Review TopicsScientific NotationUnits of Measure
What is Electricity? From the Greek word “elektron” that means “amber” There are two types of electricity: – Static Electricity - no motion of free charges – Current Electricity - motion of free charges » Direct Current (DC) » Alternating Current (AC)
2.2 Current Current is the rate of flow of charge through a conductor. – Conductor » materials with free electrons » e.g. copper, aluminum, gold, most metals – Insulator » materials with no free electrons » e.g. glass, plastics, ceramics, wood
Equation for Current I=Q/tI = the current in Amperes (A)Q = the amount of charge in Coulombs (C)t = the time measured in seconds (s)The charge of an electron is 1.6 x 10-19 C
Effect of Electric Currents on the Body0.001 A can be felt0.005 A is painful0.010 A causes involuntary muscle contractions0.015 A causes loss of muscle control0.070 A can be fatal if the current last for more than 1 second
Example Problem 2.0 How much charge will pass through a conductor in 0.1 seconds if the current is 0.5 Amperes? How many electrons are required for this much charge?
Example Problem 2.1 Determine the current in amperes through a wire if 18.726 x 1018 electrons pass through the conductor in 0.02 minutes.Example Problem 2.2 How long will it take 120 C of charge to pass through a conductor if the current is 2 A?
Example Problem 2.3 and 2.4 Write the following in the most convenient form using Table 2.1: (a) 10,000 V (b) 0.00001 A (c) 0.004 seconds (d) 630,000,000 Watts (e) 0.00006 A
Wire Gauge? AWG = American Wire Gauge AWG numbers indicate the size of the wire….but in reverse. For example, No. 12 gauge wire has a larger diameter than a No. 14 gauge wire.
2.3 Voltage Voltage is the measure of the potential to move electrons. Sources of Voltage – Batteries (DC) – Wall Outlets (AC) The term ground refers to a zero voltage or earth potential.
Digital MultimetersMeasurement Device Circuit SymbolVoltage VoltmeterCurrent AmmeterResistance Ohmeter
More on Batteries Positive (+) and Negative (-) terminals Batteries use a chemical reaction to create voltage. Construction: Two different metals and Acid – e.g. Copper, Zinc, and Citrus Acid – e.g. Lead, Lead Oxide, Sulfuric Acid – e.g. Nickel, Cadmium, Acid Paste Batteries “add” when you connect them in series. Circuit Symbol:
Equation for Voltage V=W/QV = the voltage in volts (V)Q = the amount of charge in Coulombs (C)W = the energy expended in Joules (J)
Example Problem 2.7 Determine the energy expended by a 12 V battery in moving 20 x 1018 electrons between its terminals.
Example Problem 2.8 (a) If 8 mJ of energy is expended moving 200 µC from one point in an electrical circuit to another, what is the difference in potential between the two points? (b) How many electrons were involved in the motion of charge in part (a)?
2.4 Resistance and Ohm’s Law Resistance it the measure of a material’s ability to resist the flow of of electrons. It is measure in Ohms (Ω). Ohm’s Law: V=IR V or E = voltage I = current R = resistance
Example Problem 2.9 Determine the voltage drop across a 2.2 k Ω resistor if the current is 8 mA.Example Problem 2.10 Determine the current drawn by a toaster having an internal resistance of 22 Ω if the applied voltage is 120 V.
Example Problem 2.11 Determine the internal resistance of an alarm clock that draws 20 mA at 120 V.
Equation for Resistance R =ρ Aρ = resistivity of the material from tables = length of the material in feet (ft) A = area in circular mils (CM)
Example Problem 2.12 Determine the resistance of 100 yards of copper wire having and 1/8 inch diameters.
Concept Questions How can you determine the current through a resistor if you know the voltage across it? How can you change the resistance of a resistor?
Temperature dependence of Resistance R 2 = R 1 [1 + α1 ( t 2 − t1 )]R = resistances t = temperatures α = temperature coefficient from tables
Example Problem 2.15 The resistance of a copper conductor is 0.3 Ω at room temperature (20°C). Determine the resistance of the conductor at the boiling point of water (100°C).
Resistor Color Codes 0 Black 1 Brown 2 Red Tolerance 3 Orange 5% Gold 4 Yellow 10% Silver 5 Green 6 Blue 7 Violet 8 Gray Memorize this table. 9 White
Example Problem 2.17 Determine the manufacturer’s guaranteed range of values for a carbon resistor with color bands of Blue, Gray, Black and Gold.Example Problem 2.18 Determine the color coding for a 100 k Ω resistor with a 10% tolerance.
Total Resistance for Resistors in Series R T = R1 + R 2Total Resistance for Resistors in Parallel 1 1 1 = + R T R1 R 2
Potentiometers They are three terminal devices with a knob. The knob moves a slider which changes the resistance between the terminals. Circuit Symbols:
What is the difference between E and V? E is the voltage supplied by a battery. V is the voltage measured across a resistor.
2.5 Power, Energy, Efficiency Power is the measure of the rate of energy conversion. Resistors convert electrical energy into heat energy. Equation for Power: P=IE Power Delivered by a Battery P=IV Power Dissipated by a Resistor What are some other ways that we can write this equation?
Example Problem 2.19 Determine the current drawn by a 180 W television set when connected to a 120 V outlet.
Simple Circuit Problem Using circuit symbols, draw a circuit for a 9V battery connected to a 10Ω resistor. Draw and label the direction of conventional current. Now include a voltmeter in your sketch that will measure the voltage drop across the resistors. What will it read? Include a ammeter that will measure the current through the resistor. What will it read?
Simple Circuit Problem How much power does the battery deliver? How much power does the resistor dissipate?
Note: Equations will be providedon the chalk board during theexam.However, you must know whateach variable represents andwhat units are used for each.
Example Problem 2.20 Determine the resistance of a 1200W toaster that draws 10A.
Energy and power are related: W=Pt W = energy in Joules P = power in Watts t = time in seconds
Example Problem 2.21 Determine the cost of using the following appliances for the time indicated if the average cost is 9 cents/kWh. – (a) 1200W iron for 2 hours – (b) 160W color TV for 3 hours and 30 minutes – (c) Six 60W bulbs for 7 hours.
Efficiency Po η = ×100% Pi Pi = Po + Pl 1hp = 746 W
Example Problem 2.22 Determine the efficiency of operation and power lost in a 5hp DC motor that draws 18A as 230V.
2.6 Series DC Networks Two elements are in series if they have only one terminal in common that is not connected to a third current carrying component. Total Resistance R T = R 1 + R 2 + R 3 + ... + R N Current through a Series E I= RT
Consider Figure 2.29. » E=24V, R1=2Ω, R2=4Ω, R3=6ΩWhat is RT?What is I?What is V1, V2 and V3?What is P1, P2, P3, and PE?
Kirchhoff’s Voltage Law “The algebraic sum of the voltage rises and drops around a closed path must be equal to zero.” ∑ Vrises − ∑ Vdrops = 0
Voltage-divider rule– “The voltage across any resistor in a series is some fraction of the battery voltage.” R xE Vx = RT