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					Home Lab Week 13 – Lenz-Faraday                                                                     1
Name________________________________________ Date_________________________


                                  Home Lab 13
                              Faraday and Lenz’s law
Overview
        Michael Faraday was the first to appreciate the intimate connection between electricity
and magnetism. In 1831 Faraday conducted an experiment which showed that a changing
magnetic field produced an electric current. More importantly, the reverse was true: a changing
electric current produced a magnetic field. James Maxwell was able to strengthen Faraday’s
experimental ideas with a coherent theory of electromagnetism.
                              Faraday’s experimental setup was two coils wound around a piece of
                     metal. The first coil is connected to an electrical battery while the second
                     coil is connected to a current detector called a galvanometer. Faraday
                     expected from his earlier experiments with electromagnets that the battery
                     would produce a current in the galvanometer. As the diagram (from
                     http://hyperphysics.phy-astr.gsu.edu) to the left shows, the steady current
                     produces no induced voltage or current in the galvanometer.
                              Instead he found that when he connected the battery to the first coil,
                     there is a momentary current. When he disconnected the battery, there was
                     another momentary current which causes the needle to deflect in the opposite
                     direction. Faraday found that the faster the disconnection or connection, the
                     bigger the current in the galvanometer. More importantly, the direction of
                     the current always reversed. If connecting the battery deflected the needle to
                     the right, then disconnecting the battery deflected the needle to the left.
                              Maxwell was able to produce a consistent theory of electromagnetism
                     which formalized these experimental observations.
                              In light of this, Faraday’s experiment can be explained stepwise:
                              1.      As the battery connects to the first coil, an electrical current
                     begins to flow in the coil.
                              2.      This current is changing (increasing) which produces a
                     changing magnetic field in the coil and metal bar.
                              3.      The magnetic field preferentially travels through the metal bar
                     as opposed to the air surrounding the apparatus.
        4. The changing magnetic field in the bar produces a changing electric field in the
            second coil.
        5. The changing electric field produces a voltage and current in the second coil.
        6. The current causes the galvanometer to deflect.
When the battery is disconnected, the process is the same but with a change in direction due to
the change in the first electric field from increasing to decreasing.




University of Virginia Physics Department
Home Lab Week 13 – Lenz-Faraday                                                                       2
Name________________________________________ Date_________________________

       The changing magnetic field induces a voltage in the second coil. This is summarized by
a form of the Lorentz equation for induced voltage:




        Where ΔV is the induced voltage, N is the number of coils, ΔB is the change in magnetic
field, ΔA is the change in the area of the coil, θ is the angle between the coil area and the
magnetic field, and Δt is the time interval of the change. One important feature of this equation
is the minus sign on the right side. This is given its own name: Lenz’s Law.
        Lenz’s Law can be stated more simply as the induced voltage will always give rise to a
magnetic field that opposes the change that produced it. The induced field will act like friction.
No matter what direction, friction always opposes the motion of an object. This phenomenon is
the basis of magnetic braking.
        An excellent JAVA applet that illustrates the effect of moving a magnet near a coil
can be found at http://phet.colorado.edu/en/simulation/faraday



Activity 13-1: Faraday’s experiment with a simple compass
galvanometer.
Objective: Investigate the effect of changing the current in a coil on the current in a second coil

Materials:
• Iron rod 10 cm long (in the class kit)
• Magnet wire 3.0 m (26 gauge enamel-covered copper wire) (in the class kit)
• Small compass (in the class kit)
• 9-Volt battery
• Steel wool (or a blade to scrape the ends of the magnet wire)
• Transparent adhesive tape


You should take a picture of your materials and add the image to your completed activity report.




University of Virginia Physics Department
Home Lab Week 13 – Lenz-Faraday                                                                   3
Name________________________________________ Date_________________________
   1. Cut an approximately 1.0 m long piece of magnet wire. Wrap one wire around one end
      of the iron rod leaving approximately 10 cm of wire free at each end. Secure the coil
      using transparent tape. Use steel wool or a knife blade to scrape insulation away from
      both free ends of the coil.
                                                                2. Cut an approximately 2.0 m
                                                            long piece of magnet wire. Wrap
                                                            approximately 1.0 m of wire around
                                                            one end of the iron rod leaving
                                                            approximately 10 cm of wire free at
                                                            one end. Secure the coil using
                                                            transparent tape. Continue with the
                                                            other end to wrap the compass with
                                                            magnet wire so that the wire runs from
                                                            30° to 210° (just as in the experiment
                                                            11-1) Use steel wool or a knife blade
                                                            to scrape insulation away from the free
                                                            end of the coil. Twist the two ends of
                                                            this coil together to form an electrical
                                                            circuit. Secure with tape.
                                                                3. Write your observations and
                                                            answer questions for each of the
      following: Connect one end of the first coil to the (-) side of the 9-Volt battery. Lightly
      touch remaining free end of the first coil to the (+) side of the 9-Volt battery. What
      happens immediately to the compass needle when the circuit is closed? Provide a brief
      explanation for the movement or lack of movement.




   4. While the free end is connected to (+) side of the 9-Volt battery, continue watching the
      needle. What happens to the compass needle when the circuit is closed for a while?
      Provide a brief explanation for the movement or lack of movement.




   5. Disconnect (+) side of the 9-Volt battery, continue watching the needle. What happens to
      the compass needle when the circuit is opened? Provide a brief explanation for the
      movement or lack of movement.




University of Virginia Physics Department
Home Lab Week 13 – Lenz-Faraday                                                                       4
Name________________________________________ Date_________________________


Activity 13-2: Faraday’s experiment with a transformer and LEDs.
Objective: Investigate the effect of changing the current in a coil on the current in a second coil
as demonstrated by light-emitting diodes.

Materials:
• Transformer (in the class kit)
• Volt meter (in the class kit)
• 2x red light-emitting diodes (LED) (in the class kit)
• Battery snap for 9-Volt battery (in the class kit)
• Circuit board (in the class kit)
• 9-Volt battery


You should take a picture of your materials and add the image to your completed activity report.
                                        1. Place the transformer so that the two leads of one side
                                    are in separate columns on one side of the divide of the
                                    breadboard. Place the other three leads on the other side in
                                    separate columns on the other side of the breadboard.
                                        2. Place one LED so that one leg of the LED is in a tie
                                    point of the same column as one outside leg of the
                                    transformer. Place the other leg of the same LED so that it is
                                    in a tie point of the same column as the other outside leg of
                                    the transformer.
                                        3. Place the second LED across the two tie points from
                                    step two, but with the polarity reversed (i.e. if the long leg of
                                    the first LED is in one column, then the short leg of the
                                    second LED should be in the same column. Likewise if the
                                    short leg of the first LED is in one column, then the long leg
                                    of the second LED should be in the same column.). The
       LEDs will naturally lean to the shorter side. The result should be that there is one LED
       leaning to the left and the other LED is leaning to the right.
                                                    4. Connect the black lead of the battery snap
                                                to one leg of transformer on the side away from the
                                                LEDs.
                                                    5. Touch the red lead of the battery snap to
                                                the second leg of the transformer on the side away
                                                from the LEDs. What happens immediately to the
                                                LEDs when the red lead closes the circuit?
                                                Provide a brief explanation why one LED lights
                                                and the other does not.




University of Virginia Physics Department
Home Lab Week 13 – Lenz-Faraday                                                                   5
Name________________________________________ Date_________________________




   6. What happens to the LEDs when the circuit has been closed for a while? Provide a brief
      explanation why one LED lights and the other does not.




   7. Using the voltmeter, measure the voltage across the side of the transformer nearest the
      battery. Measure the voltage on the side of the transformer nearest the LEDs. What is
      the voltage on the battery side? What is the voltage on the LED side? Provide a brief
      explanation why the voltage on the LED side is zero.




   8. Disconnect the red lead of the battery snap from the second leg of the transformer on the
      side away from the LEDs. What happens immediately to the LEDs when the red lead
      opens the circuit? Provide a brief explanation why one LED lights and the other does
      not.




   9. Given that the minimum voltage to sensibly light the red LED (Mouser TLHK5100) is
      approximately 1.4 V, what is the minimum voltage that the transformer is providing for
      the LED?




University of Virginia Physics Department
Home Lab Week 13 – Lenz-Faraday                                                                     6
Name________________________________________ Date_________________________




   10. This demonstration is clearer if the red lead is simply tapped against the transformer leg.
       One LED will light as the circuit is closed and the other will light as the circuit is opened.



Activity 13-3: Magnetic braking
Objective: Investigate the effect of a non-ferrous metal in the presence of a moving magnet

Materials:
• 2x small neodymium magnets (in the class kit)
• Aluminum plate or heavy duty aluminum foil
• Thread (~70 cm)
• Scissors
• Pendulum support (such as a ring stand with cross piece)


You should take a picture of your materials and add the image to your completed activity report.
   1. Tie the thread to an appropriate support so that the pendulum can swing freely. The path
      of the pendulum must be free of the influence of nearby ferromagnetic materials. One
      suitable procedure is to tie the thread so that the free end of thread touches the desktop or
                                                      tabletop.
                                                          2. Attach the magnets by placing one on
                                                      either side of the thread and allowing the
                                                      magnets to snap together trapping the thread
                                                      between them. Adjust the magnets so the
                                                      magnets are just above the aluminum plate
                                                      (bit do not touch the aluminum plate). Using
                                                      scissors cut the thread below the magnet so
                                                      that no thread is visible.
                                                          3. Remove the aluminum plate. Pull
                                                      the magnet over a fixed distance and release
                                                      the magnet. Count the number of swings
      until the magnet/pendulum bob stops. Describe the forces acting on the
      magnet/pendulum bob while it is in motion. Include the direction of the forces in your
      description.




University of Virginia Physics Department
Home Lab Week 13 – Lenz-Faraday                                                              7
Name________________________________________ Date_________________________
   4. Replace the aluminum plate so that it is directly under the lowest point of the pendulum
      swing. Pull the magnet over the same fixed distance and release the magnet. Count the
      number of swings until the magnet/pendulum bob stops. Describe the forces acting on the
      magnet/pendulum bob while it is in motion. Include the direction of the forces in your
      description. Provide an explanation for why the magnet/pendulum bob stops sooner.




   5. With the magnet/pendulum bob stopped at its lowest point, pull the aluminum plate along
      the tabletop from underneath the suspended magnet/pendulum bob. Describe the motion
      of the pendulum bob as the aluminum plate is pulled away. Provide an explanation for
      why the magnet/pendulum bob follows the plate.




University of Virginia Physics Department

				
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