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Understanding SI Units: Essential Concepts and Activities in Physics

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philyra-desma
20 diapositives · 346 Views · Oct 26, 2014

This guide explores key SI units, including time, mass, distance, temperature, and charge, alongside vital secondary quantities like force, energy, and power. It delves into important questions like "What is a kilowatt-hour?" and provides in-class activities, focusing on engaging examples from the Boston area on air conditioning costs. Through exercises and rules of dimensional homogeneity, this resource aims to build a solid understanding of the relationships between physical quantities and their units. Essential for students and educators in physics!

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  1. Review SI Units • Time: • Mass: • Distance: • Temperature: • Charge:

  2. Important Secondary Quantities • Force: • Energy (work): • Power:

  3. Questions: • What is a kilowatt-hour? • Where is it used?

  4. In-Class Activity #1 A study of households and businesses in the Boston, Massachusetts area found that air conditioning units were used for an average of 4600 hours per year. Determine the total annual cost of electricity required to operate a 10,000 Btu/hr air conditioning unit in the Boston area if the electric rate is $ 0.071/kWhr.

  5. Notation • The units of variables will be referred to by putting the variables in brackets • Consider the equation: v-vo = a·t • What is the equation saying?

  6. [v] will refer to the dimension of v[t] will refer to the dimension of t • In order to determine the units of an unknown (say a), we need to be able to write equations in terms of units, e.g.

  7. Rules of Homogeneity • Definition: An equation is said to be dimensionally homogeneous if all terms separated by plus and minus and equal sign have the same dimension. • Consider the previous example: • In order to be homogeneous,

  8. Rule 1 • If the dimensional quantities are replaced by their primary units the equation should reduce to an identity. • In our example, what are the primary units?

  9. Rule 2 • Dimensions do NOT add or subtract. In order to add or subtract variables, they must have the SAME units. • In our example, [v] = [vo]. If you subtract them you have no units on the left

  10. Rule 3 • Dimensions DO multiply and divide.

  11. In-Class Activities #2, #3 • If P1 = 400 W and P2 = 12 Btu/minute, what is P1+P2 in SI units? • Which, if any, of the following are correct for acceleration units:a) m/s/s b) c) d)

  12. Exercises • A relationship between Force F in (N), distance x in (m), mass M in (kg) and speed v in (m/s) is suggested as Is this equation dimensionally homogeneous?

  13. Exercises (cont) • A relationship between Force F in (N), time t in (s), mass M in (kg) and speed v in (m/s) is suggested as: Ft2 = Mv Is this equation dimensionally homogeneous?

  14. Prefixes • As you’ve already seen, we’ll deal with both very large numbers and very small numbers. • We will use scientific notation and/or engineering prefixes to represent these numbers

  15. Refresher: common prefixes T tera 1012 G giga 109 M mega 106 k kilo 103 d deci 10-1 c centi 10-2 m milli 10-3 μ micro 10-6 n nano 10-9 p pico 10-12

  16. Examples in Engineering Notation • 2x103 Volts = • .00045 A = • 1.3x10-6 C = • 10x107 Hz =

  17. Some Quantities Must Be Dimensionless • In Calculus you will see eax • ax must be dimensionless (no units) 1/1 • Example, if x is in m, a is in 1/m • If x is in s, a is in 1/s • Some dimensionless quantities need units to make sense • Example if v = vo e-atv and vo have units of volts

  18. Class Activity In an electrical circuit the current i(t) in A (Amperes) changes with time t in s according to the function i(t) = e-2t. a) How could this function be dimensionally consistent knowing that the exponential function is always dimensionless (RHS), and that i(t) is in A on the LHS? b) What are the units of the constant 2 in the exponential function?

  19. More on Units • Remember x = cos(θ)? • What are the units of θ?

  20. Angular frequency • What are the units of ω in y(t) = cos(ωt) if t has units s? • ω is called the angular frequency • ω = 2pf