Name ______Date ______
LAB 2 – Transformers and Alternating Current Power
After watching the video lab introduction, complete the following lab activity. Use this worksheet to record your results. Submit the completed worksheet via the Assignments tool on the Canvas course site. (Lab Grade: 100 points)
Introduction:
In this lab, you will construct a transformer and observe how voltage, current, and magnetic field are related in an alternating current circuit. For this activity you will use the online simulator by clicking on the link: http://phet.colorado.edu/en/simulation/generator
1. Select the “Transformer” tab. 2. In the “Electromagnet” box, under “Current Source,” select “AC.” Make sure “Show Field” and “Show Electrons” are checked. 3. Under “Indicator,” select the voltmeter.
The AC Current Supply graph displays the sinusoidal current applied to the primary coil. The x-axis represents time and the y-axis represents current.
Part I: Frequency and Current:
On the AC Supply Graph, the x-axis represents time and the y-axis represents current in the primary coil. The sliding scale showing percentages allows you to zoom in and out. Set the x-axis slider to 5% and the y-axis slider to 100%. Assume that the difference between tick marks on the x-axis is 0.5 sec. What is the frequency (f) of the AC power? Also write the equation for the current (I=I0sin[2ft + ]) substituting the correct value for frequency.
Now set the x-axis slider to 20% and keep the y-axis slider on 100%. Assuming the difference between tick marks on the x-axis is 0.5 sec, what is the frequency of the AC power? Also write the equation for the current (I=I0sin[2ft + ]).
Note that the typical frequency of a wall outlet in the United States is 60 cycles per second (cps, or Hertz, Hz). In Europe, it is 50 cps.
Part II: Magnetic Field Generated by Sinusoid:
All moving charged particles produce a magnetic field. These can be electrons moving through iron (this is how a magnet works), or electrons moving through a wire.
1. Keep the x-axis slider at 20% and the y-axis slier on 100% and still assume each tick mark on the x-axis represents 0.5 sec. In the “Electromagnet” box, check “Show Compass” and place the compass directly to the right of the primary coil.
The compass measures direction of the magnetic field generated by the current through the primary coil.
Describe the behavior of the compass needle in relation to the sign of the sinusoidal current. This demonstrates that the sinusoidal current is generating a sinusoidal magnetic field in the center of the coil.
2. Now uncheck “Show Compass” and check “Show Field Meter.” Drag the Field Meter so that the sensor (the + sign) is directly to the right of the primary coil. The meter must be directly centered in relation to the center of the coil—By must read zero so the entire magnetic field is equivalent to the Bx component.
Lab 2: Transformers and AC Power 2
Note that the sign of the magnetic field (Bx) switches when the sinusoidal current changes sign. Did you expect this based on the behavior of the compass?
Part III: Plotting Magnetic Field:
1. With the Field Meter still in place, pause the simulation at the six time points shown below by using the pause button at the bottom of the screen (start at a minimum and take the next five points separated by 90 degrees). It might take a few tries, but try to get the reader to pause as close to the six points as possible. You do not need to get all six points in one cycle.
2. Record the time and the magnetic field intensity, Bx. (Assume t=0 at the beginning of the graph and that the difference between tick marks on the x-axis is 0.5 sec.)
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3. Plot the magnetic field intensity of each of these points on the graph below. Since they represent minima, maxima, and points where the graph crosses the x-axis, you can use them to sketch a graph of the magnetic field over time. Assume each tick mark on the y-axis is 0.05 A (50 mA). Also plot the current.
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M 2 1 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Time (seconds)
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When the maxima and minima of two equivalent sinusoids are offset from one another by time, that offset is called a phase shift, . What is the phase shift between the current on the primary coil and the magnetic field that you measured?
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Part IV: Voltage on Secondary Coil:
1. The secondary coil is simply a wire attached to a resistor with a voltmeter attached. Delete the Field Meter and drag the secondary coil over so it is directly adjacent to the primary coil. What happens when the two coils get close together?
2. Record the intensity of the voltage across the resistor on the secondary coil at the same six time points as in Part III by using the pause button. You might need to move the secondary coil over a little bit so you can see the graph so you know when to hit pause, but keep the coils as close together as possible. The change in voltage is slight—this might take a few attempts. You can assume each tick on the voltmeter is 1 millivolt.
3. Plot the voltage at the six time points on the graph below. Also compare to primary coil current.
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l 5 o V 4 3 2 1 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Time (seconds)
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What is the phase shift between the current on the primary coil and the voltage that you measured? How is this different than the phase shift from Part III?
Conclusions:
1. If you replace the AC source with the DC source on the primary coil, what is the voltage amplitude and frequency of the primary coil? (Hint: Consider the applied current I=I0sin[2ft + ].)
2. What is the voltage amplitude on the resistor on the secondary coil when using the DC source on the primary coil? What does this indicate about the usability of a DC source for a transformer?
3. Based on the face that the primary coil creates a changing magnetic field that induces a current in the secondary coil, what would happen if we put a bar magnet in motion near a wire coil? To convince yourself, try out the generator simulation tool to verify this.
Lab 2: Transformers and AC Power 6