Written by Christine Deyo, July 1997

# Investigating the Magnetic Field in a Coil

Purpose
Investigate how the strength of the magnetic field inside a current-carrying slinkie is affected by the current flowing.
Procedure
1. Select a slinky and stretch it between two cardboard supports. Do not over stretch the slinky!!
2. Connect a DC power supply to the ends of the slinky.
3. Insert a Multi-meter into the circuit and connect it to measure the current.
4. Count the number of coils between the cardboard supports and record this value.
5. Measure the distance between the supports.
6. Connect the magnetic field sensor to your interface. Be sure that all cables are firmly but gently in place.
7. Support the sensor and place it in the center of the slinky.
8. Use the data collection software to collect data. Use the High (mTesla) setting to collect data using Events with Entry mode.
9. Use the power supply to vary the current and record the magnetic field strength in automatically with the data collection software.

Analysis

1. Construct a graph of Current vs. Magnetic Field Strength. If you use a calculator, you may download the data using a TI-Graph Link or enter the data in a spreadsheet or graph it on your calculator.
2. Find the equation that describes the relationship between Current and Magnetic Field Strength. Remember that y and x may not be used as variables and that units must accompany all constants. What is the meaning of the y-intercept and the slope of this graph?
3. Qualitatively explain why this relationship makes sense. Include an explanation of what current means and why doubling the current causes the resultant change in magnetic field strength.

Extension

Use the same basic setup, but this time vary the distance between the coils. Record the number of coils per meter, keeping the current constant. Explore the relationship between the magnetic field and the number of coils per unit length.

Teacher Notes

I used the CBL to collect data but it could be collected on any computer interface.

Sample data taken for current vs magnetic field. It was collected with a CBL, entered into Graphical Analysis and graphed. This data could also be analyzed easily on the calculator.

 Magnetic Field Current mTesla Amperes -0.038 0.5 0.025 1 0.059 1.5 0.099 2 0.139 2.5 0.174 3 0.223 3.5 0.271 4 0.308 4.5 0.327 5

Sample Data for the Extension
 Magnetic Field Length Coils/Length mTesla meters 1/m 0.024 1 40.0 0.04 0.9 44.4 0.052 0.8 50.0 0.076 0.7 57.1 0.085 0.6 66.7 0.111 0.5 80.0 0.156 0.4 100.0 0.225 0.3 133.3 0.322 0.2 200.0

Chris Deyo
Updated 1/13/05