Electromagnetic_Induction_VLab_2020 w answers

Virtual Lab: Electromagnetic Induction Name(s) : Date: After you finish this lab, please enter your answers in the accompanying lab quiz on eCampus for a grade. Introduction and Objectives An accidental observation by Danish physicist Hans Oersted in 1820 led to the discovery that a magnetic field is produced by an electric current. A few years later, around 1831, the Englishman Michael Faraday, and the American Joseph Henry, independently found that an electric current can be produced by a changing magnetic field. This process is known as electromagnetic induction. Faraday’s law of induction says that the induced emf ( ℰ ) or voltage is proportional to the number of turns in the coil ( N ) and the rate of change of magnetic flux (  B ): ε =− N ∆Φ B Δt ( 1 ) Magnetic flux is the total magnetic field ( B ) in a given area ( A ). It has the units of weber (Wb) or T m 2 , and is defined as: Φ B = ∫ ⃗ B∙d ⃗ A ( 2 ) Where the integral sign means “take the sum.” For a uniform magnetic field, this equation becomes: Φ B = BA cosθ ( 3 )  is the angle between the magnetic field vector B and the area vector A . The negative sign in equation (1) above indicates the direction of the induced emf. This direction is given by Lenz’s law , which states that: The induced emf has a direction that always opposes the change in flux that produces it. For example, if the external magnetic flux is made to increase in a coil, then an emf and electric current will be induced in the coil. This induced current will produce its own magnetic field. The new magnetic field produced by the induced current will have a direction that opposes the original external magnetic field. These directions can be found using the right-hand rule. Please see the figure below that explains how to use Lenz’s law . In this lab, we will explore Faraday’s law and Lenz’s law using an animation. 1

Figure explaining how to use Lenz’s law (Knight, 2017): 2

Lenz’s law states that the induced magnetic field will oppose the change in magnetic flux in the coil. When the north pole of the bar magnet is brought towards the coil, this causes the magnetic flux to increase in the coil. The induced emf produces a magnetic field that opposes this change in flux, so the induced magnetic field points to the right as shown above. The direction of the current that produces this field is counterclockwise (using the right-hand rule). 3. Next, starting with the north pole of the magnet inside the coil, move the magnet away from the coil and to the right. Move the magnet with a fast speed. This will cause the magnetic flux inside the coil to decrease. Observe the voltmeter and light bulb. Record your observations in Row 2 of Table 1 below. 4

Table 1 Number of turns in coil = 4 Orientation of magnet: north pole to the left Speed of magnet: fast Direction of magnet’s motion Speed of magnet’s motion Direction of deflection of voltmeter needle Amount of deflection of voltmeter needle Direction of induced magnetic field Direction of induced current (looking at the coil from the right) Brightness of light bulb Towards the coil Fast To the left Maximum North pole to the right Counterclock wise Bright Away from the coil Fast To the right Maximum North pole to the left Clockwise Bright 4. Repeat steps 2 and 3 above, but this time move the magnet at a slower speed. Make sure that the magnet’s motion is slow and steady. Record your observations in Table 2 below. Row 2 has been completed as an example. 5

Table 3 Number of turns in coil = 4 Orientation of magnet: north pole to the left Speed of magnet: very slow Direction of magnet’s motion Speed of magnet’s motion Direction of deflection of voltmeter needle Amount of deflection of voltmeter needle Direction of induced magnetic field Direction of induced current (looking at the coil from the right) Brightness of light bulb Towards the coil Very slow To the left About Quarter of Maximum North Pole to the Right Counterclock wise Least Bright Away from the coil Very slow To the right About Quarter of Maximum North Pole to the Left Clockwise Least Bright 6. Next, click at the bottom of the animation window to select two coils. Use the coil with less turns and record your observations for when the magnet is moved fast (as in Table 1 above). Record these observations in Table 4 below. 7

Table 4 Number of turns in coil = 2 Orientation of magnet: north pole to the left Speed of magnet: fast Direction of magnet’s motion Speed of magnet’s motion Direction of deflection of voltmeter needle Amount of deflection of voltmeter needle Direction of induced magnetic field Direction of induced current (looking at the coil from the right) Brightness of light bulb Towards the coil fast To the Left About Half to three-fourths of Maximum North pole to the Right Counterclock wise Less Bright Away from the coil fast To the Right About Half to three-fourths of Maximum North pole to the Left Clockwise Less Bright 7. Next, reset the animation to select the single coil with four turns. Change the orientation of the magnet so that its south pole faces left. Record your observations for when the magnet is moved fast (as in Table 1 above). Record these observations in Table 5 below. 8

Q2. From this lab, we see that ____. a) the larger the number of turns in the coil, the larger the induced emf but the smaller the current b) the number of turns in the coil do not affect the magnitude of the induced emf and current c) the larger the number of turns in the coil, the smaller the induced emf and current d) the larger the number of turns in the coil, the larger the induced emf and current Q3. From this lab, we see that ____. a) the induced emf produces a magnetic field that opposes the change of magnetic flux in the coil b) the induced emf produces a magnetic field that supports or increases the change of magnetic flux in the coil c) the induced emf produces a magnetic field with no specific direction, it is randomly oriented d) the induced emf produces a magnetic field that is always directed towards Earth’s north geographic pole Q4. The observations in this lab _____. a) support Faraday’s law but not Lenz’s law b) support Lenz’s law but not Faraday’s law c) support both Faraday’s law, and Lenz’s law d) support neither Faraday’s law nor Lenz’s law Q5. Faraday’s law says that ____. a) an emf is induced in a closed loop if the magnetic flux through the loop does not change ( ε = | ∆ ϕ ∆t | = 0 ) b) an emf is induced in a closed loop if the magnetic flux through the loop changes ε =− N ∆ ϕ ∆t c) an emf is induced in a closed loop only if a constant current is made to flow through the loop first d) an emf is induced in a closed loop only if the loop is non-conducting 10

Q6. Please solve the following problem. A bar magnet is moved through a conducting coil with 50 turns. The rate of change of magnetic flux is 0.03 Wb/s. What is the emf induced in the coil? Hint : use Faraday’s law to calculate the emf ( E ): ε =− N ∆ ϕ ∆t a) -120 V b) -760 V c) 0.005 V d) -1.5 V After you finish, please save this document, and enter your answers in the accompanying lab quiz on eCampus for a grade. You do not need to submit a completed lab report for this lab. Thank you. References : Knight, R. D. (2017) Physics for Scientists and Engineers A Strategic Approach (4 ed). Boston, MA: Pearson. 11