Basic Electricity. ME 120 Lecture Notes. Portland State University Mechanical and Materials Engineering

Transcription

1 Basic Electricity ME 120 Lecture Notes Portland State University Mechanical and Materials Engineering

2 Learning Objectives Successful completion of this module will enable students to Link the basic model of an atom to the flow of electricity Apply the definitions of Amp, Volt, Coulomb, Joule, Watt to unit conversions and basic problems involving current and voltage Apply Ohm s Law to simple DC circuits

3 Definition Electricity is a form of energy resulting from the existence of charged particles (such as electrons or protons), either statically as an accumulation of charge or dynamically as a current. Concise Oxford English Dictionary, revised 10 th edition

4 Definition Conductor: A conductor is a material that readily allows the flow of electricity. A good conductor has a high numerical value of a conductivity, and a low numerical value of resistance.

5 Definition Conductivity: All materials have a measurable property called electrical conductivity that indicates the ability of the material to either allow or impede the flow of electrons. Materials that easily conduct electricity have a high conductivity.

6 Definition Insulator: An insulator is a material that tends to impede the flow of electricity. An insulator resists the flow of electricity. A good insulator has a low numerical value of conductivity and high numerical value of resistance.

7 Definition Resistance: Resistance is a quantitative indicator of an objects ability to act as an insulator. A high resistance wire will impede the flow of electrons. A low resistance wire will allow a greater flow of electrons.

8 Definition Semiconductor: A semiconductor is a material with conductivity that can change between that of a conductor and insulator. The conductivity of a semiconductor can be changed by exposing it to a stimulus such as an electrical field, light, mechanical pressure, or heat.

9 Simplified Functional Differences Semiconductors can be used in devices that act like a switch.

10 Elements Pure substances are made of elements. An element consists of atoms Atoms have a nucleus consisting of protons and neutrons Electrons move in shells around the nucleus

11 Elements Number of protons determines the element Number of electrons varies State of electrical charge Is the element in a chemical bond? Number of neutrons varies with isotope

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13 Periodic Table: Copper

14 Bohr Model of the atom (Cu)

15 Electrical Current: current convention

16 How many electrons? First compute the number of atoms, N N 1cm g cm 3 1mol g atoms mol atoms Since each copper atom has one valence electron, there are valence electrons in a 1 cm cube of copper. Note: N is greater than the number of grains of sand on the earth. Compute the number of sand grains by assuming that 10 cm of sand covers all 200 million square miles of the earth s surface. Assume that each grain is 1 mm in diameter and that the packing efficiency is 68 percent.

17 Electrical current: potential

18 Electrical current: electron flow

19 Electrical current: convention

20 Definition: Charge Elementary charge 1 electron = coulomb Coulomb 1 coulomb= electrons

21 1A = 1 C s Definition: Current Electrical current is the flow of the electrons. 1 C = electrons

22 Definition: Voltage Voltage is the energy potential that would cause electrical charge to move from a region with relatively more electrons to a region with relatively fewer electrons. 1V = 1 J coulomb

23 Voltage and electrical work If the voltage between A and B is one volt, then one Joule of work is done when electrons move from A to B.

24 Voltage is a Potential A high voltage between two regions does not guarantee that a lot of electrons will flow. The electrical resistance between the two regions determines the rate of electron flow for a given voltage.

25 Ohm s Law Ohm s law describes the relationship between voltage, current and resistance, when current flows through a wire or resistor. V V = IR R I

26 Ohm s Law

27 Ohm s Law