Lab 4 - Electromagnetic Induction MANUAL

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University of North Carolina, Chapel Hill *

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252

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Physics

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Oct 30, 2023

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PHY-252 Brooks Lab 4 - Electromagnetic Induction Purpose In this experiment, you will investigate electromagnetic induction in coils of wire. You will also investigate magnetic fields produced by a bar magnet and a wire coil. Introduction Electric and magnetic phenomena are closely related. A moving electric charge (an electric current) creates a magnetic field, and a magnetic field exerts a force on a moving electric charge. These phenomena were discovered in 1820. Natural curiosity then compelled scientists to ask, “If an electric current produces a magnetic field, is it possible that a magnetic field can produce an electric current?” Ten years later the American scientist Joseph Henry and the English scientist Michael Faraday independently found that it was indeed possible. Henry actually made the discovery first, but Faraday published his results earlier and investigated the subject in more detail. Faraday discovered that under certain conditions a magnetic field does produce an electric current in a conductor. This phenomenon is expressed in Faraday’s famous Law of Induction, which today is the basis of the generation of most of the electricity used in the world. Our understanding of the mutual relationship between electricity and magnetism is among the most significant discoveries in physics during the last century and a half. Danish scientist Hans Christian Oersted made the first discovery in 1820. He discovered that a magnetic field exists in the space around an electric current. He further noted the relationship between the direction of the current and the direction of the magnetic field: the force lines form concentric circles around the current-carrying conductor. The magnetic field vector at any point is tangent to the circle at that point. The direction of the magnetic field is given by right-hand rule: Grasp the conductor in your right hand with your fingers encircling it and your thumb extended in the direction of the current. Your fingers indicate the direction of the magnetic lines. Michael Faraday reasoned that if a wire carrying a current produced a magnetic field, then a magnetic field around a wire should produce a current in the wire. However, Faraday met with failure when he attempted to reverse Oersted’s experiment. He found no effect with a stationary magnet or magnetic field.

After a lengthy series of experiments, he found the essential condition for producing a current from a magnetic field. There must be relative motion between the conductor and the field. The figure below illustrates Faraday’s breakthrough experiment. He connected a helical coil of wire to a galvanometer and brought a bar magnet near the coil. With no relative motion between the magnet and the coil, the galvanometer read zero, indicating no current. However, when he moved the magnet toward the coil, a current appeared in the coil and the galvanometer deflected to the right. As the magnet approached, the magnetic field that it created in front of it became stronger and stronger. It was this changing magnetic field that produced the current. A current also appeared when he moved the magnet away from the coil, but the direction of the current was reversed and the galvanometer deflected to the left. In this case, the magnetic field in the coil became weaker as the magnet moved away. Once again, the changing magnetic field at the coil generated a current. A current would also be created if he held the magnet stationary and moved the coil because the field lines in the coil would change as he moved the coil toward or away from the magnet. Only relative motion between the magnet and coil is needed to generate a current. It does not matter which one moves as long as magnetic field through the coil is changing. The current in the coil is called an induced current because it is induced by the changing magnetic field. Since a current in a circuit is normally produced by a source of potential difference, the coil itself behaves as if it were a source of potential difference. A source of potential difference is generally called an emf (electro-motive force), so we call the potential difference of the coil an induced emf . Thus, a changing magnetic field induces an emf in the coil, and the emf creates the current. His investigations led Faraday to the conclusion that the important factor in electromagnetic induction was the time rate of change of magnetic field lines (flux) through the coil. The induced emf ε is therefore given by t ε ∆Φ = − ∆ where Φ is the total magnetic flux through the coil, and the symbol ∆ means “a change in”. Therefore, the induced emf is the rate at which the magnetic flux is changing. Often the magnetic flux passes through a coil of wire containing more than one turn. If the coil consists of N turns all the same area, and if the same flux passes through each turn, the total induced emf is found experimentally to be N times that induced in a single loop. So, the average induced emf is:

N t ε ∆Φ = − ∆ This is known as Faraday’s Law of Induction. Since the magnetic flux is defined as the total number of magnetic field lines passing through a surface, BA Φ = where B is the magnetic field and A is the cross-sectional area of the loop. This shows that a change in flux can also result from changes in coil area, not just the magnetic field. Since the total flux depends on both of these factors, the induced emf is due to either a single factor or a combination of the two (both magnetic field and area could be changing at the same time). The negative sign in Faraday’s Law expresses another important law of electromagnetic induction. Russian Physicist Heinrich Lenz discovered what determines the polarity of the induced emf. He found that the polarity (direction) of the emf is such that the magnetic flux created by the induced current opposes the change in flux causing the emf. (Use the right-hand rule to determine the direction of the magnetic flux created by the induced current). This means that the induced current creates a magnetic field which (a) adds to the original magnetic field if the original field is decreasing, or (b) subtracts from the original magnetic field if it is increasing. Either way, the induced magnetic field tends to oppose the change in the original field. This rule is known as Lenz’s law. Consider what would happen if Lenz’s law were not true but was just the reverse. Then the induced current would produce a flux in the same direction as the original change. This greater change in flux would induce an even larger current which would cause a still greater change in flux, and so on. The current would continue to grow indefinitely, producing more power, even after the original effect ended. This would violate the conservation of energy principle. Lab Procedure The PhET we will be using in this lab is “Faraday's Electromagnetic Lab”, and can be found here: http://phet.colorado.edu/en/simulation/faraday As you go through the procedure, answer all questions in your lab report. Part A: The Electromagnet Field When you first start the PhET, there will be a compass and a bar magnet on the screen. By moving the compass around the bar magnet, you can see the magnetic field of the magnet change the direction of the compass needle. Question A1: If the red end of the compass needle is the north magnetic pole of the needle, which pole of the bar magnet does the north magnetic pole of the needle point to?

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Question A2: What happens when you move the compass to the other pole of the bar magnet? Now click on the tab at the top of the screen labeled “Electromagnet”. You should now see a battery connected to a coil along with a compass on the screen. Current flow in the coil is indicated as well. The potential difference of the battery should be set to 10 V. As with the bar magnet, you can move the compass around the electromagnet and see how the compass needle responds to the magnet field produced by the electromagnet Question A3: Which side of the coil does the north magnetic pole of the compass needle point to? Question A4: What happens when you move the compass to the other side of the coil? Question A5: Given your answers to questions 3 and 4, which side of the coil is its magnetic north pole, and which side is its magnetic south pole? Now move the slider on the battery all the way to the left. The potential difference of the battery should now be 10 V, with the positive side of the battery pointed in the opposite direction. Question A6: What happened to the magnetic field? Question A7: Now which side of the coil is its magnetic north pole, and which side is its magnetic south pole? Question A8: How does the direction of current flow in the coil affect the orientation of the magnetic field produced by the electromagnet? Part B: Current Induced by a Moving Magnet Click on the tab at the top of the screen labeled “Pickup Coil”. You should now see a bar magnet on the screen with a coil connected to a light bulb. Question B1: As you move the bar magnet on the screen, what happens to the coil and light bulb? Question B2: Does the response of the light bulb depend on how fast you move the bar magnet? If so, how? Now look on the right-hand side of the screen and find a control called “Indicator”. You should see a picture of a light bulb and a meter, with the light bulb highlighted. Click on the picture of the meter. You should now see a meter connected to the coil in place of the light bulb. This type

of meter is called a galvanometer . The amount of deflection of the galvanometer needle indicates the amount of current flowing in the coil, and the direction of the current in the coil is indicated by the right or left deflection of the galvanometer needle. Question B3: As you move the bar magnet around the coil, what happens to the galvanometer needle? Question B4: How does the direction of motion of the bar magnet affect the direction of deflection of the galvanometer needle? Now find the controls (on the right-hand side of the screen) that control the number of loops in the coil and the area of the loops. Use these controls to add and subtract loops from the coil and to change the area of the coil. Question B5: What happens to the deflection of the galvanometer needle (due to moving the magnet) when you increase the number of loops ? Question B6: What happens to the deflection of the galvanometer needle (due to moving the magnet) when you increase the area of the coils ? Now click on the tab at the top of the screen labeled “Transformer”. You should now see that the bar magnet has been replaced by an electromagnet. Move the electromagnet around the screen and observe what happens to the coil. Question B7: How does the response of the coil due to movement of the electromagnet compare to its response due to movement of the bar magnet? Question B8: Given your answer to question 7, how does the magnetic field generated by an electromagnet compare to that of a bar magnet?