SKILL: USING SIGNIFICANT FIGURES
Energy Calculations
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rounding decimals.
Significant figures are meaningful digits in a number. Calculations
can sometimes produce answers with more significant figures than
are accurately known. Scientists use rules to determine how to round
their answers. The rule for writing an answer to a multiplication
problem is shown below.
Rule: Your answer may show only as many significant figures as the
number in the problem with the fewest significant figures.
Generally, a significant figure is any digit shown except for a
zero, unless the zero is contained between two nonzero digits or
between a nonzero digit and a decimal point. For example, the
number 40.3 has three significant figures, but the number 5.90
has only two significant figures. The number 0.034 has three significant figures, and the number 0.8 has only one significant figure.
Example
A computer uses 6.5 kWh of energy per day. If the computer is
left on all the time, how much energy does it use in a year?
(1) Solve the problem.
d
ays
kWh
kW
h
E 6.5 p 365 year 2372.5 year
day
(2) Look at the number with the fewest significant figures. The
number 6.5 has two significant figures, and the number 365
has three significant figures. Therefore, the answer is only
meaningful to two significant figures.
(3) Round the answer to two significant figures.
kWh
ANSWER E 2400 year
Answer the following questions. Write your answers
using the significant figure rule for multiplication.
1. How much energy is used in a year by a computer
that uses 1.7 kWh/day?
2. An energy-efficient computer uses 0.72 kWh/day.
How much energy does it use in a week?
3. How much energy is used in one year if a 0.27 kW
computer is on for 3 hours/day? (Hint: Use the
formula E Pt.)
CHALLENGE The energy usage of a
computer is measured to be 0.058030 kWh.
How many significant figures does this
measurement have?
Chapter 12: Magnetism 439