Lab 7

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Lab 7 Magnetic Induction Part I Magnetic flux • • Recall and write down the definition of magnetic flux, the angle is between magnetic field direction and 0 and 90 degrees. • Write down the magnetic flux in each case use the symbol given in the diagram. If zero, please just write 0. a: 0 b: 90 c: 60 • Open Friday’s law simulator • Keep all as it is when you just opened the simulator, do not move anything. • Is there any flux in the coil? No • Is there any change in flux in the coil? No • Now select Show Field Lines, check if your answer is correct. • Now imagine that you are moving the magnet toward the coil. • Is there flux in the coil? Yes • Is there any change in flux in the coil? Yes • Please state Lenz’s law here: The induced electric current that flows in a direction such that the current opposes the change that induced it.

• Use the bottom wire, will there be any current according to Faraday’s law, if so, what will be the direction the current (left or right) on the bottom wire. Left • Now move the magnet into the coil, check the direction of the current, if the current flow to the left, the needle should show negative direction and vice versa. Left, negative • Now leave the magnet inside the coil, is there any current? Why not? No, the current balances out and there is no direction or movement • Now pull magnet out (move towards right) of the coil, what happens to the current direction. The current turns right, positive • Now move the magnet in and out with faster or slower speed, observe and describe the difference in the current. The faster the magnet is pulled in/out, the higher the current jump while the slower the magnet is pulled in/out, the slower the current jump will be. • In this simulation, where did the electric energy come from or who is doing the work? The electric energy came from the magnet Part II Motional EMF Open the Motional EMF simulator • Click play and observe. Suppose this purple force is a constant force applied by you on the conducting rod (red). The rod is on a frictionless conducting rail (Blue), the rail is connected to a resistor (green). Describe the motion of the rod with your constant force applied, describe what happens if you remove your (purple) force. When there is a constant force applied to the rod, it begins to accelerate. This causes a resistive force to pull on the rod which makes the rod reach a constant velocity. After the constant force is removed, the rod begins to slow down (negative acceleration) and eventually stops. If the purple force is removed, the rod stops. • Reset and run the simulation again, as you begin to move the rod to the right, will there be any current in the resistor? If so, in what direction (clock or counter- clock)

Yes, it would be in the counterclockwise direction. • As you move the rod faster to the right, what is going to happen to the current, increase or decrease, explain The current would increase because the resistive force acting on the rod would increase as the rod moves faster, making the magnetic flux through the loop of the circuit increase which leads to an increase in current. • If there is current in the red rod, will the red rod feel any magnetic force? If so, use the RHR-1 to find the force direction, will this force increase or decrease as the rod moves faster, explain. Yes, there will be a magnetic force acting on the rod in a perpendicular direction as the rod increases in velocity with the force direction being out of the page in the opposite direction. There will be a current and once the current is created, it will have a magnetic force since they interact with each other. The bigger the Emf, the bigger the current and the bigger the force as the rod moves faster. • Now, reset and run the simulation again with your purple force applied, observe the Yellow vector, is it increase or decrease? What does this yellow vector mean, is it the same force we explored in d)? The yellow vector increases when the purple force is applied and the rod accelerates, but decreases when the purple force is gone and the rod decelerates. This vector represents the resistive force acting on the rod which can also be seen as the magnetic force. This is the same force as the one we discussed in part d. The yellow force is the resistive force on the rods it increases with the area of the conducting rod traveling on the current in the opposite direction of the rod. • Eventually the yellow force (magnetic force on the red rod) will equal to the force (purple) you applied, the rod will now move with constant velocity. Now we say the rod reached terminal velocity. • Assume the rod is moving with constant velocity v=0.2 m/s, the green resistor is 0.1 ohms (assuming no resistance on rod and rail), the constant magnetic field is 2T • Measure the length of the rod (on your screen): L= 4.5 cm = 0.045 m • Calculate the induced emf using Faraday’s law. (hint: assume the part of the side length that form the closed loop is X, then the area of the closed loop is: XL, what is X/t?)

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• Emf= vBlsin90 • =0.2*2*sin90 • =0.018 V Emf= • Calculate the induced current in the loop. I=Emf/R =0.018/0.1 =0.18A • Calculate the magnetic force (yellow vector) on the rod, this is also the force you applied on the rod (purple vector). F=ILB sin 90 =0.18*0.045*2*sin90 =0.0162 N • Now, keep running the simulation, observe what happens to the red rod after you remove your purple force? What happens to the magnetic force (yellow)? Explain. After the purple force is removed, the yellow vector begins to decrease as the rod decelerates. Because a force is removed, the current begins to decrease along with Emf. As a result, the rod will move slower and the magnetic force decreases. This force decreases because as the rod’s speed decreases, it takes less force to pull the rod to a halt. So, as the speed of the rod decreases, less force is applied. Part III Difference between a motor and generator • Use the word Mechanical Energy and Electrical Energy to fill the blank below Here is the DC motor, Watch how it is turning inside a magnet, the energy transformation is from Electrical Energy to Mechanical Energy This is a simple generator we have in our lab, by turning the big wheel, the coil inside the magnets will turn, then the light bulb can be lit up. Notice the similarity in the material needed for a motor and a generator. The energy transformation is from Mechanical Energy to Electrical Energy

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