Unformatted text preview: Unit 2.1 Units and Significant Figures Unit 2: Basic Circuit Theory Learning Objectives What are engineering units and notations? How many significant figures should you use? Systems of Units uI nternational Systemof Units (SI ), formerly MKS
Fundamental Quantity Length Mass Time Electric Current Temperature Luminous Intensity Unit Meter Kilogram Second Amperes Kelvin Candela Abbreviation m Kg s A K cd Systems of Units Engineers tend to use powers divisible by 3- (engineering notation) Standard prefixes:
micro milli kilo mega m k M 10-6 10-3 103 106 109 giga G Systems of Units Suppose a calculation results in a value of 10-5 amps. What is the value in engineering notation? Engineers Use Units! The current in the resistor is 5 Five what? Amps, mA, A?
Any num ric quantity m haveunits. e ust Without units theanswe is incom te r ple . Significant figures 2000 -> 2 x 103 one significant figure Lies between 1 x 103 2 x 103 3 x 103 2000 -> 2.0 x 103 two significant figures Lies between 1.9 x 103 2.0 x 103 2.1 x 103 Precision There is a relationship between the number of significant figures and the implied precision
Number Absolute Error 5 0.5 0.05 Number of significant Figures 1 2 3 4 Precision (%) 10 1 0.1 0.01 50 or 50. 52 or 52. 52.4 52.37 0.005 General Rules-Significant Figures Zeros within a number are always significant 4308 and 40.05 each have 4 significant figures Zeros that do nothing but set the decimal point are not significant 470,000 has two significant figures 0.0081 has two significant figures Trailing zeros after the decimal point are significant 4.00 has 3 significant figures 151.0 has 4 significant figures General Rules-Significant Figures Addition and subtraction: The accuracy of the final answer can be no greater than the least accurate value The answer can contain no more decimal places than the least accurate value Example: 150.0 (1 number after decimal point) + 0.507 (3 numbers after decimal point) 150.507 rounds to 150.5 General Rules-Significant Figures Multiplication and Division The final answer to a multiplication or division problem can contain no more significant figures than the least accurate value* Example: 15.03 ( 4.87 ) = 36.837 W 1.987 Round to 36.8 W
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* EE302 and Significant Figures Use the number of significant figures specified in any problem. If none is specified, you should use three significant figures in final answers For lab measurements, three significant figures is the maximum to be expected You will be expected to use the appropriate significant figures on HW, lab reports, and exams Other Rules For decimal fractions less than one, explicitly write a leading zero for decimal fractions to emphasize the placement of the decimal point Example: Incorrect: .28 Correct: 0.28 ...
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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.
- Fall '06