Chapter 10: Magnetism

Measuring Magnet Force - The Inverse Square Law The intensity, force, or strength of most types of energy (such as light, sound, gravity, electricity, and magnetism) diminishes as we move away from the source. We have all seen the pattern created when a flashlight's beam spreads out as it is moved further from a wall in a darkened room.

. The changing intensity of this pattern is described by the Inverse Square Law. The Inverse Square Law also describes the force of attraction or repulsion that a magnet exerts on any given material under very special conditions. This law would describe a magnetic field by stating that the strength (or the intensity) of a magnetic field varies inversely with the square of the distance from the magnet. This means that if the distance (d) increases, the force strength (S) decreases by the square of the distance from the source (i.e. S is proportional to 1/d2 ). The following diagram shows this relationship. If we are interested in determining the strength of a field at a point given the strength at another point we can use the following relationship based on the Inverse Square Law

If a magnet has a strength of 32 Newtons (force strength) at 1 meter(distance to A)what is the force at 4 meters? (Distance to B) Magnetic Force Strength: Force Strength (at point B) = force strength A x (distance to A)^2(SQUARED) / (distance to B)^2 (SQUARED)

32*1^2/4^2=32*1/16 32*0.0625= 2 newtons

Which of the following is/are needed for an electric motor to work?

A. armature B. commutator C. magnet D. conductor E. all of the above

Which of the following explains why the force of a magnet decreases with distance?

A. ferromagnetism decreases with distance B. dipoles decrease with distance C. lower frequency at greater distance D. inverse square law • as follows below E. flow of electrons decreases with distance . The changing intensity of this pattern is described by the Inverse Square Law. The Inverse Square Law also describes the force of attraction or repulsion that a magnet exerts on any given material under very special conditions. This law would describe a magnetic field by stating that the strength (or the intensity) of a magnetic field varies inversely with the square of the distance from the magnet. This means that if the distance (d) increases, the force strength (S) decreases by the square of the distance from the source (i.e. S is proportional to 1/d2 ). The following diagram shows this relationship. If we are interested in determining the strength of a field at a point given the strength at another point we can use the following relationship based on the Inverse Square Law

Which of the following occurs in an electric motor works?

A. reverse square law B. switching the flow of electric current in a magnetic field • C. flow of electrons in ferromagnetic conductors D. dipole insulators in magnetic poles with free electron movement E. all of the above

Motors A generator converts mechanical motion into electric current. A motor converts electric current into mechanical motion. However, the three parts of a motor are the same as those in a generator: (1) a stationary magnet; (2) a coil, called an armature that is free to rotate between the poles of the magnet; and, (3) a device called a commutator, which changes the direction of the current in the armature.

First, a current is passed through the armature, making it an electromagnet. The armature turns until its poles are next to the opposite poles of the magnet. The armature would stop turning (due to the magnetic attraction between the opposing poles) if the direction of the current in the armature were not reversed. The timing is such that the current reverses itself just as the N pole of the armature is next to the S pole of the magnet. The N pole of the armature now becomes an S pole (due to the change in direction of the current), and is repelled by the S pole of the magnet. Due to the repulsion, the armature makes another half-turn until its two poles are again next to the opposite poles on the magnet. The process then repeats itself until the current is turned off.

The Magnetic Nature of Matter

More than 2000 years ago, an iron ore called magnetite was discovered to be able to attract small bits of iron. The term magnetism came to be applied to the force of attraction or repulsion between certain substances.

The strength of a magnet is dependent upon the number of magnetic domains that are aligned. When the magnetic domains (represented by arrows in Figure 3) are randomly arranged the material does not act as a magnet. However, when the magnetic domains in the material are lined up in the same general direction, the material does act like a magnet. The greater the alignment of the domains, the stronger the magnet. Models used to represent varying degrees of magnetic strength would look like those shown in Figure 4. The orderly arrangement of the tiny rectangles in the bottom picture of Figure 4 represents the arrangement of the domains necessary to produce a substance with north and south magnetic poles. Placing a magnetic material in the presence of a magnetic field (produced by another magnet) causes the magnetic domains in the material to align themselves like those at the bottom of Figure 4. The material, at least temporarily, becomes a magnet. We can use iron filings to represent the magnetic domains. At first, these iron filings, like the magnetic domains they represent, are pointed in random directions. When placed in the presence of a magnetic field, however, the iron filings line up in the same direction.

One can test our model by applying it to a piece of un-magnetized coat hanger wire (which is made up of ferromagnetic materials). If you obtained a length of coat hanger wire about 18 cm (7-in.) long and moved a compass along the wire you would note that the compass does not appear affected by the wire. You could then magnetize the wire by stroking the wire in one direction with one pole of a bar magnet approximately twenty-five times. If you then moved a compass along the wire you would observe that the direction the compass needle points as you move the compass along the length of the coat hanger is in the direction of the magnetic field. Ordinarily the magnetic domains in the wire point in different directions, canceling each other out, and the wire is not a magnet. However, if a strong external magnetic field (from a strong magnet for instance) is placed near the wire, the magnetic domains are influenced by this field and align themselves in the same general direction as the external field itself (i.e. the magnetic field of the wire points in the same direction as the external field). Thus, we induced a weak magnetic field in the wire that wasn't there originally. This induced magnetic field is such that the wire is attracted to the magnet. However, once the influence of the strong magnet is removed the domains in the wire revert to a random orientation. Stroking the wire with the pole of the magnet caused the domains to align themselves also, but this time the domains remained aligned (for a while at least) even after the magnet was removed. Thus, by stroking the magnet, we made the wire itself into a temporary magnet. The stroking action strengthened the magnetic field in the wire by forcing more of the domains to align. This, in turn, helped freeze them in place. The more domains that are aligned, the longer the material will retain its magnetic field. Materials with many domains (ferromagnetic materials) will quickly respond to an external magnetic field and be attracted to a magnet. Ferromagnetic materials can be easily magnetized also, as we have just done. Paramagnetic materials (like aluminum and tin) and diamagnetic materials (like copper) behave a little differently. An external magnetic field will induce a magnetic field in both the paramagnetic material and the diamagnetic material; however, the induced magnetic fields are much, much weaker than the induced fields associated with ferromagnetic materials. In the paramagnetic material, the induced field is in the same direction as the external field, but in the diamagnetic material the induced field is in the opposite direction (i.e. the diamagnetic material is actually repelled by the magnet a little but is not usually observable to the naked eye). Also, neither paramagnetic materials nor diamagnetic materials will retain the induced magnetic fields once the external field has been removed. Thus, these materials cannot be used to make permanent magnets.

Generator The voltage that a generator produces can be increased in three different ways. The first way is to increase the strength of the magnet. This, of course, increases the strength of the magnetic field, and a stronger magnetic field will generate a greater current in the wire. The second way is to increase the speed at which the coil rotates. If the coil rotates faster, the magnetic field changes faster. A faster changing magnetic field will also produce more current. The final way is to increase the number of loops in the coil of wire which, as we saw earlier, also increases the amount of current in the wire.

One complete revolution of the coil is called a cycle. The number of cycles (or revolutions) that occur each second is called the frequency. The frequency is just a measure of how many times the voltage or current changes direction each second. Frequency is measured in units called Hertz. One hertz equals one cycle (or revolution) per second. The electric current in most American homes has a frequency of 60 hertz.

Applying Magnetic Principles Determining Direction and the Earth's Magnetic Field

The Earth spins on an imaginary axis that connects the North Geographic Pole and the South Geographic Pole. Near each of these poles, the Earth also has a magnetic pole. However, the situation is a little more confusing than one might expect because the magnetic properties of these poles differ from their geographic location. The north magnetic pole is actually located in the southern hemisphere of the planet while the south magnetic pole is located in the northern hemisphere. These magnetic poles act just like the ends of a magnet. The south magnetic pole is located near Bathurst Island in northern Canada, about 1,600 kilometers from the North Geographic Pole. The north magnetic pole is located in Wilkes Land, Antarctica, about 2,570 kilometers from the South Geographic Pole. The north magnetic pole is moving 55 km per day in a North Northwest direction (away from Antarctica towards Siberia). Because the north magnetic pole on a compass actually points northwards toward the south magnetic pole, it doesn't point directly toward the geographic North Pole. The difference between the direction of the magnetic pole and the geographic pole is called the magnetic declination. The declination is different at different places on the Earth, and actually changes slightly from year to year. In order to use a magnetic compass to its fullest potential, a person must also have a declination chart. This is a chart that shows exactly what correction must be made in order to read the compass accurately. For our area, true north is approximately 6o declination west of the direction given by a compass.

Which of the following statements is correct about the Earth's magnetic field?

The north pole, of a magnet/compass, points to the south magnetic pole of the Earth.

The Law of Magnetism

This attraction and repulsion is known as the Law of Magnetism; like poles of magnets repel and unlike poles attract. When one investigates various materials to determine which are magnetic materials and which are not you may notice that all of the magnetic materials are "attracted" to the magnets and none were repelled because the magnetism in the magnetic material was induced. However, all permanent magnets have a North and a South pole. When two poles of two magnets are in close contact with each other they will either attract each other or repel each other.

The Structure of Magnets To understand magnetic materials, one first has to consider the structure of the atom. The Bohr model of the atom (an older model of the atom that works for our purposes and has been replaced with a newer one as explained in this video) includes the nucleus (dense core) containing the positively charged protons and the electrically neutral neutrons. Surrounding the nucleus and orbiting around it like the planets orbiting the sun, are the negatively charged electrons. As these electrons orbit the nucleus they rotate, in much the same way the Earth spins on its axis, thus, the electrons are said to have spin (sometimes referred to as atomic spin, or magnetic spin). These motions of the electrons produce a magnetic field (in fact, it is one of the great scientific discoveries of all time that a moving electric charge produces a magnetic field).

Thus, each moving electron generates its own magnetic field. In atoms with two or more electrons, one electron spins up and the other spins down and are paired with each other (except for the occasional lone electron that has no one to pair up with). These paired electrons occupy the same energy level, or orbit (in fact, this is why they are said to be paired). The electrons in each pair usually have opposite spins, and their magnetic fields cancel each other out. However, in atoms of magnetic elements (such as iron, nickel, and cobalt), there are unpaired electrons and their fields do not cancel each other but instead reinforce each other (the spins are in the same direction) and cause the material to be magnetic These materials are the ferromagnetic materials we spoke of earlier. (The Latin word for iron is fermium, from which we get ferromagnetic). In these materials there is also a strong interaction, or coupling, between neighboring atoms. This strong interaction results in large groups of atoms with their electron spins pointing in the same direction. These large groups of atoms are called magnetic domains. In the presence of a magnetic field the domains will align and the material can be permanently magnetized

Which of the following best indicates the Law of Magnetism?

Unlike poles attract and like poles repel

Magnetic Fields

We can determine the magnetic field surrounding a magnet by using another magnet, such as a directional compass, to physically plot the field. The simplest form of the compass is a magnetic needle mounted on a pivot in such a way that the needle can move freely. The compass needle, when placed in an external magnetic field, will align itself in the direction of the external magnetic field. As we move the compass around a magnet, the needle will stay in line with the direction of the magnetic field. We can trace the path of the field lines pointing by observing the direction shown by the compass. The field lines exit from the north pole of the magnet and curve to the south pole of the magnet. (These field lines are actually closed loops, with part of the loop inside the magnet and another part forming the magnetic field outside the magnet.) These field lines never cross. Field lines are always densest near the poles of a magnet. Can you reason why?

Permanent magnets have a surrounding magnetic field that cannot be seen but its effects can be observed. When one permanent magnet is brought into the vicinity of another permanent magnet, the magnetic fields of the two interact with each other. It is this interaction of the magnetic fields that causes the attraction and repulsion that we observe between the magnets.

When any magnetic material is brought into the vicinity of a magnet, the magnet induces a temporary magnetic field in the material causing an attraction of the material to the magnet. This is possible because of the behavior of electrons at the atomic level (which we will look at later). When a nonmagnetic material is brought into the vicinity of a magnet, the same effects do NOT happen (at least not to the same degree) and there is no resulting attraction between the material and the magnet. Our first goal is to determine which materials are magnetic and which are not.

Magnets, Electricity, Electromagnets and Oersted's Discovery In the early 1700's, reports of lightning changing the direction of compass needles and making magnets out of objects such as knives and forks led scientists to suspect a relationship between electricity and magnetism. Danish schoolteacher Hans Christian Oersted discovered the first concrete evidence of this relationship in 1820. His discovery was quite accidental. Oersted laid the current-carrying wire of an electric circuit beside a directional compass. As he did so, he happened to notice the compass needle turning. He immediately recognized that a magnetic field must have been emanating from the wire causing the compass needle to be deflected. He also realized that the magnetic field had to be produced by the current flowing in the wire because, when the current was turned off, the needle ceased to be deflected. Oersted's discovery of the relationship between electricity and magnetism led to a very important principle; when current flows in a wire (or any other conductor), it generates a magnetic field which surrounds the wire.

Wrapping the wire around a piece of soft iron can strengthen the magnetic field created by the electric current. In fact, if we wrap the wire around the soft iron core several times, we can strengthen the magnetic field tremendously. This arrangement is called an electromagnet. The iron core offers an easy path for the field inside the coil, and thus provides a minimum of magnetic resistance. In essence, the core concentrates the field and, by doing so, strengthens it. Electromagnets allow magnetism to be turned on or off at will, and are currently used in many areas of modern society.

The magnetic lines of force near a current carrying wire are

circular in a plane, perpendicular to the wire

If you were to investigate all the known materials, you would find that most materials fall in the following classifications

diamagnetic materials - these are materials that are not attracted to a magnet and are sometimes referred to as nonmagnetic materials. paramagnetic materials - these materials are weakly attracted to a magnet; however, the attraction may be so weak it is not even noticeable. These are commonly referred to as nonmagnetic materials also. ferromagnetic materials - these are materials such as magnetite, those do-dads and souvenirs we prominently display on our refrigerator doors, and any other materials that can be used to produce a "permanent magnet". These are also the kinds of materials that are most strongly attracted to a permanent magnet.

Materials that can be magnetized are called:

ferromagnetic

Moving a copper wire through a magnetic field produces:

flow of electrons

Which of the following indicates the measurement of frequency for voltage or current?

hertz

Magnetic fields are produced by:

motion of an electric charge

A generator produces electrical current by:

moving a coil of copper wire through a magnetic field

Which one of the following sets of materials contain the most nonmagnetic materials?

silver, gold, copper, brass

Which of the following sets of materials contain the most magnetic materials?

steel, iron, nickel, cobalt