Physics 42 Lab 8

The Earth’s Magnetic Field

PARTS LIST

Part / Quantity
Plastic Rulers / 1
BB Cables / 3
Dip Angle / 1
Power Supply / 1 /
Ammeter / 1 /
Tangent Galvanometer / 1 /

FIRST: Generate Data: Pre-lab.

If you did the prelab, transfer the values you found for the Earth’s magnetic field to this table. If you didn’t do the prelab, do it now: Find the Earth’s magnetic field from this NOAA magnetic field calculator. Use zip code (95404 ) first to generate the latitude and longitude. Use an elevation (.05 m) for SRJC. Record the values for th e B (given as F), BH (given as H, and its components X(+N) and Y(+E)) and BV (given as Z(+D). View your results in list format and enter the values in this table.

D (deg) / I (deg) / H (nt) / X (nt) / Y (nt) / Z (nt) / F (nt)

Part 1. Finding the Earth’s Magnetic Field with a Tangent Galvanometer

If the direction of the horizontal component of Earth’s magnetic field is BH and the external magnetic field generated by the tangent galvanometer is Bi, the magnetic needle of a compass will be directed toward BS (the total magnetic field), as shown in the vector diagram here.

By measuring  and calculating the value of Bi {Bi = 0NI/(2R)}, one can measure the horizontal component of the Earth’s local magnetic field.

  1. Measure the diameter of the galvanometer to at least a fraction of a milimeter to at least 4 significant figures. Record your uncertainty.
  1. Construct a series circuit with: the “tangent galvanometer”, an ammeter, and the 0-6 volt power supply. Connect the cables across the

contacts with the greatest number of loops (5 & 10) for a total of N = 15 loops.Be sure to connect the ammeter through the unfused 20A socket!!

  1. Orient the plane of the wires so they align with the compass needle. Rotate the compass case to zero the measuring needles (the long needles). Be patient and give the needle time to settle, and then make fine adjustments. It is critical that the field generated by the tangent galvanometer is perpendicular to the Earth’s magnetic field, so take care on this alignment!
  1. Supply a 0.5A current and measure  and record its value in the data sheet.Change current to 0.75A and measure  again. Repeat for I=1.0 A
  1. Using B = 0NI/(2R), where N is the number of turns (15) for the wire and R is the radius of the loop, calculate Bi and record its value in the data sheet. Keep 5 significant figures (for the heck of it) and express your answers in nT so you can compare your values to the data.
  1. Using the angles from your data, calculate each BH and then calculate an average value.
  1. Using the dip needle and assuming Elliot Ave is geographic due north, find the Declination and Inclination angles of the Earth’s local magnetic field.
  1. Using the dip angle and the average value of the horizontal component of the Earth’s magnetic field, calculate values for the vertical component and total magnetic field of the Earth. Show all your calculations in the provided space. Keep 5 significant figures (for the heck of it)
  1. Compare your measured and calculated values to the NOAA values with percent differences. Briefly discuss your results and possible sources of error. Please type and print it out.

Earth’s Magnetic Field Data Sheet

Diameter and radius of circular loop including uncertainty:

d (m) = ______r (m) = ______

I (A) /  (degrees) / Bi (nT) / BH (nT)
0.5
0.75
1.0

Average Be from measurement (nT) =

Quantity / Measured Value / NOAA Value / % Difference
BH (nT)
Declination
(degree)
Inclination
(degree)
Bv (nT)
B (nT)

Show Calculations of Bv and B:

Part 2. The Earth’s Magnetic Field and Pole Reversal

Go to the website for the NOAA Geomagnetism and check it out:

Read the FAQ on the Earth’s magnetic field to answer the questions below.

a. What causes the Earth’s Magnetic Field?

b. What is the dynamo effect?

c. Where is the Magnetic North Pole now?

d. How fast is it moving? What direction is it moving in?

e. How do we know the poles are changing and that one day they may flip?

f. How do we know the poles flipped in the past?