IT 219 Physics for IT Lecture Notes PDF

IT 219 Physics for IT Image result for image for physics Lecture 1 Analogue Circuits – Part 1  Outline  Units for Measurements  English standard system  SI system  Analogue Circuits – part 1  Voltage, current, resistanc...

IT 219 Physics for IT Image result for image for physics Lecture 1 Analogue Circuits – Part 1  Outline  Units for Measurements  English standard system  SI system  Analogue Circuits – part 1  Voltage, current, resistance  OHM’s law  Measuring current and voltage 2  Units for Measurements 3  Measurement You are making a measurement when you  Check you weight  Read your watch  Take your temperature  Weigh some fruits What kinds of measurements did you make today? 4  Standards of Measurement When we measure, we use a measuring tool to compare some dimension of an object to a standard. 5  Some Tools for Measurement 6  Learning Check From the previous slide, state the tool (s) you would use to measure A. temperature ____________________ B. volume ____________________ C. time ____________________ D. weight ____________________ 7  Solution From the previous slide, state the tool (s) you would use to measure A. temperature thermometer B. volume measuring cup, graduated cylinder C. time watch D. weight scale 8  Learning Check – English System What are some U.S. units (English system)that are used to measure each of the following? A. length B. volume C. weight D. temperature 9  Solution Some possible answers are (English or Standard system): A. length inch, foot, yard, mile B. volume cup, teaspoon, gallon, pint, quart C. weight ounce, pound (lb), ton D. temperature F However, conversion between the different units is difficult (foot = 12 inch, yard = 3 feet, mile = 5280 feet) 10  Using International System of Units (SI) The standards of measurement used in science are those of the SI metric system. What makes metric units easy to use? All metric units are based on multiples of 10. As a result, you can convert between units easily.  Measurements are easily understood by all scientists  Measurements are easier to convert than the English system 11  Using SI Units  The metric system was originally established in France in 1795.  The International System of Units (abbreviated SI after the French name, Le Système International d’Unités) is a revised version of the metric system.  The SI was adopted by international agreement in 1960.  Is a decimal system based on 10  Used in most of the world  Used by scientists and hospitals 12  The SI System Base units:  meter (m), gram (g), liter (L), second (s), ampere (A), Celsius (°C) Derived units:  work or energy: joule (J)  power (rate of doing work): watt (W)  1 W = 1 J/s 13  Measurement System Comparisons MEASUREMENT ENGLISH SI SYSTEM LENGTH Yard / Inch Meter / Centimeter MASS Ounce / Pound Gram / Kilogram VOLUME Quart Liter TEMPERATURE Fahrenheit Celsius / Kelvin TIME Second Second 14  Using SI Units of Length  In SI, the basic unit of length, or linear measure, is the meter (m).  All measurements of length can be expressed in meters.  For very large and very small lengths, however, it may be more convenient to use a unit of length that has a prefix as shown in the following table. 15  Using SI Units – prefixes of common use Commonly Used Metric Prefixes Prefi Symbo Meaning Factor x l giga G 1 billion times larger than unit it precedes 109 mega M 1 million times larger than unit it precedes 106 kilo k 1000 times larger than the unit it precedes 103 deci d 10 times smaller than the unit it precedes 10-1 centi c 100 times smaller than the unit it precedes 10-2 milli m 1000 times smaller than the unit it precedes 10-3 micro μ 1 million times smaller than unit it precedes 10-6 nano n 1 billion times smaller than unit it precedes 10-9 16 pico p 1 trillion times smaller than unit it precedes 10-12  Using SI Units  Example: 12.3 mW = 0.0123 W =1.23 x 10-2 W  For large distances, it is most appropriate to express measurements in kilometers (km).  The prefix kilo- means 1000, so 1 km equals 1000 m. 17  Stating a Measurement In every measurement there is a Number followed by a Unit from measuring device 18  Learning Check What is the unit of measurement in each of the following examples? A. The patient’s temperature is 102°F. B. The sack holds 5 lbs of potatoes. C. It is 8 miles from your house to school. D. The bottle holds 2 L of orange soda. 19  Solution A. °F (degrees Fahrenheit) B. lbs (pounds) C. miles D. L (liters) 20 Learning Check Identify the measurement in SI metric units. A. John’s height is 1) 1.5 yards 2) 6 feet 3) 2 meters B. The volume of saline in the IV bottle is 1) 1 liters 2) 1 quart 3) 2 pints C. The mass of a lemon is 1) 12 ounces 2) 145 grams 3) 0.6 pounds 21  Solution A. John’s height is 3) 2 meters B. The volume of saline in the IV bottle is 1) 1 liter C. The mass of a lemon is 2) 145 grams 22  Analogue Circuits – Part 1 23  The Basics  Electricity: Resulted from the flow of charges (electrons)  Circuit: Any path along which electrons can flow  Electrons will only flow if the circuit is complete with no gaps  Open Circuits are broken and do not allow the flow of electrons  Closed Circuits are complete and allow the flow of electrons 24  A Basic Circuit All electric circuits have three main parts 1. A source of energy 2. A closed path 3. A device which uses the energy If ANY part of the circuit is open the device will not work! 25  Would This Work? 26 The Central Concept: Closed Circuit  Voltage  Voltage is a measure of the potential energy that causes a current to flow through a circuit  Voltage is always measured as a difference with respect to an arbitrary common point called ground  Voltage is also known as electromotive force or EMF outside engineering 27  Potential Difference =Voltage=EMF In a battery, a series of chemical reactions occur in which electrons are transferred from one terminal to another. There is a potential difference (voltage) between these poles. The maximum potential difference a power source can have is called the electromotive force or (EMF), . The term isn't actually a force, simply the amount of energy per charge (V) 28  Why Does Current Flow?  A voltage source provides the energy (or work) required to produce a current  A source takes charged particles (usually electrons) and raises their potential so they flow out of one terminal into and through a transducer (light bulb or motor) on their way back to the source’s other terminal 29  Current and Charge Current is the rate of charge flow: 1 ampere = 1 coulomb/second (or 1 A = 1 C/s) Only measurable when circuit is closed (on) 30 Note: The “I” stands for intensity  Current (I)  An electron has a -1.602∙10-19 Coulomb charge  The rate of flow of charged particles is called current  Current = (Number of electrons that pass in one second) x (charge/electron) -1 ampere = (6.242∙1018 e/sec) x (-1.602 10-19Coulomb/e) Notice that an ampere = Coulomb/second  The negative sign indicates that the current inside is actually flowing in the opposite direction of the electron flow Electrons 31 Current  Electron Flow Vs Current Flow 32  Types of Current DC = Direct Current - current flows in one direction Example: Battery AC = Alternating Current- current reverses direction many times per second. This suggests that AC devices turn OFF and ON. Example: Wall outlet 33  AC and DC Current  DC Current has a constant value  AC Current has a value that changes sinusoidally (sine wave) Notice that AC current changes in value and direction No net charge is transferred 34  Circuit Elements - The Resistor The resistor is an element that “resists” the flow of electricity. The unit for resistance is the OHM,  35  Resistance The resistance depends on material and geometry (shape). For a wire, we have: R=L/A where  is called the resistivity (in Ohm-m) and measures how hard it is for current to flow through the material, L is the length of the wire, and A is the cross-sectional area of the wire. 36  Factors Affecting Resistance 1. The length L of the material. Longer materials have greater L 2L resistance. 1 2 2. The cross-sectional area A of the material. Larger areas offer LESS resistance. A 2A 2 1 3. The temperature T of the material. The higher temperatures usually result in HIGHER resistances. 4. The kind of material. Iron has more electrical resistance than a geometrically similar copper conductor. 37  Resistors 38  Ohm’s Law “The voltage (potential difference, emf) is directly related to the current, when the resistance is constant” Voltage vs. Current V I 10 R = constant of proportionality 9 8 R = Resistance 7 Voltage(V) 6 V = IR 5 Voltage(V) 4 3 2 1 Since R= V/I, the resistance is the 0 0 0.2 0.4 0.6 0.8 1 SLOPE of a V vs. I graph Current(Amps) 39  Ohm’s Law I=V/R I = Current (Amperes) (amps) V = Voltage (Volts) R = Resistance (ohms) Georg Simon Ohm (1787-1854) 40  Example  A 560 Ω resistor is connected to a circuit which causes a current of 42.4 mA to flow through it. Calculate the voltage across the resistor  Answer: v = iR = (0.0424)(560) = 23.7 V 41  Measuring Current  Electric current is measured in amps (A) using an ammeter or (ampermeter) connected in series in the circuit. A 42  Measuring Voltage  The ‘electrical push’ which the cell gives to the current is called the voltage. It is measured in volts (V) on a voltmeter A 43  The Voltmeter and Ammeter The VOLTMETER and AMMETER The voltmeter and ammeter cannot be are special devices you place IN just placed anywhere in the circuit. They or AROUND the circuit to must be used according to their measure the VOLTAGE and DEFINITION. CURRENT. Current goes THROUGH the ammeter Since a voltmeter measures voltage or POTENTIAL DIFFERENCE it must be placed ACROSS the device you want to measure. That way you can measure the CHANGE on either side of the device. Voltmeter is drawn ACROSS the resistor Since the ammeter measures the current or FLOW it must be placed in such a way as the 44 charges go THROUGH the device.  Open and Short Circuits  An open circuit between two points A and B means i=0.  Voltage across an open circuit: any value.  An open circuit is equivalent to R = ∞ Ω.  A short circuit between two points A and B means v=0.  Current through a short circuit: any value.  A short circuit is equivalent to R = 0 Ω. 45 Circuit Schematic Diagrams Connecting Wire For the battery symbol, the Battery LONG line is considered to be the POSITIVE terminal and the Resistor SHORT line , NEGATIVE. Capacitor Open Switch Closed Switch Lamp 46  Circuit Schematic Diagrams 47  Simple Circuit When you are drawing a circuit it may be a wise thing to start by drawing the battery first, then follow along the loop (closed) starting with positive and drawing what you see. 48  Self-Reading  Textbook Chapter 15:  Electric current p 209  Resistance and Resistivity p 212  Ammeter and Voltmeter p 223 49