PHY101 - Chapter 24
When do cosmic rays penetrate your body?
All the ti
What produces a magnetic field?
Electric charges in motion
Why is iron magnetic and wood is not?
Iron has magnetic domains, wood does not
What produces a magnetic field?
The motion of electric charges produces a magnetic field.
Is it correct to say that an electric motor extends the physics that underlies a galvanometer?
Yes. A motor is a sophisticated galvanometer.
Electromagnet
A magnet whose field is produced by an electric current. It is usually in the form of a wire coil with a piece of iron inside the coil.
Why will a magnet attract an ordinary nail or paper clip but not a wooden pencil?
A magnet will induce the magnetic domains of a nail or paper clip into alignment. Opposite poles in the magnet and the iron object are then closest to each other and attraction results (this is similar to a charged comb attracting bits of electrically neutral paper—see Figure 22.13). A wooden pencil, on the other hand, does not have magnetic domains that will interact with a magnet.
What is a magnetic domain?
A magnetic domain is a cluster of aligned atoms.
Can an electron at rest in a stationary magnetic field be set into motion by the magnetic field? What about the electron placed at rest in an electric field?
An electron must be moving across magnetic field lines in order to feel a magnetic force. So an electron at rest in a stationary magnetic field will feel no force to set it in motion. In an electric field, however, an electron can be accelerated whether or not it is already moving. (A combination of magnetic and electric fields is used in particle accelerators such as cyclotrons. The electric field accelerates the charged particle in its direction, and the magnetic field accelerates it perpendicular to its direction.)
If you place a chunk of iron near the north pole of a magnet, attraction will occur. Why will attraction also occur if you place the same iron near the south pole of the magnet?
Attraction will occur because the magnet induces opposite polarity in a nearby piece of iron. North will induce south, and south will induce north. This is similar to charge induction, where a balloon will stick to a wall whether the balloon is negative or positive.
Magnet A has twice the magnetic field strength of magnet B (at equal distance) and, at a certain distance, it pulls on magnet B with a force of 50 N. With how much force, then, does magnet B pull on magnet A?
Back to Newton's third law! Magnets A and B are pulling equally on each other. If A pulls on B with 50 newtons, then B also pulls on A with 50 newtons. Period!
Which of the following are creatures that are known to harbor tiny magnets within their bodies?
Bacteria, pigeons, bees, wasps, monarch butterflies, and sea turtles
Magnetic field
Clustered regions of aligned magnetic atoms. When these regions themselves are aligned with one another, the substance containing them is a magnet.
In preparation for an MRI scan, why are patients advised to remove eyeglasses, watches, jewelry, and other metal objects?
Currents are induced in metals by the changing magnetic field of the MRI device.
In what direction relative to a magnetic field does a charged particle move in order to experience maximum deflecting force? Minimum deflecting force?
Force is maximum when motion is perpendicular to the field; minimum when parallel to the field.
What relative direction between a magnetic field and a current-carrying wire results in the greatest force?
Force is maximum when the current is perpendicular to the field.
How is the magnetic force on a particle moving in a magnetic field different from gravitational and electric forces.
Gravitational and electric forces act along the line connecting two interacting objects, whereas magnetic forces are perpendicular to the motion of the particle and the magnetic field. Submit
Why are the magnetic fields of superconducting magnets often stronger than those of conventional magnets?
Greater electron flow produces greater magnetic field strength.
By whom, and in what setting, was the relationship between electricity and magnetism discovered?
Hans Christian Oersted in a high-school classroom noted how a current affects a magnet, thus relating electricity and magnetism.
Cosmic rays
High-speed particles that travel throughout the universe.
All atoms have moving electric charges. Why, then, aren't all materials magnetic?
How the charge moves dictates the direction of its magnetic field. (A magnetic field is a vector quantity.) Magnetic fields cancel—more in some materials than in others.
Inside a laboratory room there is said to be either an electric field or a magnetic field, but not both. What experiments might be performed to establish what kind of field is in the room?
If the field interacts with a stationary bar magnet it is magnetic; if with a stationary charge, it is electric. If an electric current is generated in a rotating loop of wire, the field is magnetic. If a force acts on only a moving charge, the field is magnetic. So any of the classes of experiments that deal with electric charge at rest and electric charge in motion might be used to determine the nature of the field in the room.
Two charged particles are projected into a magnetic field that is perpendicular to their velocities. If the particles are deflected in opposite directions, what does this tell you about them?
If the particles enter the field moving in the same direction and are deflected in opposite directions (say, one left and one right), the charges must be of opposite sign.
At the micro level, what is the difference between an unmagnetized iron nail and a magnetized iron nail?
In an unmagnetized nail, the magnetic domains have a random orientation so that the net magnetism adds to zero. In a magnetized nail, many of the magnetic domains are aligned.
Why is the magnetic field strength greater inside a current-carrying loop of wire than about a straight section of wire?
Inside the loop the lines are more concentrated.
Magnetic force (between a magnetic field and a moving charged particle)
It is a deflecting force due to the motion of the particle: The deflecting force is perpendicular to the velocity of the particle and perpendicular to the magnetic field lines. This force is greatest when the charged particle moves perpendicular to the field lines and is smallest (zero) when it moves parallel to the field lines.
Magnetic force (between magnets)
It is the attraction of unlike magnetic poles for each other and the repulsion between like magnetic poles.
When iron-hulled naval ships are built, the location of the shipyard and the orientation of the ship in the shipyard are recorded on a brass plaque permanently attached to the ship. Why?
Just as a nail is magnetized by beating on it, an iron ship is beat upon in its manufacture, making it a permanent magnet. Its initial magnetic field orientation, which is a factor in subsequent magnetic measurements, is in effect recorded on the brass plaque.
How is the rule for the interaction between magnetic poles similar to the rule for the interaction between electrically charged particles?
Like poles repel, unlike poles attract, and the force is proportional to the inverse square of the distance between two poles
What are magnetic pole reversals?
Magnetic pole reversals are reversals of the north and south poles, common throughout Earth's history.
In what way are magnetic poles very different from electric charges?
Magnetic poles cannot be isolated; electric charges can.
What is the source of magnetic force?
Moving electrons are the source of magnetic force.
A strong magnet attracts a paper clip to itself with a certain force. Does the paper clip exert a force on the strong magnet? If not, why not? If so, does it exert as much force on the magnet as the magnet exerts on it? Defend your answers.
Newton's third law again: Yes, the paper clip, as part of the interaction, certainly does exert a force on the magnet—just as much as the magnet pulls on it. The magnet and paper clip pull equally on each other to make up the single interaction between them.
In what direction relative to a magnetic field does a charged particle move in order to experience maximum deflecting force? To experience minimum deflecting force?
Perpendicular for maximum force, parallel for minimum force
What is different about the magnetic poles of common refrigerator magnets and those of common bar magnets?
Refrigerator magnets have narrow strips of alternating north and south poles. These magnets are strong enough to hold sheets of paper against a refrigerator door, but they have a very short range because the north and south poles cancel a short distance from the magnetic surface.
What two kinds of motion do electrons in an atom appear to have?
Revolution and spin
Many dry cereals are fortified with iron, which is added to the cereal in the form of small iron particles. How might these particles be separated from the cereal?
Separation is easy with a magnet (try it and be amazed!).
In a mass spectrometer (see Figure 34.14), ions are directed into a magnetic field, where they curve and strike a detector. If a variety of singly ionized atoms travel at the same speed through the magnetic field, would you expect them all to be deflected by the same amount, or would different ions be bent different amounts? Defend your answer.
Singly charged ions traveling with the same speed through the same magnetic field experience the same magnetic force. The extent of their deflections then depends on their accelerations, which in turn depend on their respective masses. The least massive ions are deflected the most, and the most massive ions are deflected the least. (See Figure 34.14 in Chapter 34 for a diagram of a mass spectrograph.)
Name at least six creatures that are known to harbor tiny magnets within their bodies.
Six creatures are bacteria, pigeons, bees, butterflies, sea turtles, and fish.
What is the cause of the aurora borealis (the northern lights)?
The Earth's magnetic field guides trapped charged particles to follow field lines toward the poles, where they collide with the atmosphere, causing a glow.
Why do astronauts keep to altitudes beneath the Van Allen radiation belts when walking in space?
The Van Allen radiation belts are filled with swarms of high-energy charged particles that can damage living tissue. Astronauts, therefore, make an effort to keep below these belts.
Why are there probably no permanently aligned magnetic domains in Earth's core?
The core is too hot to allow atoms to maintain a constant magnetic orientation.
A proton moves in a circular path perpendicular to a constant magnetic field. If the field strength of the magnet is increased, does the diameter of the circular path increase, decrease, or remain the same?
The diameter of the circular path decreases.
Will either pole of a magnet attract a paper clip? Explain what is happening inside the attracted paper clip. (Hint: Consider Figure 22.13.)
The domains in the paper clip are induced into alignment in a manner similar to the electrical charge polarization in an insulator when a charged object is brought nearby. Either pole of a magnet will induce alignment of domains in the paper clip: Attraction results because the pole of the aligned domains closest to the magnet's pole is always the opposite pole.
Why does a piece of iron in a current-carrying loop increase the magnetic field strength?
The electromagnetic field of the current-carrying loop is amplified because it aligns the domains in the iron.
In Chapter 22, we learned that the direction of the electric field about a point charge is radial to the charge. What is the direction of the magnetic field surrounding a current-carrying wire?
The magnetic field is directed along concentric circles surrounding the wire.
Why is the magnetic field strength greater inside a current-carrying loop of wire than about a straight section of wire?
The magnetic field of each segment of wire in the loop, due to electrons moving in the wire, adds together inside the loop, thereby making the field become bunched-up.
What happens to the direction of the magnetic field about an electric current when the direction of the current is reversed?
The magnetic field reverses direction at every point. A clockwise pattern of concentric circles becomes a counterclockwise pattern of concentric circles and vice versa.
In Chapter 22, we learned that the direction of the electric field about a point charge is radial to the charge. What is the direction of the magnetic field surrounding a current-carrying wire?
The magnetic field takes the form of concentric circles about a current-carrying wire.
Why is iron magnetic and wood not?
The magnetic fields of individual iron atoms are strong enough to align the magnetic fields of neighbor atoms. The atoms in wood have much weaker magnetic fields.
By what mechanism do the iron filings shown about the magnet in Figure 24.2 align with the magnetic field?
The mechanism of alignment involves two factors: First, each filing is turned into a tiny magnet by the magnetic field of the bar magnet, which induces domain alignment in the filing. Second, a pair of equal and opposite torques act on each filing whenever it is not parallel to the magnetic field lines. These torques rotate the filings into alignment with the field lines like little compass needles.
Why isn't it advisable to make a horseshoe magnet from a flexible material?
The poles of the magnet attract each other and cause the magnet to bend, even enough for the poles to touch if the material is flexible enough.
A loudspeaker consists of a cone attached to a current-carrying coil located in a magnetic field. What is the relationship between vibrations in the current and vibrations of the cone?
The vibrations in both the coil and the speaker cone have identical frequencies at any instant.
What happens when Dr. Hewitt places a current- carrying wire between the poles of the magnet for the first time?
The wire jumps down.
What happens when Dr. Hewitt turns the magnet around and places the current-carrying wire between the poles of the magnet for the second time?
The wire jumps up.
What relative direction between a magnetic field and a current-carrying wire results in the greatest force?
The wire should be perpendicular to the field.
The force between electrically charged particles depends on the magnitude of each charge, their separation distance, and what else?
Their motion
If you had two bars of iron—one magnetized and one unmagnetized—and no other materials at hand, how could you determine which bar was the magnet?
To determine only by their interactions with each other which of two bars is a magnet, place the end of tar + at the midpoint of bar 2 (like making a "T"). If there is an attraction, then bar + is the magnet. If there isn't, then bar 2 is the magnet.
What is a galvanometer called when it has been calibrated to read current? When it has been calibrated to read voltage?
When calibrated for current, a galvanometer is an ammeter; when calibrated for voltage, a voltmeter.
Will a pair of parallel current-carrying wires exert forces on each other?
Yes. Each will experience a force because each is in the magnetic field generated by the other. Interestingly, currents in the same direction attract, and currents in opposite directions repel.
We know that a compass points northward because Earth is a giant magnet. Will the northward-pointing needle point northward when the compass is brought to the Southern Hemisphere?
Yes. The compass aligns with Earth's magnetic field, which extends from the magnetic pole in the Southern Hemisphere to the magnetic pole in the Northern Hemisphere.
Magnetic domains are composed of clustered _________
aligned iron atoms Submit
The magnetic field about a straight length of current-carrying wire is _________.
circular in shape
What kinds of appliances work on the principle of deflection of a current-carrying wire in a magnetic field?
electric motor
Compared with a simple galvanometer, an electric motor is _________
much the same with small modifications
The figure shows the path of a charged particle moving in a magnetic field directed into the screen.
negative F = qv * B
he magnetic field about Earth takes a shape _________.
similar to that of an interior bar magnet
When an iron rod is placed inside a current-carrying coil of wire ________
the coil becomes a stronger electromagnet
