Lab 04: Magnetic Forces and Fields - University of Central ... · Lab 04: Magnetic Forces and...
2
Lab 04: Magnetic Forces and Fields INTRODUCTION Magnets are fun! Who doesn’t love playing around with magnets? As a result, we all learned as children several fundamental properties of magnets and magnetic fields. For example, we all know that a magnet will not stick to just anything. But what will a magnet stick to? Metal? Any metal, or just specific types? We also know that a magnet doesn’t have to actually touch the metal object for the force to exist (which is why we can slip the grocery list between the magnet and the fridge). Does this action at a distance force behave like gravity and the electrostatic forces we have studied? As children, however, we may never have connected the concepts of electricity and magnetism. They are fundamentally related, and we will begin to look at how we know a relationship exists, and what the nature of that relationship is. Ferrofluid! Magnetic fields have never been more beautiful! OBJECTIVES ๏ Observe the magnetic force between two magnets, or between a magnet and another non-magnetic object ๏ Map the magnetic field of a bar magnet ๏ Measure the field strength of a neodymium magnet magnet as a function of distance and determine the relationship ๏ Create a magnetic field using electric current ๏ Use a magnet to exert a force on a current-carrying wire ๏ Induce electric current in a wire coil using a permanent magnet ๏ Demonstrate the principles of an electric generator and dc motor PROCEDURE This lab consists of a series of short demonstration-type activities that are primarily conceptual in nature. You should follow the specific directions and answer the questions in your lab notebook, using diagrams or sketches wherever appropriate. One exercise, Magnetic Field Strength, will require you to record some measurements and construct a graph of your results. MAGNETIC POLES Use the selection of magnets provided to demonstrate to yourself the existence of magnetic poles, and the nature of the force (attractive or repulsive) between poles. Under no circumstances should you place the magnets on or around the computer. Please be very careful of this. Record and/or sketch your observations in your lab notebook. MAGNETIC FIELD MAPPING Use a compass to map the field of a bar magnet. This should be done with no other magnets in the vicinity, especially strong magnets. On a large sheet of blank paper, trace the outline of the magnet. Label the poles, then replace the magnet on the paper. Use the compass to determine the location of field lines, and trace the lines from pole to pole. Check your compass (which end of the needle points Notice the parallels with electric fields..also notice they differences north?) and use it to verify that the poles of your magnet are labeled correctly (it is possible that your magnet is backwards; the bar magnets need to be re-magnetized periodically, and occasionally it happens that a magnet is placed in the polarizer upside down). 1. Compare this magnetic field map to an electric field map. What distribution of charge would give you a field map that looks like this? 2. We know the field map is a two-dimensional representation of the actual 3-dimensional field. Think about the electric field created by a single positive point charge. Could you create a magnetic field having this same geometry? How (or, if not, why not)? OERSTED'S EUREKA Hans Christian Oersted was the first to make the connection between electric current and magnetic field. He noticed that a magnet brought near a current-carrying wire caused a deflection in the wire. ๏ Create a simple circuit in which a wire connects the terminals of the power supply. Adjust the power supply to 2V, then switch off. Position the magnet so the wire runs straight between the poles, as shown in the picture. Switch the power back on and observe what happens to the wire. ๏ Remove the magnet, and adjust the power supply to 3V. Power off, replace the magnet. Turn the power back on and observe. ๏ Power off, turn the magnet upside down and replace. Power on, note the deflection of the wire. ๏ Change the direction of the current by swapping the positive and negative leads at the power source (turn the power off before swapping). Again, note what happens. 3. Sketch the directions of the magnetic field, current, and deflection in relation to each other. 4. What effect does increasing the current in the wire have on its deflection? 5. What happened when the magnet was turned upside down? Why? 6. What effect does reversing the direction of the current have on the deflection of the wire? The first of many examples of right-hand-rule. PHYS 1420: College Physics II Summer 2017 Page 01
Transcript of Lab 04: Magnetic Forces and Fields - University of Central ... · Lab 04: Magnetic Forces and...
Lab 04: Magnet ic Forces and F ie ldsINTRODUCTION
Magnets are fun! Who doesn’t love playing around with magnets? As a result, we all learned as children several fundamental properties of magnets and magnetic fields. For example, we all know that a magnet will not stick to just anything. But what will a magnet stick to? Metal? Any metal, or just specific types? We also know that a magnet doesn’t have to actually touch the metal object for the force to exist (which is why we can slip the grocery list between the magnet and the fridge). Does this action at a distance force behave like gravity and the electrostatic forces we have studied? As children, however, we may never have connected the concepts of electricity and
magnetism. They are fundamentally related, and we will begin to look at how we know a relationship exists, and what the nature of that relationship is.
Ferrofluid! Magnetic fields havenever been more beautiful!
OBJECTIVES
๏ Observe the magnetic force between two magnets, or between a magnet and another non-magnetic object
๏ Map the magnetic field of a bar magnet๏ Measure the field strength of a neodymium magnet magnet
as a function of distance and determine the relationship๏ Create a magnetic field using electric current๏ Use a magnet to exert a force on a current-carrying wire๏ Induce electric current in a wire coil using a permanent
magnet๏ Demonstrate the principles of an electric generator and dc
motor
PROCEDURE
This lab consists of a series of short demonstration-type activities that are primarily conceptual in nature. You should follow the specific directions and answer the questions in your lab notebook, using diagrams or sketches wherever appropriate. One exercise, Magnetic Field Strength, will require you to record some measurements and construct a graph of your results.
MAGNETIC POLES
Use the selection of magnets provided to demonstrate to yourself the existence of magnetic poles, and the nature of the force (attractive or repulsive) between poles. Under no circumstances should you place the magnets on or around the computer. Please be very careful of this. Record and/or sketch your observations in your lab notebook.
MAGNETIC FIELD MAPPING
Use a compass to map the field of a bar magnet. This should be done with no other magnets in the vicinity, especially strong magnets. On a large sheet of blank paper, trace the outline of the magnet. Label the poles, then replace the magnet on the paper. Use the compass to determine the location of field lines, and trace the lines from pole to pole. Check your compass (which end of the needle points
Notice the parallels with electric fields..also notice they differences
north?) and use it to verify that the poles of your magnet are labeled correctly (it is possible that your magnet is backwards; the bar magnets need to be re-magnetized periodically, and occasionally it happens that a magnet is placed in the polarizer upside down).1. Compare this magnetic field map to an electric field
map. What distribution of charge would give you a field map that looks like this?
2. We know the field map is a two-dimensional representation of the actual 3-dimensional field. Think about the electric field created by a single positive point charge. Could you create a magnetic field having this same geometry? How (or, if not, why not)?
OERSTED'S EUREKA
Hans Christian Oersted was the first to make the connection between electric current and magnetic field. He noticed that a magnet brought near a current-carrying wire caused a deflection in the wire.
๏ Create a simple circuit in which a wire connects the terminals of the power supply. Adjust the power supply to 2V, then switch off. Position the magnet so the wire runs straight between the poles, as shown in the picture. Switch the power back on and observe what happens to the wire.
๏ Remove the magnet, and adjust the power supply to 3V. Power off, replace the magnet. Turn the power back on and observe.
๏ Power off, turn the magnet upside down and replace. Power on, note the deflection of the wire.
๏ Change the direction of the current by swapping the positive and negative leads at the power source (turn the power off before swapping). Again, note what happens.
3. Sketch the directions of the magnetic field, current, and deflection in relation to each other.
4. What effect does increasing the current in the wire have on its deflection?
5. What happened when the magnet was turned upside down? Why?
6. What effect does reversing the direction of the current have on the deflection of the wire?
The first of many examples of right-hand-rule.
PHYS 1420: College Physics II Summer 2017
Page 01
MAGNET CREATES AN ELECTRIC CURRENT!Oersted was also the first to notice (or at least, the first to note that he noticed) that a magnet could be used to induce current through a wire.๏ Attach the
galvanometer to a coil of wire, and pass the bar magnet inside of the loop of wire. Record both the number of loops in the coil and the reading on the galvanometer dial (this device can measure either the impressed voltage or induced current; do not worry about the units for this).
๏ Repeat for the remaining coils. Try to use the same magnet speed each time.
๏ Choose the coil for which you got the best results, and attach the galvanometer. Compare the readings when you change the magnet speed from very slow to very fast.
๏ Compare a weaker magnet with a stronger magnet, using the same coil (the best one) and the same magnet speed.
7. For the same magnet speed, how does increasing the number of coils affect the impressed voltage (induced current)?
8. For the same number of coils, how does increasing the magnet speed affect the impressed voltage (induced current)?
9. For the same number of coils and the same speed, what is the effect of replacing the magnet with a stronger one and repeating the experiment?
CURRENT CREATES A
MAGNETIC FIELD!๏ Wind one lead from the
hand crank generator around the magnetic compass several times, and secure with a bit of clear tape.
๏ Clip the end of the lead to the clip on the other lead. Position the compass so that the needle aligns parallel with the windings.
๏ Turn the generator crank clockwise and observe. Reverse the direction and crank counterclockwise.
10. What happens when you crank clockwise? Why?
11. What effect does reversing the direction of the crank have on the compass needle? Why?
Magnets create current?!?Shut the front door!
MAGNETIC FIELD OF A LONG STRAIGHT WIRE
Use the compasses to establish the geometry of the magnetic field generated when current passes through a long straight wire.12. What happens to the
direction of the field if the current is reversed?
13. Think about the moving charge. Imagine a single charge, right now, in the plane of the compasses as shown on the left. The magnetic field lines are concentric circles centered on the wire. For that single charge right there in the plane, what does the electric field it
creates look like? Sketch the 2-dimensional representation of both fields (E and B) in the plane.14. From your sketch, what is the relationship between the
electric and magnetic fields?
MOTORS AND GENERATORS
๏ Connect the leads of the generator to either side of the bulb.
๏ Turn the crank clockwise and note what happens. Turn the crank counter-clockwise and note.
๏ Observe what happens as you crank the handle (either direction) faster or slower.
๏ Clip the leads of one generator to the leads of another. Turn the crank of one generator clockwise and observe the other crank. Adjust the speed with which you turn the crank and observe.
๏ Turn the crank counterclockwise and repeat the above observation.
๏ Reverse the leads and observe.
15. Does changing the direction you crank change the effect on the bulb? Why or why not?
16. What is the effect of cranking slower or faster? Explain.
17. What happens to the second crank when you turn the first? Why?
18. What is the relationship between the speed of first crank and the speed of the second crank?
19. Explain what happens to the second crank when you reverse the direction of the first.
20. What happens if you reversed the leads instead of the direction of the crank?
Currents create magnetic fields?!? Who knew?
Can you use the generatoras an electric motor?
PHYS 1420: College Physics II Summer 2017
Page 02
