Chapter 2, 3, & 4

1. Chapter 2, 3, & 4 Dr. Farid Farahmand CET 263

2. Outline • Reviewing basic concepts: Voltage, Current, and Resistance • Ohm’s law • Power and Energy

3. Atom Structure • An atom is the smallest particle of an element that retains the characteristics of that element. • An atom has a nucleus, consisting of positively charged particles called protons, and uncharged particles called neutrons. • The basic particles of negative charge, called electrons, orbit the nucleus.

4. Electron Shell and Valence Electrons • Electrons orbit the nucleus at discrete distances from the nucleus. • Orbits are grouped onto energy bands known as shells. • Electrons with the highest energy exist in the outermost shell, known as the valence shell, and its’ electrons are called valence electrons. • Valence electrons are relatively loosely bound to the atom. • If a valence electron acquires enough external energy to leave the atom, the process is known as ionization. • The escaped electron is called a free electron.

5. Categories of Materials • Conductors readily permit current flow, due to a large number of free electrons in the material. • Insulators have few free electrons, and tend not to permit current to flow through them. • Insulators have no free electrons • Semiconductors have relatively small number of free electrons • limited number of electrons can escape

6. Electric Field! - + - + Electrical Charge • The charge of an electron and that of a proton are equal in magnitude but opposite in polarity. • The force acting between charges is called an electric field. • Electrical charge (Q) is measured in coulombs (C). • By definition: One coulomb is the total charge possessed by 6.25 x 1018 electrons. Q = (number of electrons)/(6.25 x 1018)

7. 1 C Voltage • The unit of voltage is the volt (V). • By definition: • One volt is the potential difference (voltage) between two points when one joule of energy is used to move one coulomb of charge from one point to the other. The potential difference is one Volt! 1 Joule of Energy

8. Voltage Sources • A battery is a type of voltage source that converts chemical energy into electrical energy. • Solar Cells convert light energy into electrical energy. • Generators convert mechanical energy into electrical energy. • Electronic power supplies? • They do not produce electrical energy, but they transform the ac voltage from the wall outlet into a constant dc voltage for use in our circuits.

9. Current definition The movement of free electrons from negative to positive is electrical current (I). Electrical current is the rate of flow of charge I = Q/t The Unit of Current is Amp One amp is the amount of current that exists when a number of electrons having a total charge of one coulomb move through a given cross-sectional area in one second. Current One C of charges/sec

10. Applying 1 V 1 A of current is generated There must be 1 ohm of resistance! Resistance • The property of a material that restricts the flow of electrons is called resistance. • The unit of Resistance is Ohm (). • By definition • One ohm of resistance exists if there is one ampere of current in a material when one volt is applied across the material.

11. Resistors and Conductance • Resistors are used to limit current or divide voltage. • Conductance is the reciprocal of resistance. • G = 1/R • The unit of conductance is siemens (S). • Common resistors are carbon-composition, carbon film, and metal film • Variable resistors are designed so that their resistance values can be changed with manual or automatic adjustment. • A potentiometer is a variable resistor used to divide voltage. • A rheostat is a variable resistor used to control current.

12. Color-code bands on a resistor • 1st band is the first digit of the resistance value. • 2nd band is the second digit of the resistance value. • 3rd band is the multiplier (number of zeros). • 4th band indicates the tolerance. Examples: http://www.ccsu.edu/technology/farahmand/ccsu/courses/cet233/cet_233.htm#Resistors

13. Resistor color code

14. Resistor color code

15. Ohm’s Law • Ohm’s law describes mathematically how voltage, current, and resistance are related. I = V/R • Hence, current and voltage are linearly proportional. • In resistive circuits, with a constant resistance, if voltage increases or decreases by a certain percentage, so will current. • On the other hand, current and resistance are inversely related. • With constant voltage, if resistance is reduced, current goes up; when resistance is increased, current goes down

16. Ohm’s Law Given R=10 ohm; how will this graph look like?

17. Ohm’s Law

18. Ohm’s Law Given V=10 V; How will this graph look like?

19. Ohm’s Law

20. Voltage and Current Measurements • To measure voltage, the voltmeter is placed in parallel across the component; that is, one lead is place on each side of the component. • To measure current, the ammeter must be placed in series with the component; that is, it must be in line with the current path.

21. 304.2μ 4.469 6.509 Voltage and Current Measurements • Analyzing the circuit: • Total resistance = 10+15+22+3.3=50.3K • I=V/R = 0.298 mA • What is the measurement error? About %2 error! • Where does the error come from? • What is the measurement error across 15K resistor? • To measure voltage, the voltmeter is placed in parallel across the component; that is, one lead is place on each side of the component. • To measure current, the ammeter must be placed in series with the component; that is, it must be in line with the current path. Check the numbers! Make sure you understand how we got them!

22. 304.2μ 4.469 6.509 Current Measurements

23. 304.2μ 4.469 6.509 Voltage Measurements

24. Energy and Power • Energy is the ability to do work - joules (J) • Power is the rate at which energy is used - in watts (W) • By definition: One watt is the amount of power when one joule of energy used in one second. Power = energy/time P = W/t

25. Kilowatt-hour • The kilowatt-hour (kWh) is frequently used as a unit of energy. One kWh is used when one thousand watts is used for one hour. • Power utilized over a period of time represents energy consumption. W = Pt • Energy can also be expressed as watt-seconds (Ws), watt-hour (Wh).

26. Power in an Electric Circuit • There is always a certain amount of power in an electric circuit, and it is dependant on the amount of resistance and the amount of current, expressed as: P = I2R

27. Energy Conversion and Voltage Drop in Resistance • A resistor used in a circuit must have a power rating in excess of what it will have to handle. • As electrons flow through resistors, some of their energy is given up as heat. • The same number of electrons entering a resistor will exit it, only their energy will be less, so the voltage exiting a resistor is less than the voltage entering the resistor. • This is called voltage drop across a resistor.

28. Power Loss and Efficiency • The output power of an electronic power supply is always less that the input power, because some of the input power is used to operate the power supply circuitry. • An important characteristic of electronic power supplies is efficiency, which is the ratio of output power to input power. POUT = PIN – PLOSS Efficiency = Pout/Pin