Download Analog Galvanometers

Name:
Lab Partner(s):
Date lab performed:
Dr. Julie J. Nazareth
Physics 133L
Section:
Analog Galvanometers
Part A: Galvanometer Characteristics
Table 1: Galvanometer Characteristics
Resistance of series resistor, K (Ohms)
Voltage at full scale deflection, V (volts)
Resistance at half scale deflection, h (Ohms)
Current necessary for full scale deflection, Ig ( A)
Show calculation with units: Current necessary for full scale deflection of galvanometer, Ig
V
Ig =
=
K+r
€
Part B: Voltmeter
Show calculation with units: Multiplier resistance, M (for a voltmeter with a full scale deflection
for 3.0 volts)
Eqn 1. M =
Vmax
−r =
Ig
WARNING: Set the multiplier resistance to your calculated value BEFORE connecting the
power supply as shown in Figure 5-6 in the lab manual.
Table 2: Using the Galvanometer as a Voltmeter
Power Supply Voltage,
Galvanometer
Galvanometer
V (Volts)
Reading (divisions) Voltage, Vgal (Volts)
3.0
±
±
2.5
±
±
2.0
±
±
1.5
±
±
1.0
±
±
0.5
±
±
0.0
±
±
***Galvanometer voltage, Vgal = (# divisions) (Vmax/500 divisions)***
Part C: Ammeter
Show calculation with units: Shunt resistance, s (for an ammeter with a full scale deflection for
3.0 A)
rI g
Eqn 2. s =
=
I max − I g
Lab: Analog Galvanometers
Updated 02/06/2017
Data & Reporting score:
Show calculation with units: Length of piece of #22 copper wire with resistance, s, calculated
above. (#22 copper wire resistance = 0.00053 Ω/cm) Show full calculation below, not just
answer.
Wire length = resistance / (resistance per length) =
Have instructor sign off on your wire length BEFORE you make your shunt resistor with the
copper wire.
__________ Instructor initials.
NOTE: The above length is for the bent part of the wire shunt resistor. You need 2-3 cm on
either side of the bent part to attach the alligator clips from the leads. LOOK TO SEE IF
THERE IS ANY PRECUT COPPER WIRE 4-6 CM LONGER THAN YOUR CALCULATED
LENGTH. SEE THE INSTRUCTOR IF YOU CANNOT FIND THE LENGTH YOU
NEED IN THE PILE OF PRECUT WIRE SEGMENTS. Connect your wire shunt resistor to
the galvanometer following the lab manual directions (pages 5-5 to 5-6) and Figure 5-7 BEFORE
connecting the power supply.
Table 3: Using the Galvanometer as an Ammeter
Power Supply Current,
Galvanometer
Galvanometer
I (A)
Reading (divisions)
Current, Igal (A)
3.0
±
±
2.5
±
±
2.0
±
±
1.5
±
±
1.0
±
±
0.5
±
±
0.0
±
±
***Galvanometer current, Igal = (# divisions) (Imax/500 divisions)***
Analysis: Identifying any systematic errors
BE SURE TO READ/FOLLOW THE ANAYLSIS DESCRIBED IN THE LAB MANUAL
UNDER THE SECTION TITLED: Analysis and systematic errors. Use the analysis and
examples discussed on those pages to answer questions 1-3 below. The computer graphs
and these questions are worth a significant proportion of the lab grade for this report.
Graphs: You should use a computer program such as LINEFIT to plot the following graphs.
NO HAND-DRAWN GRAPHS FOR THIS LAB REPORT. Use a computer program to
calculate a best-fit line (least squares type fit). Record the resulting slope and y-intercept in
Table 4 – round to the thousandths place (e.g., 1.013). Use the same scale (and units) for both yand x-axes.
•
•
Graph 1: Power supply voltage versus galvanometer voltage, V vs. Vgal
Graph 2: Power supply current versus galvanometer current, I vs. Igal
Lab: Analog Galvanometers
Updated 2/06/2017
(“y” vs. “x”)
(“y” vs. “x”)
(Phy 133L)
If you complete both graphs during class period, show the instructor your results on screen
and have the instructor initial here. You do not have to print out the graphs for your report with
the instructor’s initial below.
Instructor’s initial for graphing/least-squares fit during class:__________________
If you do not finish the graphs during class (and have the instructor’s initial), then you must
print out both of your graphs and include them with your lab report. Label and title your graphs
appropriately either with the computer or by hand after you have printed them out. Include your
computer-calculated best-fit line on your graph. [Screen print for LINEFIT is ok – write axis
labels, and title by hand.]
NOTE: If you choose to ignore any data points for cause (outliers) when the computer (or
calculator) calculates the slope and y-intercept, be sure to record this on your data sheet and
circle the ignored data points on your graph. (Yes, graph all data and only circle the points on
the graph if you have treated them as outliers to be ignored in your calculation of the best-fit
line).
***Round your slope values to the thousands place (e.g., 1.013)***
Table 4: Graphing Results
Slope
Y-intercept values
Y-intercept units
Graph 1
(Voltage)
Graph 2
(Current)
Questions: Answer the following questions in the space provided.
[Questions below directly copied or modified from the Physics 133L Laboratory Manual
(revision Fall 2014) Analog Galvanometer Experiment]
1. a. Do either or both graphs have a non-zero intercept? Circle one of the following.
Both graphs
Just the voltage graph
Just the current graph
Neither graph
b. If yes, which meter (galvanometer, power supply voltmeter, power supply ammeter) is most
likely to have caused your observed non-zero intercept values? Circle one of the following.
Galvanometer
power supply voltage reading
power supply current reading
c. How much is this meter misadjusted? Be specific. Give values in divisions, volts and/or
amps as appropriate for your answers in 1a and 1b.
Lab: Analog Galvanometers
Updated 2/06/2017
(Phy 133L)
2. Consider your voltmeter graph. Slope rounded to the thousandths place = ____________
(a) Does the slope of the voltmeter best-fit line differ from 1.000? Yes
(b) Are your voltages reading less than or
be? (Circle one of the underlined choices.)
more than
or
or
No? (Circle one)
exactly what
they should
(c) The observed voltage read on the galvanometer (Vgal) is _________________ proportional to
Ig, the galvanometer current for a full-scale deflection.
Choose “directly” or “inversely” to fill in the blank in the above sentence.
(d) By what percent did you misread Ig in your experiment? (Use the results of your voltage
graph to calculate the misread in Ig following the reasoning in the Analysis and Systematic
Errors section of the Analog Galvanometer lab in your Lab Manual.) Show your
calculation(s) below or state your reasoning.
Ig is ____________% too large/too small
[Fill in the numerical value and circle whether it is “too large” or “too small]
3. Consider your ammeter graph. Slope rounded to the thousandths place = ____________
(a) Does the slope of the ammeter best-fit line differ from 1.000? Yes
(b) Are your measured current values (Igal) too large or
they should be? (Circle one of the underlined choices.)
too small
(c) Does this mean that your s, the shunt resistant you used, is
exactly right ? (Circle one of the underlined choices.)
(d) By what percent was your shunt resistance, s, off?
Show work below or state your reasoning.
or
No? (Circle one)
or
too large or
exactly what
too small or
_________ %
(e) Equation 2 tells us that s is _______________ proportional to Igr.
Choose “directly” or “inversely” to fill in the blank in the above sentence.
(f) We can rewrite this to say that r is proportional to s/Ig. Use the percent misreads in s and Ig
to estimate the misread in r following the reasoning in the Analysis and Systematic Errors
section of the Analog Galvanometer lab. [Remember, if something is say 2% too small, we
write that as 98% of what it should be, or .98 in decimal form, while something that is 2%
too large is 102% of what it should be, or 1.02 in decimal form.] - continued on next page -
Lab: Analog Galvanometers
Updated 2/06/2017
(Phy 133L)
Question 3f – continued –
By what percent did you misread r in your experiment?
Show your calculation(s) below or state your reasoning.
r is ____________% too large/too small
[Fill in the numerical value and circle whether it is “too large” or “too small]
4. Consider a digital multimeter like the desk version shown in Figure 5-9 on page 5-8.
(a) What changes inside the digital multimeter when you push the button to change from the
200V range to the 20V range? Circle one of the following.
voltage of the thing you are measuring
multiplier resistance (M)
shunt resistance (s)
(b) Use algebra (not your data) and the maximum voltages of the ranges listed in part (a) to
calculate by what factor it changes. [Show your work or state your reasoning. You may
assume that r << M to cancel a term.]
(c) Does it
increase
or
decrease
? (Circle one)
5. Consider a digital multimeter like the desk version shown in Figure 5-9 on page 5-8.
(a) What changes inside the digital multimeter when you push the button to change from the 20
mA range to the 200 mA range? Circle one of the following.
current of the thing you are measuring
multiplier resistance (M)
shunt resistance (s)
(b) Use algebra (not your data) and the maximum currents of the ranges listed in part (a) to
calculate by what factor it changes. [Show your work or state your reasoning. You may
assume that Ig << Im to cancel a term.]
(c) Does it
increase
or
decrease
? (Circle one)
Don’t forget to write your summary! (Start with an introductory sentence stating the
purpose/goal(s) of the lab. Did your readings of voltage and current from your galvanometer
come close to the power supply readings? Consider uncertainty. If you had any systematic
errors, discuss the probable source(s) of these errors. Consider your answers to questions 1-3,
and any misreads you had in Ig and/or r. Be specific and concise.)
Lab: Analog Galvanometers
Updated 2/06/2017
(Phy 133L)