INDUCTION: Week 1

A. Helmholtz Coil

1. Record the number of turns in the first pickup coil (to which a golf tee is attached). Determine its cross-sectional area. Set the oscillator driving the Helmholtz coils to 1 kHz - sine wave output. The current through the H-coils and the frequency can be measured on channel 1 of the scope as the voltage drop across a known resistor. Adjust the H-coil current to produce an easily measurable pickup coil voltage (measured on channel 2) when the axes of the coils are parallel. Calculate the peak magnetic field at the center of the H-coils from the current I passing through them.

2. Vary the angle theta between the coils from 0 to 180 degrees and measure the corresponding induced voltages. Plot V-ind vs cos theta. Obtain B-peak from the slope of the curve and compare with the value from part 1.

3. Set the coil axes parallel. Vary the applied frequency from 150 Hz to 1500 Hz while keeping the H-coil current (and thus B-peak) constant. Measure the corresponding induced voltages. Plot V-ind vs f. Obtain B-peak from the slope of the curve and compare with the calculated value.

4. Reset the frequency to 500 Hz. Use a triangular wave oscillator output. Calculate dB/dt. Measure the induced voltage and compare with the expected value. Discuss the actual waveform on the pickup coil.

5. Keep the frequency at 500 Hz. Use a square wave oscillator output. Discuss the waveform on the pickup coil.

The following equations will be useful in the analysis of your results:

The magnetic field in Tesla produced at the center of the Helmholtz coils is

where N is the number of turns in one of the coils, I is the current in Amperes, and R is the radius of the coils in meters.

 

The voltage induced in the pickup coil is

 

 

where N' is the number of turns in the pickup coil and A is the cross-sectional area of the pickup coil in m^2.

 

For a sinusoidal applied field