Interpreting an XY Scatter Graph
Example Ohm’s Law
Data
If we set up a circuit with
some resistor and an
ammeter (to measure
current) in series. And supply
voltages 1,2,3,4, and 5 volts
respectively, the currents (in
milliAmperes) shown were
measured.
Highlight the data, make an XY Scatter
chart and choose Layout #9.
Label axes (including units), eliminate
legend, change title.
Current versus Voltage
• In our set up, we controlled voltage.
Furthermore we used the convention that the
variable we controlled is plotted on the x-axis.
• Now we want to compare the fit line
(Trendline) from Excel with the equation from
our theory – Ohm’s Law.
Excel: y = 0.2807x + 0.0019
Theory V=IR
Rearranging
Identification (Part 1)
Identification (Part 2)
Right click on the Trendline, choose
Format Trendline, check Set intercept
Identification (Part3)
Units of slope
• Slope is “rise over run” so its units are mA/V.
• The resistance was the reciprocal of slope, so
its units are V/mA.
• The m means milli means 10-3.
• A factor of 10-3 in the denominator
corresponds to a factor of 10+3 in the
numerator. 10+3 goes with kilo or k.
• A volt/ampere is an ohm. Thus we have units
for the resistance in kilo-ohms.
Interpolation
• Our equation y = 0.2812x or I = 0.2812V
allows us to guess what the current would be
for voltages that we did not measure.
• For instance, we would expect the current
corresponding to a voltage of 2.5 to be
0.2812*2.5 = 0.703 mA.
• When we use the Trendline to estimate a new
value within our range (here 1-5 volts), this is
called Interpolation.
Extrapolation
• We can also use our results to estimate what
the current would be for voltages outside the
range we measured.
• For instance, we would expect the current
corresponding to a voltage of 6 to be
0.2812*6 = 1.6872 mA.
• When we use the Trendline to estimate a new
value outside our range (here 1-5 volts), this is
called Extrapolation.