University of Huddersfield
School of Computing & Engineering
NIE2299 Electrical Principles 2 (Session 2014-15)
Assignment – Electromagnetics
Word limit: 2,500
Submission Deadline: 12.00 (noon) Friday 1 May 2015
Submission Method: electronic word document to Turn-It-In
Submission method in the event that Turn-It-In is not available: hard copy* to the Student Support Office (SSO)
*For hard copy submission to the SSO (CW1/01) students must download a coversheet from the following link: http://ecover.hud.ac.uk/assignments/ to go with their assignment. The cover sheet contains a barcode which SSO staff will scan. Students will then receive an electronic receipt by email.
Safety: read this before starting any practical work
In the following exercises you will be using some powerful magnets. Please note, and act on, the following precautions:
- IF YOU HAVE ANY METALIC IMPLANTS THAT MAY BE MAGNETIC (I.E. IMPLANTS WHICH MAY CONTAIN IRON, NICKLE, COBALT, GADOLINIUM, NEODYMIUM OR SAMARIUM) THEN MAKE THIS KNOWN TO THE TUTOR AND DO NOT TAKE PART IN THIS ASSIGNMENT BEFORE A RISK ASSESSMENT HAS BEEN UNDERTAKEN
- IF YOU HAVE A HEART PACEMAKER THEN MAKE THIS KNOWN TO THE TUTOR AND DO NOT TAKE PART IN THIS ASSIGNMENT BEFORE A RISK ASSESSMENT HAS BEEN UNDERTAKEN
- SWALLOWING A MAGNET IS DANGEROUS. THEY MAY CAUSE LIFE-THREATNING DAMAGE TO THE STOMACK AND/OR INTESTINES DUE TO FORCES ON THE MAGNETS FROM NEARBY MAGNETIC MATERIALS. NEVER PUT ANY MAGNET IN YOUR MOUTH. SWALLOWING TWO MAGNETS IS ESPECIALLY DANGEROUS.
- NEODYNIUM MAGENTS (SUCH AS THE ONES USED IN THESE EXERCISES) ARE POWERFUL. HANDLE THEM CAREFULLY AND BE AWARE OF NEARBY FERROUS OBJECTS AND OTHER MAGNETS. THE MAGNET COULD CAUSE BLOOD BLISTERS AND OTHER INJURIES IF SKIN AND/OR TISUE IS TRAPPED BETWEEN TWO MAGNETS OR A MAGNET AND A MAGNETIC MATERIAL.)
This (assessed) assignment forms one component of NIE2299. It aims to deepen your conceptual understanding of magnetostatic forces and fields. There are two 3-hour timetabled periods for you to undertake the practical and simulation work. If you complete all the practical work in the first timetabled session then you should use the time in the second timetabled session for writing up your report. The tutor will be on hand in the sessions to assist you (as far as is compatible with the explanations you give being your own work).
You may not be able to explain all of your experimental observations until close to the end of autumn term (i.e. after you have completed the lecture course). This is why the deadline has been set as it has. You are advised to start writing the report as soon as possible, however, adding explanatory material as you learn it. (You need to manage your time and leaving the entire report until you have covered all the relevant material in the lectures would risk overloading yourself with work at a time when several deadlines for submission of work across multiple modules might coincide.)
The word limit on your report (excluding title page, equations, words within figures and appendices but including words in figure captions) is 2,500. You must state clearly the word count at the start of the report. Appendices may be included for your own purposes (e.g. revision) but will not be read by the assessor.
In your report you will need to provide figures that describe various equipment configurations you have used in the experiments. If you have access to a camera (e.g. incorporated into your mobile phone) then taking a picture of the equipment for pasting it into your report is a particularly effective and efficient way of recoding this information. If you do not have access to a camera then line diagrams are an entirely acceptable alternative.
- Practical work
2.1 Magnetic shielding
You have been given three square plates of material: steel, aluminium and PVC (plastic). You have also been given a pair of small, but powerful, neodymium magnets. One of the magnets is attached to a threaded stud.
Determine which of the three materials is magnetic and describe how you made this determination. Determine whether a non-magnetic conductor can be used to ‘block’ or ‘shield’ a magnetic field and describe how you made this determination.
2.2 Falling magnets
You have been given a copper pipe. Determine if copper is magnetic and describe how you made this determination.
Take the copper pipe and hold it vertically. Drop the non-studded magnet down each pipe. Record your observations. In particular record the nature of the motion of the magnet and compare this with how it falls outside, and well away from, the pipe. When you have completed a sufficient proportion of the course explain why, and how, the motion of the falling magnet is modified by the pipe.
2.3 Homopolar motor
Using only an AA battery, a piece of conducting wire and the studded magnet construct a motor. [Hint: use the wire to make a simple rotor that can balance and rotate freely on one end of the battery and use the magnet to provide a suitable magnetic field. Record (using the camera on your mobile phone or a sketch) the structure of your motor. In your report explain how the motor works including the direction of rotor rotation. (Note: You may need to wait until you have covered the theory for this in the lectures before you can explain the motor principles fully. To explain the direction of rotation you will need to establish the polarity of the magnet. You can do this by hanging the magnet by a thread and observing its orientation in the Earth’s magnetic field.)
Acknowledgement: the simulation software used in the following work was developed for educational use by the University of Colorado under the PHET interactive simulations project.
3.1 Compass needles
Start the simulation magnets_and_electromagnets_en (located in a folder called NIE2299 on the root of the d: (data) drive.)
Click (if necessary) on the ‘Bar Magnet’ tab. The ‘Show Field’ and ‘Show Compass’ boxes should be ticked and all other boxes should be un-ticked.
Drag the bar magnet around by clicking, holding and dragging. Note how the field (as represented by the field of compass needles) changes. Drag the large compass around and note the changing direction of the compass needle (such that it agrees with the local field as represented by the small compass needles). The compass needles are themselves small bar magnets free to rotate in the field of the large bar magnet. Is the red end of the compass needle a north (-seeking) or a south (-seeking) pole. Explain your answer.
3.2 Flux density decay law
Tick the ‘Show Field Meter’ box. The field meter measures magnetic flux density. The four readings on the metre are the magnitude of the flux density (in gauss), the x-directed component of the flux density, the y-directed component of the flux density and the angle (in degrees) between the flux density vector and the x-direction. Gauss (G) is the cgs (centimetre, gram, second) unit of flux density which has been superseded by the mks (metre, kilogram, second) unit i.e., the tesla (T). 1 G = 10-4 T. The gauss is still widely used in the USA, however (and occasionally elsewhere).
Use the field meter to estimate (as accurately as you are able) the index n in the magnetic flux density decay law (of the form B µ 1/rn) with distance by measuring the flux, B1 and B2) at two distances, r1 and r2, from one end of the magnet on the line of symmetry along the length of the magnet. Comment on whether this is consistent with an inverse square law and whether you would expect it to be. [Hint: take the log of the ratio of the two measurements, use the logarithm rule log(ax) = x log(a), and rearrange the formula to give n explicitly.]
3.3 Earth’s magnetic field
Un-tick the field meter box and tick the ‘Show Planet Earth’ box. The north poles on magnets are more strictly called north-seeking poles. (North is a word of geographical, not electromagnetic, origin.) Is the magnetic pole of the Earth located in the arctic a north-seeking pole or a south-seeking pole? Explain your answer.
3.4 Coil current and coil resistance
Select the ‘Electromagnet’ Tab. Set the ‘Current Source’ to DC. Adjust the number of turns in the coil of the electromagnet to one. Use the field meter to find the magnetic flux density inside the coil. Assuming that the single-turn coil has a radius on 1 cm, calculate the current flowing through the coil and thus the resistance of the coil.
Start the simulation faraday_en (located in a folder called NIE2299 on the root of the d: (data) drive.)
Select (if necessary) the ‘Pickup Coil’ tab.
Drag the bar magnet into the coil. Observe the lamp (which lights when there is a flow of current) as you drag the magnet and as the magnet lies stationary inside the coil. Drag the magnet through the coil at different speeds. Explain (qualitatively) your observation.
Replace the lamp with the meter by clicking on the meter icon in the Pickup Coil box on the lower left of the simulation interface). Move the magnet into and out of the coil. What do you notice about the polarity of the current (or voltage) indicated in the meter?
Click on the ‘Transformer’ tab.
Set the ‘Current Source’ (if necessary) to DC.
Drag the electromagnet (the coil attached to the current source) inside the pickup coil. Observe the lamp (which indicates a flow of current) as you drag the electromagnet inside the coil and as the electromagnet lies stationary inside the pickup coil. Explain (qualitatively) your observation. How many ways can you find of causing the lamp to light? Describe each of these ways and explain (qualitatively) its effect.
3.6 Effect of frequency on induction
Set the ‘Current Source’ to AC. Using the meter as the indicator assess (qualitatively) the intensity of the induced voltage in the pickup coil and configure the coils to maximise this voltage. Change the frequency of the source from 50% to 100% and find the factor by which the peak voltage in the secondary winding changes. You may find that pausing the simulation (using the pause button at the bottom of the simulation window) and then advancing the simulation in discrete steps (using the advance button to the left of the pause button) makes estimating the peak deflection of the meter pointer easier. Explain your observation quantitatively.
3.7 Phase change in a transformer
Maximise the number of coils in both electromagnet and pickup coil. Change the AC current supply frequency to a low enough value such that you can identify the times in the cycle where voltage across the pickup coil load resistor (and therefore the pickup coil current) is zero. Do these times coincide with those when the AC current supply is zero? Explain your answer.