Download Tangent Galvanometer Lab

Tangent Galvanometer Lab
Name:
Period:
AP Physics
Moving charges produce magnetic fields. The most common place where we encounter moving
charges is with the electrons flowing through a wire in an electric circuit. The field of an individual
strand of wire circles around the wire. Wrapping that wire into a loop has the effect of strengthening the
field in the center of the loop, where the fields from each section of the loop come together and reinforce
each other, creating a larger net field. In this lab we will look at the field produced by a wire coiled into
loops.
To investigate the strength of the field produced by the coils we will compare the strength of the field
produced by the coils to the strength of the earth’s magnetic field in our classroom. We will do this
using a simple compass placed at the center of the coils. When no current flows through the coils, the
only effect on the compass is the earth’s magnetic field and it will point north. When current flows
through the coils, it creates a magnetic field in the center of the coil which points perpendicular to the
loop of wire. If we align the coils so they are on the north-south line, the field produced by the coils will
point either east or west. That field will deflect the compass away from north. The stronger the coils
field is the more it deflects the compass from north.
In fact the compass will line up with the total field at its
location in the center of the coils. This total field is a
combination of the earth’s magnetic field and the coils
magnetic field. Since magnetic fields are vectors, we can
break down the total field into two components: the northsouth component which is the earth’s magnetic field and the
east-west component which is the coil’s magnetic field.
The only complexity lies in the fact that we don’t know the magnitude of the total magnetic field,
though we measure its direction. However we do know the magnitude of the earth’s magnetic field and
it remains constant. We can use that information to determine the magnitude of the total magnetic field
and then find the magnitude of the coil’s magnetic field. In this lab we will determine the strength of the
magnetic fields for different settings of the tangent galvanometer and its power supply.
Materials:
tangent galvanometer
power supply
wires
ammeter
compass
Procedure:
Part I: Field and Current
0. As a class, measure the earth’s magnetic
field parallel to the surface of the earth.
BE =
T
1. Center the compass on the platform in the
middle of the tangent galvanometer
2. Rotate the entire tangent galvanometer
unit until the coils are lined up with the
needle of the compass. Then rotate the
compass so that the needle is initially
pointing north.
3. Connect the power supply to the tangent
galvanometer between the two connectors labeled “5 turns”. Add an ammeter in series with the tangent
galvanometer. Connect the ammeter so it reads up to 500 mA of current. Have your teacher inspect
your circuit before you turn it on.
4. Turn on your power supply and gradually increase the voltage until you get a 10° deflection on the
compass. Read the current on the ammeter and record your value
5. Continue to increase the voltage and record the current for every 10° of deflection (20°, 30°, 40°, etc)
up to 60°. Then turn the power supply so the voltage is 0 V and turn it off.
6. Based on the value of the Earth’s magnetic field and the deflection, calculate the size of the total
magnetic field and the magnitude of the coil’s magnetic field for each deflection. Record your results
Data Table: Current and Magnetic Fields at Different Angular Deflections
Number of Loops:
Diameter of Loops:
Angular Deflection Current (mA) Total Magnetic Field (T) Coil Magnetic Field (T)
10°
20°
30°
40°
50°
60°
7. Construct a graph of coil’s magnetic field (BC) as a function of the current through the coils.
8. Write the equation for your graph
 NI
9. Based on our theoretical equation: BC = 02r , what should the slope of your line be? How close
were your actual results? If they are different, why do you think they are different?
Part 2:Field and Coils
10. Take your experiment as set up for Part 1. Turn on your power supply and turn up the voltage until
the current through the coils is exactly 0.2 A. Measure the deflection on the compass and record your
value in the data table. Turn the power supply down to 0 V and turn it off.
11. Reconnect the wires to the tangent galvanometer so that they are connected between the two leads
labeled “10 turns.” Turn on your power supply and turn up the voltage until the current through the
coils is exactly 0.2 A. Measure the deflection on the compass and record your value in the data table.
Turn the power supply down to 0 V and turn it off.
12. Reconnect the wires to the tangent galvanometer so that they are connected between the two leads
labeled “15 turns.” Turn on your power supply and turn up the voltage until the current through the
coils is exactly 0.2 A. Measure the deflection on the compass and record your value in the data table.
Turn the power supply down to 0 V and turn it off.
Data Table: Angular Deflection and Magnetic Fields for Different Amounts of Coils
# of
Current
Loop
Angular
Total Magnetic Coil’s Magnetic
Coils
(A)
Diameter (m)
Deflection
Field (T)
Field (T)
5
0.2 A
10
0.2 A
15
0.2 A
13. Construct a graph of coil’s magnetic field (BC) as a function of the number of coils.
14. Write the equation for your graph
 NI
15. Based on our theoretical equation: BC = 02r , what should the slope of your line be? How close
were your actual results? If they are different, why do you think they are different?