PHY 242 - Laboratory LABORATORY 9: FARADAY'S LAW Objectives: identify equipment and conditions that produce electromagnetic induction compare and contrast how both a light bulb and a voltmeter can be used to show characteristics of the induced current predict how the induced current will change when the conditions are varied identify the basic ingredients of generators and transformers Materials Required: Computer with Excel and access to simulation Faraday's Electromagnetic Lab: https://phet.colorado.edu/sims/cheerpj/faraday/latest/faraday.html?simulation=faraday Java simulation runs in a web browser via a technology called Cheer - not compatible with iPads). The original simulation (runs faster but needs Java software to be installed on your computer) can be found at https://phet.colorado.edu/en/simulation/legacy/faraday Software Requirements: Windows Macintosh Chromebook Linux iPad Mobile Phone Chrome, Edge Chrome, Safari Chrome Not recommended Safari Not recommended Introduction: English physicist Michael Faraday discovered magnetic induction in 1830. Faraday's Law was later incorporated into the more comprehensive Maxwell's equations that explain the relationship between electricity and magnetism. Magnetic induction makes possible the electric motors, generators and transformers that form the foundation of modern technology. Whenever a coil is placed in a varying magnetic field, an EMF gets induced in the coil (induced emf). A current is induced by moving a magnet, by moving the coil, and/or by rotating the coil relative to magnetic field, therefore the induced emf depends, not only on how the magnetic field changes with time, but also on how the geometric relation between the loop and magnetic field may change as well. For example, even in a uniform and constant magnetic field, changing the shape of a conducting loop or its orientation relative to the magnetic field lines will produce an electric field and hence a current. The appropriate combination of geometry and magnetic field needed to describe the induced electric field, when either changes, is called magnetic flux. Faraday's Law of Induction $= BA cos estates that the induced voltage (emf) in a circuit is proportional to the rate of change over time of the magnetic flux through that circuit. We can increase the emf by increasing the number of loops in the circuit. The induced voltage in a coil with two loops will be twice that with one loop, and with three loops it will be triple. This is why real motors and generators typically have large numbers of coils. An important application of Faraday's Law of Induction is the transformer, invented by Nikola Tesla. In this device, alternating current, which changes direction many times per second, is sent through a coil wrapped around a magnetic core. This produces a changing magnetic field in the core, which in turn induces a current in second coil wrapped around a different part of the same magnetic core. Transformers make possible the electric grid we depend on for our industrial and technological society. Transmission lines operate at hundreds of thousands of volts to transmit more power within the current-carrying limits of the wires. This voltage is stepped down repeatedly using transformers at distribution substations until it reaches our houses. Activity 1: The emf due to a Pick-up Coil 1. Start the Faraday's Electromagnetic Lab PhET simulation, and chose the Pick-up Coil tab. Explore the simulation. Use the incandescent light bulb as an indicator and the slider to increase the field strength to 25%. For the number of loops, select 1, and for the loop area 25%. Make sure that you check the Show Field and Show Electrons boxes. 2. Does the coil or the bar magnet appear to be creating the magnetic field? 3. Move the bar magnet around the outside of the coil bulb apparatus. What is happening to the coil bulb? 4. Move the bar magnet back and forth inside the coil of wire. What is happening to the coil/bulb? 5. Try moving the magnet at different speeds (very slowly and rapidly). Describe what happens for each case. 6. Rank the arrangements and motions shown below from most effective to least effective in terms of lighting the bulb, allowing for ties. For example, if A were most effective, B were least effective, and C and D were equivalent to one another, the ranking would be A > C = D > B. A. Transverse B. Transverse C. Longitudinal D. Longitudinal External Internal Internal External 2Increase and decrease the strength of the bar magnet (using the slider). What effect does changing the magnet's strength have? 8. Set the bar magnet strength at 75% and move the magnet. Increase the number of loops to 3. What effect does this have? 9. Increase the loop area to 100% and move the magnet. What effect does this have? 10. What combination of magnet strength, number of loops, and loop area gives you the greatest induced current? 11. Move the bar magnet through the coil and observe the motion of the electrons in the forward arc of the coil loops. Report (draw in the box) the correlations of magnet motion and electron motion. For example, when the magnet approaches from the left, north pole first; electrons move downward, and when the magnet departs to the right, south end last; electrons move upward. N S N 12. How do the electrons move when the magnet approaches from the right, south pole first, and departs to the left, north end last? S N - N 13. How do the electrons move when the magnet approaches from the left, south pole first, and departs to the right, north end last? N 14. How do the electrons move when the magnet approaches from the right, north pole first, and departs to the left, south end last? N N S Activity 2: Magnetic Field Created by a Current 15. Switch to the Electromagnet tab. Explore the simulation. Use the DC current source. Make sure that you check the Show Field and Show Compass boxes.16. Move the electromagnet around the screen and describe what the compass does. 17. Move the compass around the electromagnet in order to determine the North and South poles. 18. Does changing the number of loops appear to have an effect on the rate of motion of the electrons in the wire? 19. Gradually decrease the voltage of the battery from 10 V to 5 V. What effects does that appear to have on things? 20. What happens when the voltage is 0 V? 21. Continue to slide the voltage bar to the left. What does that do? 22. Change your current source from DC to AC and describe what the compass does. 23. Observe the electrons in the AC current source and compare their movement to those in the DC current source. Explain the difference between DC and AC in terms of electron movement. Activity 3: The Transformer 24. Start the simulation in the Transformer tab and explore it. Use the DC current source. Make sure that you check the Show Field, Show Electrons, and Show Compass boxes. For the pick-up coil indicator use the incandescent light bulb. In this simulation one coil is making a magnetic field (PRIMARY) and the other is picking it up (SECONDARY). 25. When you start the simulation, and there is current flowing in the battery coil, is the light bulb lit? 26. Now move the battery coil back and forth. What happens with the light bulb? 27. Change the current source to AC to replace the battery with an AC voltage. KEEP IT STILL! Describe what is happening to the magnetic field and to the light bulb. 428. Replace the light bulb in the pickup coil indicator with a meter. Investigate the AC voltage controls. The left slider controls the peak voltage. The bottom slider controls the frequency (number of wave patterns per second) 29. Changing only the peak voltage from 30% to 30% and then 100% in the primary coil, what happens to the voltage meter indicator in the secondary coil? Use the incremental steps button at the bottom of the simulation to better read the meter indicator. 30. Changing only the frequency from 30% to 50% and then 100% in the primary coil, what happens to the voltage meter indicator in the secondary coil? 31. What parameters should you change to get the maximum voltage in the secondary coil? Activity 4: The Generator 32. Start the simulation in the Generator tab and explore it. Use the DC current source. Make sure that you check the Show Field, Show Field Meter, and Show Compass boxes. For the pick-up coil indicator use the incandescent light bulb. In this simulation one coil is making a magnetic field (PRIMARY) and the other is picking it up (SECONDARY). 33. Turn the faucet on, just enough to get about 10 RPM on the turbine. Increase the rotation to about 20, 60, even 100 RPM. What effect does that appear to have on the compass, the wire and the bulb? 34. Increase and decrease the number of loops. What effect does this have? 35. Increase and decrease the bar magnet's strength. What effect does this have? References: CC-BY license, PRET Interactive Simulations, University of Colorado Boulder, http://phet colorado.edu 5