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Unformatted text preview: Unit 2: Basic Circuit Theory
Unit 2.2: Physical Basics 1 Learning Objectives Charge Current and Voltage Energy and Power Ideal basic circuit element Passive sign convention 2 Charge Charles Coulomb Fundamental electric quantity Charge of an electron: qe = -1.602 x 10-19 C Charge of a proton: -qe = +1.602 x 10-19 C Obviously, qe is very small when measured in Coulombs. It takes many, many electrons to produce a meaningful amount of electric current.
3 More Properties of Charge Charge is conserved. Charge may be transferred from one part of a circuit to another, but the total amount of charge does not change. This is a result of conservation of matter. Charge is due to electrons and protons. Electrons and protons are neither created nor destroyed in an electric circuit.
4 Current Andr Marie Ampre Current (i) is the rate at which charge is passing through an area i = q/ t where i = the current in Amperes q = the charge in Coulombs t = the time in seconds 1 Ampere = 1 Coulomb/second 1 Ampere = 6,240,000,000,000,000,000 electrons per sec (6.24 x 1018) 5 Current Flow Current is a signed quantity Physicists: Current flow in a conductor is due to electrons flowing from the negative terminal to the positive terminal of a voltage source Electrical Engineers: Use the nomenclature that current is due to positive charges flowing from the positive terminal to the negative terminal of a voltage source
6 Current Flow in Metallic Conductors Outer electrons are loosely bound to nucleus in metals e- e- e- 7 Current Flow in Metallic Conductors Electron moves to right. Effective positive charge moves to left. Net effect is electrons are flowing from negative to positive terminals. Same as positive "charge" flowing from positive to negative terminals. +
eee- 8 Current is a Signed Quantity In electrical engineering, the positive direction of current flow is that of positive charges. Current requires a reference direction. Otherwise, it is undefined and meaningless! 9 Voltage Allesandro Volta Energy is expended to move the charges in an electric circuit The total energy per unit of charge associated with the motion of charge between two points is called voltage. v = w/ q where v = the voltage in Volts w = the energy in Joules q = the charge in Coulombs 1 Volt = 1 Joule/Coulomb
10 Voltage (Contd.) Voltage is also known as potential difference The voltage between two points in a circuit indicates the energy required to move charge from one point to another An electric field is what moves the charge Does not necessarily mean that the charges are moving between these two points
11 Voltage references Voltage requires: Definition of two points in space Reference polarity. Otherwise it is undefined and meaningless! vx = 3V Circuit Element + vx = 3V - Circuit Element 12 Voltage References Voltage is a signed quantity. Reverse the polarity Reverse the sign. The reference polarity is arbitrary.
Circuit Element v'x = -3V + Circuit Element + vx = 3V - 13 Example Uniform electric field
B We move 1 coulomb from A to B and get 1 Volt A Move two coulombs: do you get , 1 or 2 volts? Answer: 1 Volt v = w/ q = 2/2 Twice as much work, but also twice as much charge 14 Thought for the day
If you don't have confidence, you'll always find a way not to win. Carl Lewis 15 ...
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This note was uploaded on 03/22/2008 for the course EE 302 taught by Professor Mccann during the Fall '06 term at University of Texas at Austin.
- Fall '06