Part 1. Find Magnetic North

1) Turn off all the electrical devices at your station. (Currents in these devices cause magnetic fields!)

2) Distribute your compasses around the flat work area; feel free to keep adjusting their precise locations. See if they all agree. Describe your observations. If there are any "outliers", disagreeing with the others, try to come up with a possible explanation. (Again, feel free to keep moving them around.)

3) As precisely as you can, measure the angle between your lab bench and magnetic north. Draw a clear and detailed diagram defining your angles, so that someone who wasn't in the room could understand exactly which way magnetic north lies, relative to a drawing of the lab bench.

(From the drop-down menu, select "Insert", then "Drawing", then "New". This will allow you to make a free-form drawing.)

4) Look at a map of the building on your phone or a print-out campus map, and determine whether your direction of Magnetic North is reasonable. Explain your reasoning.

5. What can you say about the magnetic fields in the lab room, when all the devices are turned off?

Part 2. Magnetic Fields caused by a Long Straight Current

1) Distribute your compasses around the vertical wire running through the center of your platform, as shown.

2) Disconnect all wires from your DC power supply. Turn it on. Turn the current control knob all the way down (full counterclockwise), and the voltage control knob all the way up (full clockwise). In this lab you will be adjusting the current control only.

3) Connect the positive and negative outputs of your DC power supply to the OUTSIDE connectors, as shown in the diagram. When you turn up the current, this will send current around the large vertical loop. Some of this current will run right through the center of the circular platform.

4) Slowly start turning up the current control knob, to 1.0-1.5 Amps; watch the compasses carefully. Describe your observations. Explain what must be happening to the local magnetic fields.

5) The direction of the magnetic field caused by a long straight wire can be determined with the "right hand rule". Your right fingers curve in a partial-circle around your straight thumb. Line up your curved right fingers with the magnetic fields produced by the wire; use your thumb to determine what direction the current is flowing through the wire. Explain your conclusions.

6) Turn down the current control knob to zero. Make a prediction for what will happen when you reverse the current direction and then turn it back on.

7) Now do the experiment described in (6). Specifically, reverse the connections on the loop of wire. Then, sowly turn up the current and watch the compasses. Compare your observations to your prediction, and explain your observations. If your prediction was incorrect, explain what you forget to take into account.

8) What is happening to the Earth's magnetic field as you turn up the current in the loop? Justify your answer with both reasoning and experimental evidence.

Estimating the Earth's Magnetic Field

Note: in this experiment we are only measuring the horizontal component of the Earth's magnetic field, not the full magnitude. (In reality there is also a vertical component - a "dip angle" that is different at different locations along the Earth's surface.)

1) Turn off your current. Remove all but one compass from your platform. Move it as close to the wire as you can without it touching. When you are done, measure the distance from the center of the compass to the central wire.

r = distance from compass center to wire	Error estimate

2) The magnitude of the magnetic field produced by asingle long straight wire with a current "I", is

|B| =0I2 r

where0 is a constant with the approximate value of0=4 10-7 Tesla meters/Amp,

and "r" is the distance from the center of the wire to the location where you are calculating the generated field.

3) The wire used in this experiment is actually a 50-loop coil, so you can think of it as 50 straight wires, each one making some magnetic field. Use this fact, and the above equation, to estimate the magnetic field you can produce at the center of your compass, with the equipment you have, if the current is set to 1.0 Amps. Show your calculations and your reasoning. (Hint: The field due to each strand of wire will add up with the field produced by every other strand of wire.)

Now you can make a measurement of the horizontal component of the Earth's magnetic field. The key is that the field due to the wire adds up with the field due to the Earth to produce a total magnetic field as seen by the compass. This will deflect the compass away from magnetic north, depending on the relative strength and direction of the two sources of magnetic fields.

Before proceeding, make sure your current is turned to zero.

4) Position the compass such that the magnetic field of the Earth (see Part 1) is perpendicular to the magnetic field produced by the current in the wire (see Part 2). Move the compass around the wire until you get to this location, staying as close to the wire as possible without touching (try to maintain your same measured value of "r".) Use drawings and sentences in the below space to convince your lab instructor that you have chosen the right location. (This would be a good time to check in with your lab instructor to see that you are approaching this correctly.)

5) If two magnetic fields are perpendicular to each other, then when they are equal magnitude, the total field will point at 45-degrees relative to each individual field source. See the below diagram to understand why this would be true.

6) Slowly turn up the current in your wire until you think that the magnetic field of the earth is equal in magnitude to the magnetic field produced by the long straight wire. Use the above vector diagram as a guide. (Hint: the total field will *only* be at 45 degrees when both of these vectors are equal in magnitude.) TURN YOUR CURRENT TO ZERO AFTER YOU HAVE MADE ALL YOUR FINAL MEASUREMENTS.

Measured Current when BEH=BC	Error estimate

7) You now have everything you need to estimate the horizontal component of the Earth's magnetic field. (Use the above equations, and don't forget the 50-wire factor.) Explain your reasoning and show your calculations; give your final answer in Tesla.

Part 3. Quantifying Earth's Magnetic Field With Smartphones

• Smartphones have magnetometers that measure the magnetic field around the phone in three different directions x-y-z.On your smartphone download the app: "Physics Toolbox Sensor Suite"
• Open the app. Tap on the menu option( ). This will open a list of tools
• Select Magnetometer from the list. Set your phone on a table (move as far as possible from any other electronics) and let the graph form as the app collects data.

1. From the graph that forms, notice the red, green, and blue lines indicating the X, Y, and Z-axis respectively.
   1. Use the pause button to pause the readings at least 8 times (the more the better.) Each time you pause, record the values for the magnetic field on the X,Y, and Z directions
   2. Take the average of your X, Y, and Z values. These are your best estimates for the X, Y, and Z components of Earth's magnetic field.
   3. For vectors that have 3 components, we use an extension of the Pythagorean theorem such that the total field strength will be:B=Bx-avg2+Bz-avg2+Bz-avg2
   4. Calculate the strength of Earth's magnetic field.

1. From the graph that forms, notice the white line. This line indicates the total value of Earth's magnetic field.
   1. Use the pause button to pause the readings at least 5 times (the more the better.) Each time you pause, record the values for the total magnetic field. Don't do this at the same time you are collecting data from number 1.
   2. Take the average of your total field data.
   3. Take the average of the values found in number 1.d. (value found using x-y-z) and 2.b. (value found using white line).This is your final measured value for Earth's Magnetic Field.

Final Value for Earth's Magnetic field:

1. Hold your phone vertically (still away from any other electronics) and wait for the lines on the graph to flatten out again. Move your phone as shown in the video named "Magnetic field phone motion" found on https://www.physdataexperiment.net/sjsu-2b-lab (make sure you are logged into google with your sjsu account.) Do this about 8 times and then take a screenshot (quickly).
   1. Sketch the graphs you see in the X, Y, and Z orientations and describe what you observe (don't just include the screenshots- Make your own sketches).
   2. Describe what happens to the total magnetic field.

1. Walk around with your phone in different directions(don't get too close to other electronics). Using these observations along with the observations from number 3, explain how using the magnetic field sensors to measure Earth's magnetic field can be used to know where an object (an airplane or a person with a phone!) is oriented and where it is going.

1. Now, wait for the lines on the graph to flatten out again and move your phone very close to another electronic device that is turned on.
   1. What do you observe happening to the magnetic field?
   2. Knowing that in electronics, we have currents, do you see a relationship between the currents in electrics and the magnetic field reading? Explain.

1. Take a moment to find the values from the National Center for Environmental Information's "NCEI Geomagnetic Calculator" fromhttps://www.ngdc.noaa.gov/geomag/calculators/magcalc.shtml?useFullSite=true .

These are values from theoretical models developed by scientists. Using this website you can input SJSU's address and they will locate your coordinates. Then you can click on "calculate" and the website will display the strength of the magnetic field for your location.

1. How does your Measured value for Earth's magnetic field ( from 2.iii) compare to the value you find online? Do a percent difference calculation.

1. What are some factors that keep us from having a more accurate value? Try to be as specific as possible indicating what are the sources that might interfere with our measurements.

Reflection

Reflect upon what you have learned about magnetic fields.