physc100- 20A Lab8-Magnetic Fields PDF

Preview

Summary

physc100- 20A Lab8-Magnetic Fields...

Description

Name: Shereen Bakshi

ID: 1541605

Date: 14/05/2020

Lab 8 – MAGNETIC AND ELECTRIC INTERACTIONS Introduction We can’t always see interaction between objects; however we should have a good understanding of ‘non visible’ magnetic and electric interactions. This lab will look at magnetic fields and induction and the electric force. Section 1: Magnetic Fields When a changing magnetic field is brought near a wire, an electric field is induced in the wire. If the wire forms a closed circuit, a current will be produced. This current will in turn generate a magnetic field to oppose the change which produced it (Lenz’s law). Thus, varying magnetic fields can create electric currents, and electric currents create magnetic fields. Our direct experience with magnetic fields may be limited. The following demonstrations are designed to enhance your magnetic intuition. Download the simulator from https://phet.colorado.edu/en/simulation/legacy/faraday Note this section relies on this download. If the above download doesn’t work try this interface they’ve set up: https://phet.colorado.edu/sims/cheerpj/faraday/latest/faraday.html?simulation=farada y (note this may take a couple of mins to load) If you still cannot access this simulator, no problem - please complete Section 3 in replacement for Section 1. You should still complete section 2. You should be familiar with how the magnetic field of a bar magnet are typically represented on paper. An example of this is shown below in Figure 1a, where the North and South poles of the magnet are clearly labelled and lines (blue in this case) are drawn to represent the magnetic field. These are field lines. The magnetic field has a direction, so the field lines all have an arrow on which points away from North and towards South. The spacing of the lines is important as this represents how strong the magnetic field is. This is similar to contour lines on a map such as Figure 1b, where lines closer together represent a stronger gradient/steeper mountain. Looking at the Bar Magnet tab, the field in the simulator is represented by compass arrows where the red ends are North polarized (so point towards South – opposites attract), and the silver ends are South polarized (so point North).

a

b

https://www.teachoo.com/10694/3113/Magnetic-Field-Lines/category/Concepts/ https://www.walkinglegends.co.nz/assets/Uploads/Related/Tongariro-NP-Trail-map.pdf Now select the Pickup Coil tab in the simulator. The menu of the right-hand side of the simulator allows you to change settings. Set the bar magnet strength to 100%. Ensure the Show field box is selected. 1. The aim of this activity is to, in words, propose relationships between the movement of the bar magnet and the amount of electricity generated. You’ll also look at how the number of coils and area of the loop affect the electricity generated. To look at the amount of energy being generated you can visualize this using the light bulb, you can use the voltmeter to visualize the direction of the current. a. What happens when you move the magnet back and forth through the centre of the coil? According to Lenz’s Law, a current must be induced in a way to oppose the change of magnetic field. Since the magnetic field pointing towards right is stronger, current will be induced in the coil which will be in the opposite direction (i.e. towards left). According to the Ampere ’s RightHand rule, the current points upwards. b. Position the magnet stationary in the coil, what voltage is generated? When the magnet is held stationary near, or even inside, the coil, no current will flow through the coil. Since there is no flow of current, no voltage is generated. Therefore, Voltage is zero. c. How does the speed you move the magnet back and forth affect the electricity generated? Moving the magnet faster increases the brightness of the bulb as more electricity is generated due to the greater kinetic energy being converted into electrical energy. While moving the magnet slower, due to less movement and lesser kinetic energy being produced the electricity generated is low. d. What is the effect when you do this with one loop compared to three loops?

While doing it with one loop the electricity generated is lesser and the brightness is dim whereas, while doing the same with three loops electricity generated is much higher and its brighter at the same speed. e. Just put the North end of the bar magnet back and forth into the coil, flip the polarity (using the button in the menu) and then do the same movement with the South end. Using the voltmeter, how does this affect the direction of the induced current? While moving the north end forward into the coil we see that the direction of the current flow in the coil is downwards. On moving it backward, the current starts moving upwards. However, while moving the south end forward into the coil, the current flows upwards. While moving it backward the current flows downwards. When the magnet is moved one way (into the coil), the needle deflects one way, when the magnet is moved the other way (out of the coil), the needle deflects the other way. Therefore, in both cases it’s the opposite and the voltmeter reading keeps fluctuating while moving the magnet back and forth. f. Move the magnet near but not through the coil. Do you still see a current generated? What does this tell you about how the current is created, e.g. it is caused by magnet entering the coil, or is it something to do with the magnetic field? Yes, current is generated but it is very low. The current generated is to do with the magnetic field which changes at a higher rate when the magnet enters the coil thereby, generating high current in the coil.

Now select the Electromagnet tab. An electromagnet is a magnet where the magnetic field is produced by the flow of electricity. The strength and polarity of the magnetic field are dependent on the electricity. The purpose of this section is to understand how alternating current (AC) and direct current (DC) are different and how this affects the magnetic fields produced by an electromagnet. 2. Switch between the current sources of AC and DC within the simulator. You can add the Field meter (using the select box) to help you with this section. a. By looking at the flow of electrons within the coils to help, what is the difference between AC and DC? When set to DC the current flows in one direction only. On changing the current source to AC, the magnetic field direction changes periodically and therefore the current flows in both directions. The rate of change of the magnetic field direction depends on the frequencies. b. What is the effect on the magnetic field produced by the flow of electrons around the coils? When the flow of electrons in the coil is in one direction (DC) the magnetic field remains static. Whereas, when the flow of electrons in the coil is in both directions (AC), the direction of the magnetic field constantly changes.

c. Use the field meter to propose a relationship between the number of coils, N, and the magnetic field, B (the number of loops can be changed on the right between 1-4). Hint: position the field meter at a fixed point and change the number of coils/loops, how does the reading change. B (magnetic field) is directly proportional to N (number of coils). Now select the Transformer tab. A transformer is an electrical device consisting of two or more coils which transfer electrical energy through induction caused by changing magnetic field. Transformers are used to convert from a higher voltage to a lower voltage (or vice-versa). 3. You should have observed in part 1 that the magnet had to be moving to generate energy – with this in mind looking at the transformers tab you should see an electromagnet (coil attached to a power source), and a second coil connected to a light bulb. You will use the magnetic field created from the electromagnet to induct a voltage in the second coil and power the lightbulb. a. It is possible to power the lightbulb using DC and AC current, find the conditions under which each can create a current in the lightbulb’s coil. Yes it is possible in both the cases. To be able to create a current in the lightbulb’s coil the AC and DC source of current should constantly move around the coil connected to the bulb therefore, allowing the electrons to move in the coil. b. Using the AC source – how are the electromagnet and lightbulb coil arranged when the inducted current is maximum? Since alternating current would induce magnetic field with alternating direction in the coil, there will be a constant change of magnetic flux linkage experienced by the lamp coil. According to Faraday’s law there would be a changing emf and hence changing current across the lamp coil. c. Fix the coils with the electromagnet supplied with AC, is sitting inside the other coil. Change the number of coils on both the electromagnet and the lightbulb coil to a few different options. Describe the relationship between the number of coils on each and the amount of energy created (amount of energy can be assessed using the ‘strength’ of the light created or swapping to look at the voltmeter). The number of coils is directly proportional to the amount of energy created. Now select the Generator tab. A generator is a device that converts mechanical power or movement into electrical power. 4. In the simulation turn on the tap (using the slide bar on the tap), a. Describe how electricity is generated to power the lightbulb When the tap is turned on the mechanical energy is converted into electrical energy. This happens as the water keeps turning the wheel and magnet continuously that causes the change in the magnetic field strength experienced by the coil. According to Faraday’s law, when there is a change in magnetic flux linkage there is an induced emf. Since the circuit is completed with the light bulb, a current is induced that flows through the light bulb. So, the light bulb gets continuous electric supply.

b. Describe the relationship between the flow rate of water and the voltage induced Greater flow rate of water causes higher turning rate of the bar magnet. This will cause changes in magnetic flux linkage which in turn will cause an induced voltage. The flow rate of water is directly proportional to the induced voltage.

Section 2: Electric Fields Any electrically charged object creates a force on any other electrically charged object. Like magnets, opposites attract, that is a positively charged object will be attracted to a negatively charged object. Counter to this that means that similar charges repel each others, so two positive charges will repel. Due to this, a force is created that either pushes or pulls the object to either attract or repel them. This force is given by Coulomb’s law: 𝑘𝑞 𝑞 1.1 𝐹 = 𝑟12 2 9 Where k is Coulomb’s constant 𝑘 = 8.988 × 10 𝑁𝑚2 𝐶 −2 , q are the charges on object 1 and 2 respectively, in Coulombs (C), and r is the distance (m) between them. 1. Show that equation 1.1 is dimensionally consistent.

2. If a charge of q1 = 5 µC is positioned at a distance of 40 mm from a second charge of q2 = 3 µC. What is the force created?

3. The same magnitude force is felt on q1 and on q2. On the diagram below (or a sketched version) show the force vectors (size and direction) experienced by both charges.

4. If the charges were change to be, q 1 = -5 µC and q 2 = -3 µC, how would this change the force? The force won’t change, the magnitude of force and direction will remain the same. 5. If the charges were changed to be, q 1 = 5 µC and q 2 = -3 µC, how would this change the force? The magnitude of the force will remain the same but the direction would be different as charges will attract eachother. Open the simulator below and select Macro Scale: https://phet.colorado.edu/sims/html/coulombs-law/latest/coulombs-law_en.html 6. Using the simulator, check your answers to the above questions 2-5. Did your answers agree? Yes the values match. F= 84.3 N When working on the atomic scale the same rules apply, the only difference is we express the charge of bodies in fundamental units of electric charge, e (𝑒 = 1.6 × 10−19 𝐶). where e is the charge of a single proton or -e is the charge of a single electron. Imagine now you have two bodies of charges, q 1 = -6e and q2 = 3e and a separation distance of 40 pm. 7. Use equation 1.1 to find the electrical force created.

8. Check your answer using the Atomic Scale simulator, does you answer agree? (note you can also switch between scientific and non-scientific notation if you want to practice this) Yes, the answer matches. F = 2.60 X 10^-6 N...