Physics Lab 212P-9
Faraday's Law & Induced EMF
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Software List Science Workshop
Equipment List (all items marked with * are in the student kit, others are supplied at the time of the lab)
Science Workshop interface + voltage probes High inductance coil Stand and clamps 5 foot long copper pipe Cardboard tube Meter stick or ruler *bar magnet *Hookup wires with alligator clips *Compass *tape Lab Activity: Faraday's Law & Induced EMF
Activity 1: Understanding Faraday's Law of Induction
The aim of this activity is to understand how a magnet moving near a coil of wire induces an emf.
First, set up the experiment shown below (most of it may already be set up for you -- if so, check the set up to make sure it is correct.) Examine which terminal of the induction coil is connected to the positive ("red") voltage probe and which one is connected to the negative (black) voltage probe. Remember that you are going to induce an emf in the coil; examine the induction coil closely and note down how the coil is wound. Discuss and note down the relationship between the sign of the voltage measured by the voltage probes and the sense (clockwise or anti- clockwise) of the current when looking down at the coil from above.
Bar magnet
Induction coil Interface box
Analog channel A Cardboard tube Start Science Workshop. Click & drag the "analog plug" icon above to analog channel A and select the voltage probe measurement option when prompted. Click & drag the "Graph" icon shown above to analog channel A. Double click anywhere on the x-axis of the graph. When you do this, you will get a screen that allows you to set the parameters for the x-axis. Change the time settings to read "milliseconds" and set the maximum value to 3 seconds. Double click anywhere on the y-axis of the graph. When you do this, you will get a screen that allows you to set the parameters for the y-axis. Set the maximum and minimum values of the voltage to +1 and -1 V, respectively. Double click the "sampling options" icon. The default rate at which the program measures data is 10 Hz. Change this to 1000 Hz. Next, select "time" for "Stop condition;" this will open up a menu called "select stop condition;" enter "3" seconds. This means that data acquisition will stop after 3 seconds. Click OK. Now, you are ready to take data!
Q1. Before you do the experiment, discuss amongst your group what you expect to observe as the magnet passes through the induction coil. Assume that the magnet is dropped with its N pole facing down. Sketch a qualitative expected plot of Voltage vs. Time below. Pay attention to whether voltages are positive or negative at some instant of time. Now, carry out the following experiment: Use a compass to check which pole of your bar magnet is "North". Position the cardboard tube so that the top is approximately 10 cm from the top of the induction coil. Hold the bar magnet with the N pole facing downwards, ready to be dropped through the tube. Then, start recording data by clicking on the "REC" icon. Drop the magnet and observe the graph of voltage versus time.
Q2. Sketch the graph below, with some indication of the values of the voltages observed. (Alternatively, you may print the graph and include it with your lab report.)
Q3. Use Faraday's Law to explain the shape of the graph. Remember the shape of the magnetic field lines of a bar magnet from your reading and lectures.
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df Now, if we rewrite e = - in the following form, e Чdt = - df and integrate both sides of the dt equation, we arrive at f = -тe Чdt , i.e. the flux is equal to the negative of the area under the voltage versus time graph. So, using Science Workshop analysis the previous graph to determine the incoming and outgoing flux. In Science Workshop, click the previous graph to make it active. Click the “Statistics” () icon to open the Statistics area on the right side of the graph. Click the “Autoscale” icon to rescale the graph to fit the data. In the Statistics area, click the “Statistics Menu” icon and select Integration from the menu. In the Graph display, use the cursor to click-and-draw a rectangle around the first peak of the voltage plot. The area for the first peak will appear in the Statistics area.
Integration (first peak) = ______Vs
In the Graph display, use the cursor to click-and-draw a rectangle around the second peak of the voltage plot. The area for the first peak will appear in the Statistics area.
Integration (second peak) = ______Vs
Q4. Is the incoming flux equal to the outgoing flux? Explain what you expect and compare it to the values calculated above.
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Optional: Tape two magnets together, end to end, with identical poles in contact, i.e. N to N or S to S, and repeat the above analysis for flux in the different regions of the graph. How are they related? Q5. Repeat the previous experiment with the magnet dropped from different heights: e.g. 2 cm, 5 cm, 15 cm, 25 cm and a few more positions for a total of seven or more different heights. Note down the values of any maximum or minimum voltages you observe in each case. Describe how the voltage versus time plots change when you change the initial height from which the magnets are dropped. Does the value of the incoming or outgoing flux change as the drop height is varied? Also, qualitatively explain your observations. Use sketches to illustrate the data.
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Q6. Creative exercise: Suppose you want to carry out a crude analysis of the dependence of the induced EMF E on the initial height h of the bar magnets. Set up appropriate assumptions and then derive a simple relationship between E and h. Comment on whether your observations agree with this analysis. Activity 2: Induced EMFs in metals
In the lecture, you were probably shown demonstrations in which the motion of a magnet near a non-magnetic metal created a force between the magnet and the metal. This too is a consequence of Faraday's Law. To experience this for yourself, use a bar magnet and the copper pipe provided in the lab. First, confirm for yourself that the copper is not attracted to the bar magnet. Then, hold the copper pipe vertically and drop the bar magnet through the pipe (use a backpack or have a lab partner hold his or her hand underneath the pipe to catch the magnet). Observe whether the motion of the magnet is slowed down because of the pipe. You can easily check this by dropping another metal object (e.g. a penny) outside the pipe at the same time as the bar magnet. Then answer the following questions:
Q7. Use Faraday's Law to explain your observations.
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Q8. Explain your observations using energy conservation. (Key ideas: kinetic energy, gravitational potential energy, and energy dissipation in a conductor.)
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______Q9. Creative exercise: Suggest a way in which you could make slight modifications to the copper pipe so that the motion of the bar magnet would not be impeded. (Imagine that you were given an electric drill or a cutting tool like a milling machine.) Explain with a figure why your scheme would work.
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Q10. Suggest practical applications for the effect you observed in this experiment.
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