experiment 46

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Experiment 46 Magnetic Fields and Electomagnetic Induction

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Name_______________________ Lab___________ Instructor___________________

Experiment 46

Magnetic Fields and Electromagnetic Induction

Object:

The object of the lab is to study the magnetic field of both permanent magnets andelectromagnets as well as the phenomenon of electromagnetic induction.

Apparatus:

Small magnet, small magnet on string, small magnet on stick, compass, 2 Pasco 400-turn wirecoils, steel weights, power supply, Voltage probe, Magnetic Field Sensor with Pasco box and

computer, ruler, modeling clay, pendulum stand.

Part 1: Magnetic Field of a Permanent Magnet

In this first part you will examine the magnetic field around a permanent magnet. In particularyou will measure the strength of the magnetic field as a function of the distance away from themagnet. The magnets you will be using are rare-earth magnets made from neodymium and are

typically quite strong permanent magnets.The simplest tool for studying the magnetic field is a compass. The north pole of a compass will

point in the direction of the magnetic field. Remember that magnetic field lines point towards thesouth pole of a magnet and away from the north pole. Place the compass in front of you and pick

up the loose magnet. Move it around the compass to establish the location of the north and southpoles of the magnet. Make a sketch of the magnet with the north and south poles labeled. Draw a

couple field lines at each of the poles.

For this next step, you want to measure the strength of the magnetic field as a function of the

distance from the magnet. You will use a magnetic field sensor that is interfaced to the computerfor readout. See the instructions for starting up DataStudio at the end of the lab writeup. You will

also find instruction for setting up a digital display. Once the sensor is set up, place the magnet

and the probe on or near the ruler as shown (see Figure 1) with the north pole of the magnetpointing directly at the pole. Use the modeling clay to hold the magnet in place at the same

height as the probe. The probe should be set to measure the field parallel to the direction of theprobe (Axial setting).


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Figure 1

Measure the field at the distances shown in Table 1. The distance should be measured from the

face of the magnet to the dot on the magnetic field sensor (this dot represents the location of theactual sensor).

Table 1

Distance(cm) Magnetic Field (g)Trial 1 Trial 2 Average

1.0

2.0

3.0

4.0

When completing the table, perform the measurement 3 times just to make sure the values arereasonably reproducible. Each lab partner should complete at least one trial (set of 4

measurements). Calculate the average field at each distance.Follow the directions at the end of the lab to create a data table and graph of position versus field

strength using DataStudio. Print the graph, label it and include it with your writeup. Use theprinter designated by your lab TA. It will typically be labeled by the room number of your lab.

Part 2: Magnetic Field of a Current-Carrying Coil

Next we want to examine the magnetic field created by a current carrying coil. Wire up the coil

to the power supply as shown in Figure 2. Turn on the power supply and turn the current andvoltage down to zero if it is not already there. If the green CC/CV light is lit, push the button to

turn it off. Increase the current from 0A to 1.5 A.Do not run the coil above 1.5 A for anextended period of time (more than about half a minute) or the coil may heat and burn out. Do

not exceed 3.0A.


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Figure 2

With a current running through the coil, bring a compass near. Establish which end of the coil isthe north pole. The large piece of steel in the center of the coil helps strengthen and direct the

field. We will refer to it as the core later in the lab. For certain parts of the experiment, it will beremoved (but not right now).

Turn the current back down to zero and reverse the wires. What does that do to the direction ofthe magnetic field? (Record your observation)

Using a setup similar to Part 1 (see Figure 3), use the magnetic field sensor to record the strengthof the magnetic field as a function of distance.

Figure 3

Record the data in Table 2. As before take 3 measurements.


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Table 2

Current _______________

Distance(cm) Magnetic Field (g)Trial 1 Trial 2 Average

1

2

3

4

Calculate the averages. Do notworry about entering the data to create a graph.How does this data compare to the data from Part 1?

Place the coil and magnetic sensor in a fixed position relative to each other. Measure the fieldstrength of the coil at this position with several different currents. Put your data in Table 3.

Table 3

Distance ____________ cm

Current(A) Magnetic Field (g)

0.5

1.0

1.5

2.0

What do you observe about the relationship between current and field strength?


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Part 3: Magnetic Force

This part is purely observational. You have several steel blocks or masses on your table. Placethe electromagnet coil flat over one of the blocks. Turn up the current on the electromagnet and

lift the block. Slowly turn down the current to the point where block drops and record thecurrent. Repeat this for one of the other blocks.

Part 4: Magnetic Field

In this next section we look at the combined magnetic field due to both the electromagnet and the

permanent magnet.Place the south pole of the permanent magnet facing the electromagnet at a distance of about

6cm. Use the modeling clay to support the magnet such that it is at the same height as the centerof the electromagnet. Set the current in the electromagnet to 1.5A.

Set the probe to measure the field perpendicular (radial) to the end of the probe (youll be

inserting it in from the side this time). It would be good to move the sensor away from allmagnets and tare the sensor again.

Figure 4

What is the strength of the field half way in between the permanent magnet and theelectromagnet?

Move the sensor along the line between the two magnets. Does the field ever go to zero? If so at

what location?

Turn the permanent magnet around so that the north pole now faces the electromagnet. What is

the strength of the field half way in between the permanent magnet and the electromagnet?

Move the sensor along the line between the two magnets. Does the field ever go to zero? If so at

what location? (for example, at what distance from the face of the electromagnet?


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Why are the two measurements of the field at the center different depending on the orientation of

the permanent magnet?

Part 5: Magnetic Induction Permanent magnet

Attach the second coil to the voltage probe as shown and set up the voltage probe as discussed inthe Data Studio directions at the end of the lab. Create a graph for the voltage from the coil as a

function of time by dragging Voltage and dropping it on the Graph setting below. Since theinduced emf does not stay non-zero for very long, you will be using the computer to find quick

spikes on the induced emf. Make the graph window large (grab the bottom corner and expand thewindow or use the expand window icon in the top right corner of the graph window). Start a new

data run to collect the graph. At this point you can click on the scale to fit button on the top leftof the graph window to rescale the graph. Erase data sets and re-run new data as necessary.

In this section we use the permanent magnet to create an induced emf. Remove the core of thecoil and place it to the side. Take the magnet that is attached to the wooden stick and pass it

quickly into the coil, hold it in the center for a second, and then quickly pull it back out again.Zoom in on the graph and draw a quick sketch of the voltage (emf) as a function of time. Label

the region where you (1) insert the magnet, (2) hold the magnet in place, and (3) pull the magnetback out.

What happens when you move the magnet faster or slower?


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Pick two points that represent the beginning and end times for three complete oscillations. Usingthe cursor find and record here the time associated with the beginning and end. From these

values calculate the period of oscillation of the pendulum (show your calculations).

Remember that the period of a pendulum depends on its length. Measure the length of the

pendulum and calculate the period of the pendulum: T = 2!L

g. Show your calculation.

What is the percent difference between the period calculated using the length and the periodmeasured using the time measurement from the coil?


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Instructions for DataStudio

Starting Up and Connecting Magnetic Field Sensor1. Start the computer.

2. Make sure the Pasco 750 USB device is turned on.

3. Wait for the Pasco 750 USB device to be found by the laptop. Most people do not wait

long enough at this step.

4. Start the program called DataStudio

5. The program will show a box with four tasks, You want to click Create Experiment

6. The Pasco Box image will show up in the center. If it indicates that it is not connected it

is either off or you did not wait long enough at step two. If this is the case, shutdown and

restart DataStudio

7. Click the same port on the computer screen that the magnetic sensor is connected on the

Pasco box (typically Channel A). A list of sensors should appear, scroll down and select

magnetic sensor, then hit Ok button.

8. Settings for magnetic field sensor in DataStudio: on the screen select Magnetic FieldStrength 1x, a sampling rate of 10 Hz, and Low (1X). These settings are typically the

default settings.

9. Settings for magnetic field sensor:

a. Make sure the Range Select setting on the sensor is at 1x.

b. The magnetic sensor has a tare button that zeroes the background signal. Make

sure any magnets are far enough away from the sensor so as not to influence it,

then touch the tare button. You can periodically check the zero by moving all

magnets and coils away and reading the value. If the value is not zero, press the

tare button.

c. The sensor can either read the component of the magnetic field parallel to theplastic arm (axial) or perpendicular (radial) to the arm. Move the button closest to

the probe end to axial for Parts 1 and 2 of the lab.

The probe in the picture below is set to axial mode (top switch) and 1x Range

(switch closest to the cable).


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!"#Close the setup window.

!!#In the upper left of the screen, there is an Entry that says Magnetic Field Strength. Drag

that icon to the lower left and drop it on the word digit. (See the annotation in the

screenshot below). This will create a numeric readout for the magnetic field sensor.

The screenshot below shows the experiment setup with a Magnetic Field Sensor in

Channel A set to 1X and a 10Hz Sampling Rate.

Starting, Stopping and Deleting Runs

!# %& '()*( ()+,-. /)()0 12,1+ 34()*(5 &* (678 1(*29:

;# %& '(&7 ()+,-. /)()0 12,1+ 34(&75 &* (678 1(*29#

-/8* (=8 3DE78*,?8-(5 /*&79/&F- ?8->

Entering Data and Creating a Plot

In DataStudio you can enter data into a table by hand and then plot that data. You will use this

ability to create a plot for each of Part 1 and Part 2.To make a Table and Graph with the data you collected:

1. Click Experiment in the upper right.

2. Select New Empty Data Table

3. A two-column data table should appear.

4. Enter Your data


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5. Once you have entered your data, drag Editable Data and drop it on Graph to create a

graph of the data.

6. Select File Print and print to the printer noted on the board by your Lab TA (typically

the printer is labeled by the room number of your lab).

See the screen shot below for a sample table. Ignore the extra data points in the table.

Setting Up the Voltage Sensor

To set up the Voltage Sensor so you can measure the induced emf(voltage) in the circuit:!# G2,1+ 348(>75

;# G2,1+ &- G=)--82 H )-/ '8281( 3I&2().8 48-'&*5 J)22 (=8 F)6 )( (=8 C&((&?K#

experiment 46

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