1. Interpreting an XY Scatter Graph

2. Example Ohm’s Law

3. 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.

4. Highlight the data, make an XY Scatter chart and choose Layout #9.

5. Label axes (including units), eliminate legend, change title.

6. 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 = x
Theory V=IR

7. Rearranging

8. Identification (Part 1)

9. Identification (Part 2)

10. Right click on the Trendline, choose Format Trendline, check Set intercept

11. Identification (Part3)

12. 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 goes with kilo or k.
A volt/ampere is an ohm. Thus we have units for the resistance in kilo-ohms.

13. Interpolation
Our equation y = x or I = V 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 *2.5 = mA.
When we use the Trendline to estimate a new value within our range (here 1-5 volts), this is called Interpolation.

14. 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 *6 = mA.
When we use the Trendline to estimate a new value outside our range (here 1-5 volts), this is called Extrapolation.