thought of as the driving force behind the current (though it isn’t really a force).  Voltage is the energy per unit charge.  Current flows through electrical elements when a voltage appears across the terminals of the element, similar to when water flows through a pipe when a pressure difference appears across the pipe. –Voltage is an “across” variable. We talk about pressure differences and voltage differences.  Voltage is related to potential energy. Voltage is defined as the electrical potential energy that a charge has by its position in space. –If you pull two charges apart, you put potential energy into the system –That potential energy can be converted into other forms of energy –Energy can neither be created or destroyed, only transferred

Voltage 2  A mass m, h meters above the Earth has potential energy mgh. A mass at h=0 has 0 potential energy. A charge at electrical ground has 0 potential engergy. Voltage is the potential energy per unit charge, or V= W/Q.  If we move a charge from point A to point B, and put a given number of joules of work into the charge, we will recover exactly the same number of joules from the charge if it moves back from point B to point A. If we move the charge through any closed path or circuit, there will be no net energy input to the system and no net energy recovered from the charge.

Voltage and Batteries  Batteries are voltage sources.  Batteries can be thought of as charge pumps. –They take a charge and though chemical reactions pump them up to a certain voltage, or potential energy level.  As the charge flows through the circuit, this potential energy can be used by the circuit to do work. The charge loses energy as it goes through the loads. –Heat up a filament –Make a motor turn.  Energy gained from the battery = energy lost by the loads. –Law of conservation of energy

Ground  Reference voltage from which all other measurements are measured – the potential of the Earth. –Defined as having 0 V potential energy with respect to the rest of the circuit.  In physics equations, ground level is used as the point of 0 potential energy when lifting a weight, another thing electrical systems have in common with mechanical systems.  In wiring for houses, the ground is physically connected to the Earth – a place of 0 potential energy when compared to the rest of the wiring.  Ground provides a return path for the current back to the source because all the ground points are electrically the same point and provide a zero resistance path

Resistance  Opposition to the flow of current.  When there is current through any material that has resistance, heat is produced by the collisions of electrons and atoms.  Can be thought of as partially closed valve in our pipe system – it restricts the flow of water.

Circuits  Consists of a voltage source, a load, and a path for current between the source and the load. –A load is a device on which work is done by the current through it.  Open circuits versus closed circuits.

Review  Charge –Definition: Fundamental property of matter based on the absence or excess of electrons. –Symbol: Q –Measured in Coulombs. –1C = total charge of 6.25 X electrons –2 types, positive and negative

Review  Current –Definition: Charge in motion –Symbol: I –Measured in amperes (A). –I = Q/T, current is the amount of charge that passes a point in a given amount of time. Or, i(t) = dq / dt –Current is a “through variable”. –The water that’s flowing through a pipe.

Review  Voltage –Definition: Energy per unit charge. –Symbol: V – V= W/Q, voltage is the energy per unit charge. Where, W is the energy expressed in joules, Q is in Coulombs. –Voltage is an across variable. –The pressure that pushes the water through the pipe.

Ground and Resistance  Ground –Definition: Reference point that is at 0 volts with respect to all other points in the circuit.  Resistance –Opposition to the flow of electrons. –The resistance caused by the collision of electrons and atoms causes heat to be given off.