Conceptual Physics Chapter 24 - Think and Explain

What kind of force field surrounds a stationary electric charge? What additional field surrounds it when it moves?

Any electric charge establishes an electric field around itself. If it is moving, a magnetic field also appears. Since the charge has mass, there is also a gravitational field.

What changes in cosmic-ray intensity at Earth's surface would you expect during periods in which Earth's magnetic field passed through a zero phase while undergoing pole reversals?

Cosmic ray intensity at the Earth's surface is greater. Incoming cosmic rays move at an angle to the Earth's magnetic field, and are deflected away. When the Earth's magnetic field passes through a zero phase, this protection diminishes, and more cosmic rays reach the surface.

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?

50 N. Newton's third law. Whenever object A exerts a force on object B, we find that object B exerts an equal and opposite force on object A.

Residents of northern Canada are bombarded by more intense cosmic radiation than residents of Mexico. Why is this so?

Charged particles moving in a magnetic field are deflected the most when they move at right angles to the field lines, and deflected the least when they move parallel to the field lines. Cosmic rays (high-energy charged particles) that are heading toward regions near the Earth's poles (such as northern Canada) are moving more parallel to the Earth's magnetic field lines, and are deflected less. For regions closer to the equator (such as Mexico), incoming cosmic rays move more nearly at right angles to the Earth's magnetic field, and are deflected away.

Why do astronauts keep to altitudes beneath the Van Allen radiation belts when walking in space?

Cosmic rays that are deflected by Earth's magnetic fields can be trapped in the Van Allen radiation belts. These high-energy charged particles are unhealthy for astronauts.

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?

First, one could put a bar magnet in the room and see what happens. Does it align with a magnetic field? Second, one could put a stationary electric charge in the room. A magnetic field exerts no force on a stationary charge. However, in an electric field E, a charge q feels a force of magnitude q×E. A stationary charge will be set into motion.

One way to shield a habitat in outer space from cosmic rays is with an absorbing blanket of some kind, which would function much like the atmosphere that protects Earth. Speculate on a second way for shielding the habitat that would also be similar to Earth's natural shielding.

If a magnetic field were established around the habitat, it could shield astronauts from cosmic rays by the same principle by which the magnetic field of the Earth shields the surface.

Will a superconducting magnet use less electric power than a traditional copper-wire electromagnet of the same field strength? Defend your answer.

It will use less power. There is no electrical resistance in the superconducting magnet, so there is no heat loss. It will take less power (in fact, no power) to keep the current moving, making it more efficient than a traditional copper-wire electromagnet.

All atoms have moving electric charges. Why, then, aren't all materials magnetic?

Magnetic field is a vector quantity. Though all atoms contain moving charges, in some atoms the resulting magnetic fields cancel each other, and the material is nonmagnetic.

In what sense are all magnets electromagnets?

Magnetic fields are caused by electric charges in motion. Even for "permanent magnets" where no electric current is obvious, the field is still created because electrons are in motion: orbiting the nucleus, or "spinning" around their own axes. Thus, we may say that all magnets are electromagnets.

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?

No and yes, respectively. A magnetic field exerts no force on a stationary charge. In an electric field E, a charge q feels a force of magnitude q×E. A stationary electron will be set into motion.

One way to make a compass is to stick a magnetized needle into a piece of cork and float it in a glass bowl full of water. The needle will align itself with the horizontal component of Earth's magnetic field. Since the north pole of this compass is attracted northward, will the needle float toward the north side of the bowl? Defend your answer.

No. Over the diameter of the bowl, the Earth' magnetic field may be considered to be constant. Though the north pole of the needle is attracted northward, the entire needle doesn't float northward because the other end is attracted southward. The net force on the needle is zero.

Why isn't it advisable to make a horseshoe magnet from a flexible material?

Opposite magnetic poles attract each other. If the magnet is too flexible, it will bend, possibly allowing the poles to touch, and weakening the nearby external magnetic field.

Why aren't permanent magnets really permanent?

Over time, heat and physical jiggling will cause the domains to randomize and go out of alignment.

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?

Place the end of one bar at the midpoint of the other. The magnetic field of a bar magnet is strongest at the ends Place the end of bar A at the midpoint of bar B (making the letter "T"). If there is a relatively strong attraction, bar A is the magnet, and if not, bar B is the magnet.

What is different about the magnetic poles of common refrigerator magnets and those of common bar magnets?

Refrigerator magnets are made with alternating stripes of north and south poles. They are relatively weak because the differing-direction magnetic fields caused by the stripes cancel out when you are only a short distance away.

The north pole of a compass is attracted to the north magnetic pole of Earth, yet like poles repel. What does this indicate about the polarity of Magnet Earth at its poles.

The Earth must have a magnetic south pole located in the Northern Hemisphere, and a magnetic north pole located in the Southern Hemisphere.

In preparation for an MRI scan, why are patients advised to remove eyeglasses, watches, jewelry, and other metal objects?

The MRI machine uses a large magnet, and those objects might contain iron and be attracted to the machine.

In what position can a current-carrying loop of wire be located in a magnetic field so that it doesn't tend to rotate?

The axis of the current-carrying loop of wire should be lined up or parallel with the magnetic field.

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?

The charges are of opposite sign. Oppositely charged particles feel forces in opposite directions in an electric field. The same is true of identically-directed particles in magnetic fields. If these particles are deflected in opposite directions (say, one to the east, and one to the west), the charges must be of opposite sign.

A straight current-carrying wire is horizontal and oriented south to north. When a compass needle is placed below or above it, in what direction does the compass needle point?

The compass needle points perpendicular to the wire (either east or west).

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 decreases. When the field is strengthened, the proton is pulled into a tighter, smaller-diameter circular path. If the field were weakened, the proton would feel less force, and move in a straighter, larger-diameter path.

A beam of electrons passes through a magnetic field without being deflected. What can you conclude about the orientation of the beam relative to the magnetic field?

The electron beam is traveling parallel (or anti-parallel) to the magnetic field. The magnetic force on the moving charges is zero when it moves parallel to the magnetic field lines.

what is the magnetic effect of placing two wires with equal but oppositely directed currents close together or twisted about each other?

The magnetic fields created by the 2 wires will nearly cancel. When you reverse the direction of a current, the direction of the magnetic field it creates will also reverse.

A magnetic field can deflect a beam of electrons, but it cannot do work on the electrons to change their speed. Why?

The magnetic force that acts on a charged particle is always perpendicular to the particle's motion. There is no component of the force that acts along the instantaneous direction of motion, so the field cannot speed up, or slow down, a moving charged particle.

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?

The north pole of the bar magnet induces south poles on the closer side of a piece of iron. In the same way, the south pole induces north poles in the iron. This causes the attraction.

Why will a magnet attract an ordinary nail or paper clip but not a wooden pencil?

The north pole of the bar magnet induces south poles on the closer side of the nail or paper clip. In the same way, the south pole induces north poles in the nail or paper clip. This causes the attraction. However, a wooden pencil does not have magnetic domains that would be affected by the permanent magnet.

Historically, replacing dirt roads with paved roads reduced friction on vehicles. Replacing paved roads with steel rails reduced friction further. What recent step eliminates rail friction of vehicles? What friction remains after rail friction is eliminated?

The technique of magnetic levitation eliminates rail friction, because the train doesn't even touch the rail. Drag forces (forces due to friction with air) remain.

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 occur at the same frequency. When the current in the coil increases, it becomes a stronger electromagnet, and feels a stronger force from the permanent magnet. When the current decreases, the coil feels less force. Thus, the mechanical vibrations of the speaker occur in step with the variations in the current.

By what mechanism do the iron fillings shown about a magnet align with the magnetic field.

There are two factors involved. First, the iron filings turns into tiny magnets because their domains align in the field. Second, if the filing is not parallel to the external magnetic field lines, a net torque acts on it and rotates it until it aligns along the field lines, just like a tiny compass needle.

In a mass spectrometer, 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.

They would be bent by different amounts. The singly-charged ions have the same charge and the same speed in the same direction, and experience the same magnetic force. However, the amount they bend depends on their accelerations, which depends on their masses. The lighter ions will be deflected more than the heavier ones.

A friend tells you that refrigerator door, beneath its layer of painted plastic, is made of aluminum. How could you check to see if this is true (without any scraping)?

Try testing the door with a magnet. Aluminum is not magnetic, so if it sticks, s/he is mistaken. However, if it doesn't stick, that doesn't prove that the door is made of aluminum, because there are many nonmagnetic materials.

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?

When the ship is manufactured, the iron hull is heated and struck during the lengthy manufacturing process. The heating and the impacts allow the magnetic domains in the hull to align with the Earth's magnetic field, and the hull becomes magnetized. The hull's magnetic field strength and orientation may be of interest for historians, or for later magnetic measurements, and so it is recorded on a plaque.

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.

Yes, it exerts a force of equal magnitude. Newton's third law. If the magnet exerts a force on the paper clip, we find that the paper clip exerts an equal and opposite force on the magnet.

Will a pair of parallel current-carrying wires exert forces on each other?

Yes. Each current-carrying wire creates a magnetic field around it. Each current experiences a force because it is in the magnetic field generated by the other current-carrying wire.

When a magnet exerts force on a current-carrying wire. Does the current-carrying wire exert a force on a magnet?

Yes. Newton's third law. Whenever object A exerts a force on object B, we find that object B exerts an equal and opposite force on object A.

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 lines up with the Earth's magnetic field. This field points from the north magnetic pole located in the Southern Hemisphere to the south magnetic pole located in the Northern Hemisphere.

Do the poles of a horseshoe magnet attract each other? If you bend the magnet so that the poles get closer together, what happens to the force between the poles?

Yes. The force will increase. Opposite magnetic poles attract each other. When opposite poles get closer, the attractive force is greater.

Will either pole of a magnet attract a paper clip? Explain what is happening inside the attracted paper clip.

Yes. The north pole of the bar magnet lines up magnetic domains in the paper clip, in such a fashion that the domain's south pole are on the closer side. In the same way, the south pole induces north poles in the paper clip. This causes the attraction.

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?

You can use a magnet to lift the iron particles out of your breakfast cereal.

To make a compass, point an ordinary iron nail along the direction of Earth's magnetic field (which in the Northern Hemisphere, is angled downward as well as northward) and repeatedly strike it for a few seconds with a hammer or a rock. Then suspend it at its center of gravity by a string. Why does the act of striking magnetize the nail?

You made a bar magnet by jiggling the magnetic domains and letting them align with the Earth's field.