MAGNETISM UM Physics Demo Lab 07/2013

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Pre-Lab Questions

How are magnetic poles similar to electric charges?

How are magnetic poles fundamentally different than electric charges?

EXPLORATION

Materials

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1 battery board

1 alligator lead card

1 bolt

1 spool of copper wire

2 ceramic Magnets

1 compass

1 fur strip

1 Petri dish of paper-clips

1 small piece of steel (for scraping off wire insulating enamel)

1 wooden block (pedestal for compass and magnets and wire scraping backstop)

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Magnetic Forces and Poles

The two ceramic magnets are marked with anorange square on one side. On top of the wooden block, place the two magnets a few inches apart, standing on edge with the orange squares facing each other. Move the magnets toward each other and observe how they affect each other. Record your observations below.

Now repeat experiment (1) with the “orange square” side of one magnet facing the unmarked side of the other. Again, record your observations.

Let’s define magnetic “poles” as an analogy to electric charges. How does the behavior of the magnets compare with that of charged pith balls? Based on your observations, how many types of magnetic poles are there?

A compass needle is a permanent magnet, like the ceramic magnets we have been observing. Like the ceramic magnet, a compass needle has a “north” and a “south” magnetic pole. A north magnetic pole is one that seeks the geographic North Pole of the Earth. By convention, the north pole of a compass needle is painted red and the south pole is painted white.

Place one of the ceramic magnets on the wooden block with the orange square facing upward. Bring the compass near and observe the behavior of the needle. Based on your observations, is the “orange square” side of the ceramic magnet a magnetic north or south pole? Explain.

Repeat experiment (4) with the unmarked side of a ceramic magnet facing upward. Is the unmarked side of the ceramic magnet a magnetic north or a magnetic south pole? Explain.

Stand the wooden block on end and place the compass on the blockaway from any metal objects or magnets and observe how it is oriented. If you cannot escape metal objects (for example, the table tops we work on are made of steel), hold the compass in your hand. What is the orientation of the Earth’s magnetic field based on what you observe with a compass (where does it point in the room)? Which type of magnetic pole points toward geographic north? Based on these observations, what is the magnetic polarity of the Earth’s geographic North Pole? Discuss with your group and explain your conclusions.

Based on your conclusions from (6), fill in the magnetic polarity of the Earth on the diagram below.

Figure 1: Earth’s Geographic and Magnetic Poles

Magnetic Forces and Magnetic Materials

Bring the ceramic magnet to the paper clips in the Petri dish, and then remove the magnet. Observe and explain what happens. Are the paper clips and magnet the same thing? Replace the paper clips to the dish.

Take the bolt and bring it near the paper clips. Compare the effect of the bolt on the paper clips to that of the ceramic magnet. Discuss with your group and explain what you’ve observed. Are the bolt and the ceramic magnet the same thing?

Bring the head end of the bolt near the compass. Compared to the ceramic magnet, what effect does the bolt have on the behavior of the compass needle? Discuss with your group and explain what you’ve observed.

Take the strip of fur and rub the bolt. Bring the bolt near the paper clips and observe what happens. How does this compare to what you observed for static electric charge? Discuss with you group and record your conclusions.

Electromagnetism

Take the spool of wire and useall the wire to wind a uniform coil around the bolt.Leave at least 10 cm (≈ 4 inches) of extra wire at the beginning and finish of your coil for making connections. Consult the prototype at the front of the classroom for clarification.

Place the wire on top of the wood block and scrape 2 cm (≈ 1 inch) of insulating enamel off the end of each of the leads coming from your coil. The copper wire is coated with insulating enamel which must be removed to reveal expose the copper metal before a good electrical connection can be made.

Now that you have finished winding the coil and have scraped 2 cm of insulation off the ends of the 10 cm wires, build a circuit with a 3V battery and a switch in series with the bolt-coil assembly.

Close the switch and bring the compass slowly near the rounded head of the bolt. Open the switch. How is this different than what you observed with the bolt in step (10)? Why is it different? What has changed? Discuss with you group and record your conclusions.

Given what you’ve observed, is the head end of the boltacting as a north or a south magnetic pole? Explain your answer.

Close the switch again and bring the compass near the tail of the bolt. Open the switch and observe the effect on the compass. Discuss your observations with your group. Is this consistent with your pole assignment for the head of the bolt?

What is causing the bolt to act as a magnet?

Challenge Work:

1. Slide a paper clip along the side of the ceramic magnet in one direction only. Then bring the paperclip near the compass needle. What do you observe? How are magnets magnetized?

Everyday Applications

Earth’s magnetosphere
Aurora at northern latitudes
Junk yard electromagnets used to lift cars and to do evil deeds in James Bond films
Home alarm systems use electromagnetic contacts to see if windows or doors are opened.
Magnetic tips are used on power drills and hand tools to hold ferromagnetic screws more easily.

APPLICATION: A CURRENT IN THE PROXIMITY OF A MAGNET

Materials

1 Battery Board

1 magnetic force “swing set”:

Cenco stand
bent wire “swing”
soda straw insulator
2 small binder clips for wire routing
2 small gauge clip leads (red and black)
wooden block
two ceramic magnets

Wire a 3 V battery in series with a switch and connect it to the magnetic force “swing set” apparatus, shown below with the positive battery terminal connected to the red wire on the swing set.

Figure 2: The magnetic force “swing set”.

Place the ceramic magnets, stuck together to make a single magnet, with their north pole (“orange square”) ends upward on the wooden block under the wire “swing” as close as possible to the wire, while still allowing the wire to swing freely above the magnet. Recalling that the conventional electric current flows from the positive terminal of the battery to the negative terminal;therefore orient the wire swing and magnet so that looking down on the magnet the current flows toward the top of the page as shown in the diagram below.

Figure 3: Orientation of the current and the magnetic field

Close the switch. In which direction does the wire move (left/right)?

Direction of wire motion:______

Based on your observations for (1), assemble your vector model to show the orientation of the current (IL), the magnetic field (B) and the magnetic force on the wire (F). All of the vectors should be perpendicular to each other. Note that the magnetic field points upward out of the plane of the paperin Figure 3. Show your vector model to your instructor before proceeding to (3).

Now turn the magnet over so the orange square is down against the wooden block. This has the effect of pointing the magnetic field into the page in Figure 3. Sketch the orientation of the magnetic field and the current below. Using your vector model predict the direction the wire will move when the switch is closed (left/right). Close the switch and observe and record the direction the wire moves(left/right).

Predicted direction of wire motion:______

Observed direction of wire motion:______

Now reverse the leads of the battery so that the current now flows toward the bottom of the page in Figure 3. Again, sketch the orientation of the magnetic field and the current below. Again, using your vector model predict the direction the wire will move when the switch is closed (left/right) and then observe and record the motion of the wire when the switch is closed (left/right).

Predicted direction of wire motion:______

Observed direction of wire motion:______

Finally, again flip the magnet so the orange square is upward as shown in Figure 3. Remember that the current is now flowing toward the bottom of the page. Sketch the orientation of the magnetic field and the current below. Using your vector model predict the direction the wire will move when the switch is closed (left/right) and then observe and record the motion of the wire when the switch is closed (left/right).

Predicted direction of wire motion:______

Observed direction of wire motion:______

Based on your observations for (3)-(5), does your vector model correctly predict the orientation of the magnetic field, current and magnetic force?

Challenge Work:

Supposing you didn’t have your vector model available and you wanted to predict the direction of the magnetic force given the direction of the magnetic field and the current in a wire. Can you suggest a method for doing this without a vector model? That is, can you make an equivalent vector model with something you always have “handy”? Explain your method!

Summary

Magnets have two poles north and south, that cannot be separated.
Like magnetic poles repel, opposite magnetic poles attract.
The north geographic pole of the Earth is a magnetic south pole.
The magnetic field of the Earth points down as well as north at the latitude of Ann Arbor (dip needle demonstration).
Only moving charges (currents) produce a magnetic field and only moving charges experience magnetic forces.
Magnetic field lines indicate the direction and magnitude of the force on one magnet due to the presence of another magnet.
The direction of the force is tangent to the field line, attractive for opposite poles, repulsive for like poles.
The magnitude of the magnetic force is proportional to the local density of field lines.
The direction of movement of a current-carrying wire due to the magnetic force depends on the direction of current and the orientation of the magnetic field.
The magnitude of the magnetic force on a current-carrying wire decreasesas the distance between the magnet and the wire increases, proportional to 1/(distance)3.

Final Clean-up

Please disconnect all alligator leads and reattach them to the clip card. Save the bolt wound with wire, you will use it next time. Label your electromagnet.

Bibliography and recommendations for further reading:

Kurtus,Ron, "Magnetism," School for Champions, (accessed June 12, 2006).

MAGNETISM

Every magnet has two poles: a north and a south pole. Electric charges can be isolated individually, as with a single proton or a single electron. Unlike electric charges, the two opposite poles of magnets are always found in pairs. Even when you induce magnetism, as with an electromagnet, the resulting magnet has two poles.

In electrostatics we observed that charge could be built up from friction, rubbing materials with compatible electronegativities produced an excess of positive or negative charge. This is not the case with magnetism. Magnetic fields behave differently than electric fields.

Magnetic fields are represented graphically byfield lines. Widely spaced lines (low density) indicate a weak field; closely spaced lines (high density) indicate strong fields. If a small magnet is in a magnetic field, it will try to orient itself parallel to the local field line. The presence of a magnetic field can induce magnetism in some materials such as iron, causing small iron objects to behave like bar magnets. These two effects are readily demonstrated by observing the behavior of iron filings in the presence of a bar magnet.

Magnetic fields are caused by electric currents. The simplest current configuration which can be isolated in a small region of space is a closed loop of current. A loop of current produces the characteristic field pattern of a bar magnet withnorth and south poles, as shown in Figure 1. The direction of the north pole can be determined by wrapping the fingers of the right hand in the direction of the current in the coil as shown in Figure 2.This field shape is called a “dipole” field, because it inherently gives rise to two inseparable poles. This is the reason why magnetism and static electricity behave in fundamentally different ways. Static (nonmoving) electric charge can be isolated onto a single charged object. A compact magnet confined to a region of space can only be produced by a current (the flow of electric charge) in a closed loop. This is why it is possible to isolate individual positiveand negative charges (“electric monopoles”) but never a single north or south magnetic pole (a “magnetic monopole”).

Figure 1: The equivalence between the magnetic field of a current loop and a bar magnet.

Due to the motion of their electrons, atoms inside a ferromagnetic material can be viewed as many small current loops. If the ferromagnetic material (soft iron for example) is exposed to a strong external magnetic field, these atomic “Amperian current loops” can be brought into alignment and the bulk material becomes magnetized. A large magnet is therefore really just the sum of many small magnets. These microscopic internal magnets, called magnetic domains, are regions within the material where the small current loops within the atoms of the material align in the same direction. When these small internal magnets are aligned along a preferred direction (usually due to exposure to a strong external magnetic field) the object is magnetized and has become a permanent magnet.

As we have observed, current passing through a wire creates a magnetic field. When a current-carrying wire is looped many times around a ferromagnetic core (a material susceptible to being magnetized), the device is an electromagnet. Even without the presence of the ferromagnetic core, the coil will produce a bar-magnet field. Such a coil, consisting of many turns of wire wrapped around a cylinder is called a solenoid.

Figure 2: Determining North for a Current Loop Using the Right Hand

Figure 3 is an artist’s conception of Earth’s magnetic fieldextending out into space. The Earth is a giant magnet whose magnetic field extends over the planet. This is why it is used for navigation with compasses (small magnets); it provides a fixed direction useful for geographic orientation at any point on Earth. As you have seen, magnetic fields exert forces on electric currents which are comprised of moving charges. The Earth’s magnetic field extends well out into space and deflects charged particles from the sun, called solar wind, by exerting magnetic forces on the fast moving solar wind particles (mostly positively-charged protons) in exactly the same way that your table top magnet exerts forces on the charges moving in a wire. Notice that the presence of the solar wind (really a complicated distribution of electric currents) changes the Earth’s magnetic field, distorting the shape of the field lines away from the shape of an ideal bar magnet.

Figure 3: Earth’s Magnetosphere, (NASA/CXC/SAO, 2006)

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