Teacher s notes Induction of a voltage in a coil: A set of simple investigations

Size: px

Start display at page:

Download "Teacher s notes Induction of a voltage in a coil: A set of simple investigations"

Hugh Rogers
6 years ago
Views:

Faraday s law Sensors: Loggers: Voltage An EASYSENSE capable of fast recording Logging time: 200 ms Teacher s notes Induction of a voltage in a coil: A set of simple investigations Read This activity

1 Faraday s law Sensors: Loggers: Voltage An EASYSENSE capable of fast recording Logging time: 200 ms Teacher s notes Induction of a voltage in a coil: A set of simple investigations Read This activity is a demonstration of Faraday's experiments into induction. It is a self contained piece of work that uses the Lascells Faraday s law apparatus to create a simple to run practical, and set of investigations. The reliability of the apparatus coupled with fast data logging guarantees good repeatable results within the time constraints of a standard lesson. The high repeatability of the work makes collaborative work a strong possibility. The investigation uses a Voltage sensor; this simply produces better results in terms of the graph shape and size. You can easily add a Current sensor to show, graphically, the traces of current and voltage (use the ±100 ma Current sensor). Apparatus 1. An EASYSENSE logger capable of fast logging. 2. A Smart Q Voltage sensor ±1 V. 3. A Smart Q Current sensor ±100 ma (optional). 4. Lascells Faraday s law apparatus. 5. Laboratory stand and finger clamp (to hold the acrylic tube). 6. Ruler (expandable tape is better). 7. Soft pad to catch the magnet, or small box filled with tissue. Note: It would be a good idea to check the polarity of the supplied magnet against one with known polarity (or a directional compass) to identify and mark the North pole. Set up of the software Set up the software using the information shown below. This is based on the North of the supplied magnet falling first and with the black lead of the sensor connected to the black plug of the coil, red lead of the sensor connected to the white 300 turns on the coil. Recording method Total recording time Interval between samples Trigger on Level Graph 200 ms 50 μs Channel: Voltage Rises above Trigger value Pre trigger +100 mv 50% Notes For the sample data, the acrylic guide tube was clamped so the top of the tube was aligned with the top of the wire coil. Height measurements were taken from the top of the tube to the top of the fixed plastic clip of the wire coil. 1

2 With the supplied magnet, expect a voltage of approximately 130 mv with the smallest drop height. A soft landing for the magnet needs to be provided. Repeated hard blows to a magnet will reduce its strength. Using pre-trigger allows data to be collected but not recorded until the trigger is reached. The polarity of voltage induced depends upon which pole of the magnet is moving through the coil. In this experiment the trigger is +100 mv. If the magnet is dropped "wrong pole" first the recording may only capture half the event. Using pre-trigger, 50% of the data is collected before the trigger event. Results Trigger of +100 mv 50% pre-trigger data The above screen shows a typical set of data, drop height 3 cm. Same data with axis limits changed (via the Sensor settings display option). If you change the axis limits try to keep them even about the zero line if possible, this helps with interpretation, i.e. have the maximum and minimum limits the same. 2

3 Values is used to find the maximum voltage of the first peak. Using Area to find the area of the first peak. 3

4 Study of the graph should lead to conclusions, Area of Voltage (mv.s) of each peak is the same but the peak voltage is different. The difference in peak voltages is attributed to changes in the velocity of the two poles as they pass through the coil. The peaks are asymmetrical, but become more symmetrical as the velocity increases. When there is no velocity change between the two poles the peaks will be symmetrical. Dropping the magnet S pole first reverses the polarity of the peaks but does not change the characteristics e.g. peak voltage, area shape. By using Overlay and increasing the distance between the drop point and the coil, the relationship between the height of release and induced voltage can be studied. Changes in induced voltage with 3 cm height increments. Use Zoom to select the area of the peaks to find peak voltage for each drop height. Use Values to find the value. Note how the peak moves to the left with increase in height. 4

5 Sample results table The analysis will proceed in three stages:- 1. Measure the values of the maximum emfs 2. Calculate the values of v 2 and v 3. Draw graphs of Max emf vs v 2, and max emf vs v. Draw a line of best fit as needed. Drop height Peak voltage Peak voltage Peak Velocity m.s -1 first peak mv second peak difference Drop height should be recorded to the mm and should be the actual drop height and not the value it should have been, e.g. you may suggest increasing the height by 3 cm for each run, in reality the height may be 2.9, 3.2, 6.1, 9.0, etc. Peak difference should get less as velocity increases. Increase in peak velocity should decrease as drop height increases, it is not linear Induced emf vs magnet velocity for a coil 2.50 y = x velocity m/s Induced emf Volts Plot of velocity vs. emf. The graph shows a good linear relationship, with the x.y intercept effectively at 0, 0. We must conclude that the velocity of the magnet is directly linked to the voltage produced. The calculation of velocity uses; 2 2 v = u + 2as Where; u = the initial velocity, as the object is stationary the initial velocity is zero. a = gravity (9.81 m/s -2 ) s = displacement (distance from drop to coil in metres) 5

6 v 2 = 2as Or by taking the square root of v 2 to get v and substituting g for a, and s for h, we get, v = 2gh Using a Current sensor If you use a Current sensor, use the ±100 ma sensor. Recording method Total recording time Interval between samples Trigger on Level Graph 200 ms 200 μs Channel: Current Rises above Trigger value Pre trigger +10 ma 50% All other aspects of the work remain the same. Typical result of current when a magnet passes through the Lascells Faraday s coil. Coil at furthest drop distance, down the tube, of approx 35 cm. 6

7 Faraday s law Sensors: Loggers: Voltage An EASYSENSE capable of fast logging Logging time: 200 ms Induction of a voltage in a coil: A set of simple investigations If a magnet is passed through a coil of conducting wire a voltage is induced (created) in the coil. The faster the magnet moves through the coil the greater the strength of the induced voltage. The Lascells Faraday s law apparatus is used with a Voltage and or Current sensor and a data logger to provide a simple and effective method for investigating Faraday s laws of induction. The advantage of the data logger system over an oscilloscope to measure the voltage pulse is that the data is saved and analysed by the powerful tools in the EasySense software. The system gives a high degree of repeatability and simplicity of use, within minutes of getting the apparatus out of the cupboard you are producing results. Q Drop distance s What you need 1. Lascells Faraday s law apparatus. 2. A retort stand and finger clamp (to hold the acrylic tube). 3. An EASYSENSE logger capable of fast logging. 4. A Smart Q ±1 V Voltage sensor. 5. A Smart Q ± 100 ma Current sensor (optional). 6. Ruler. 7. Soft pad to catch the falling magnet. Note: Mark one end of the supplied magnet to ensure consistent orientation of the magnet when dropping down the tube. 1

8 What you need to do Connect the Voltage sensor to input 1 of the logger. From the EasySense software s Home screen select Graph. Use the logging wizard to set up the logging time and parameters as described in the table below. Total recording time Interval between samples Trigger on Level 200 ms 50 μs Channel: Voltage Rises above Trigger Pre trigger value +100 mv 50% Part 1: Collecting the basic induced voltage detail 1. Assemble the apparatus as shown with the Voltage sensor across the terminal marked O (black terminal) and the white terminal marked 300. Make sure there is something soft for the magnet to fall on. Grip the tube with the finger clamp as close to the top as possible. 2. It is possible that the coil may act as an aerial and pick up electrical noise. To test for this, select Test Mode from the Tools menu to check for background electrical activity. If the voltage readings are erratic try moving the apparatus to a location where the voltage has the smallest variation. 3. The experiment uses fast data recording with a trigger to make sure the samples are collected when the magnet is passing through the coil, and all subsequent recordings start at the same relative time and position. Select the Overlay icon so that you can see all sets of data on the same graph axis. 4. Select Start. Make a note of which pole of the magnet will enter the coil first and drop the magnet down the guide tube (e.g. N pole first, then S pole first on the second run). 5. Wait for the graph to appear in the software. Check it looks similar to a single trace on the example (page 4). Repeat if necessary. 6. Use Save As to save the data. Results and analysis You will have recorded the basic induced voltage pulse as the magnet passes through the coil. Use Values to find the maximum voltage value of both of the peaks. Call the value of the first peak H 1 and the value of the second peak H 2. Calculate the ratio of H 1 over H 2. Use Add Text to label the graph with the pole, height of the magnet drop, etc. Questions 1. How did you test the polarity of the magnet? Describe the method you used. 2. Explain why the values of the two peaks measured (H 1 and H 2 ) are not the same. 3. Does the ratio of H 1 over H 2 remain constant if you change the position of the drop height? 4. Why are the peaks opposite in direction? 5. Use the Area function (Analysis menu) to find the area under the two peaks. The area is related to flux change. Is the incoming flux change equal to the outgoing flux change? 6. Are the two peaks the same shape? Describe and explain any differences. 7. What do you think will happen to the shape of the peaks if the drop height increases? Part 2: What happens if the number of turns in the coil changes? Repeat part 1 but have the sensor connected to the black zero and the white 150 turn terminal. 1. What has the reduction in turns done to the voltage impulse? 2. Is there any relationship between turns of the coil and the voltage impulse? 3. If you were to investigate this further, what other characteristics of the coil could you change? 2

9 Part 3: Investigate the relationship between height dropped and induced voltage Use the same basic set up as in part 1. Make sure you have Overlay selected. 1. Make a note of the height from which the magnet is dropped (measure from the top of the guide tube to mid height of the coil). Hold the magnet for dropping with the end just barely inside the guide tube. It is important that the drop position is constant through the experiment. 2. Select Start and drop the magnet, wait for the graph to be drawn. 3. Move the coil clip to a new height (you should aim for a minimum of 5 different magnet drop heights). 4. Select Start for the next recording and release the magnet. 5. Repeat for as many heights as you have decided to record. 6. Use the Values tool to find the peak emf. 7. Plot a graph of peak emf vs. drop height (you could use Transfer to Excel from the File menu to export the data direct to Excel). 8. Describe the relationship between height dropped and emf. Example graph showing changes in voltage with 3 cm height increments. 3

10 Part 4: Investigate the relationship between the velocity at which the magnet passes through the coil and the induced voltage Use Zoom and then Values to find the peak voltage for a data set. Data set (2)s peak Data set (3)s peak Data set (1)s peak v is the velocity of the falling magnet. The analysis will proceed in three stages:- 1. Measure the values of the maximum emfs 2. Calculate the values of v 2 and v 3. Draw graphs of Max emf vs v 2, and max emf vs v. Draw a line of best fit as needed. 1. Measure the values of the maximum emfs Select Values and measure the value of the maximum emf of the first peak for each data set. Note down the values in a results table. 2. Calculate the values of v 2 and v. This calculation gives the velocity 2 for the first voltage maximum in each data set. Use the formula: - v 2 = u 2 + 2as (as the falling mass is being accelerated by gravity). u = 0 (the magnet has no velocity before it is released) a = g = 9.81 m/s 2 s = displacement, i.e. distance from base of magnet at release to the centre of the coil, in metres. v 2 = 2gs v 2 = 2 x 9.81 x s Calculate the values of v 2 and v and enter into a results table or a spreadsheet program such as Excel. Drop height s (m) Max emf mv v 2 (m/s) 2 v m/s 3. Draw the graphs of Max emf vs. v 2, and max emf vs. v. Using the data from the results table, draw graphs, Max emf vs. v 2, and max emf vs. v either manually on paper or using a spreadsheet program. Draw lines of best fit as needed. 4

11 Questions 1. What do the two graphs tell you about the relationship between the velocity of the magnet and the induced emf? The second statement in Faraday s law is: The size if the induced emf is proportional to the speed with which the wire cuts the magnetic field. 2. Do your results demonstrate the validity of this law? Explain your reasoning. 3. In each data set the size of the peaks (H 1 and H 2 ) are not the same. a. Which one is bigger? b. Explain why. 4. Does the ratio of H 1 over H 2 remain constant? 5. Why are the peaks opposite in direction? 6. Use the Area function (Analysis menu) to find the area under the two peaks. The area is related to flux change. Is the incoming flux change equal to the outgoing flux change? Errors Discuss any sources of error in the experiment. Extensions 1. Does it make a difference which pole of the magnet passes through the coil first? 2. Does turning the coil over make a difference? 3. Does the strength of the magnet have an effect? 4. Does increasing the number of turns in the coil have any effect? 5. Investigate the change in magnetic field and the induced voltage (use a Magnetic field sensor mounted alongside the coil). 6. Consider the direction of the induced emf in the coil, and the magnetic pole going through. Use this information to demonstrate the validity of Lenz s law. 5

Magnetic Induction Kit

Magnetic Induction Kit Investigating Faraday=s laws of electromagnetic induction DO094 Revision History 1) 003 Original. ) 3.7.07 Corrected EMF definition. Added Appendix. Reformatted. Copyright 003-007