Physics exam discussion questions

(Chapter 20) can an iron rod attract a magnet? Can a magnet attract an iron rod? What must you consider to answer these questions?

A magnet can attract an iron bar, so by Newton's third law, an iron bar can attract a magnet. Another consideration is that the iron has domains that can be made slightly magnetic by an external magnetic field, as opposed to a substance like plastic or wood.

(Chapter 16) when determining an electric field must we use a positive test charge or would a negative one do as well?

A negative test charge could be used. For purposes of defining directions, the electric field might then be defined as the OPPOSITE of the force on the test charge, divided by the test charge. Equation (16-3) might be changed to E F q , q 0 .

(Chapter 14) a 22C day is warm while a swimming pool at 22C feels cool. why?

Air is a poorer conductor of heat than water by roughly a factor of 20, and so the rate of heat loss from your body to the air is roughly 20 times less than the rate of heat loss from your body to the water. Thus you lose heat quickly in the water, and feel cold. Another contributing factor is that water has a high heat capacity, and so as heat leaves your body and enters the water, the temperature rise for the water close to your body is small. Air has a smaller heat capacity, and so the temperature rise for the air close to your body is larger. This reduces the temperature difference between your body and the air, which reduces the rate of heat loss to the air as well.

(Chapter 19) what is the main difference between an analog voltmeter and an analog ammeter?

An analog voltmeter has a high value resistor in series with the galvanometer, so that the actual voltage drop across the galvanometer is small. An analog ammeter has a very low value resistor in parallel with the galvanometer, so that the actual current through the galvanometer is small. Another significant difference is how they are connected to a circuit. A voltmeter goes in parallel with the component being measured, while an ammeter goes in series with the component being measured.

(Chapter 18) when electric lights are operated on low frequency ac they flicker noticeably. Why?

At only 10 Hz, the metal filament in the wire will go on and off 20 times per second. (It has a maximum magnitude of current at the maximum current in each direction.) The metal filament has time to cool down and get dim during the low current parts of the cycle, and your eye can detect this. At 50 or 60 Hz, the filament never cools enough to dim significantly.

(Chapter 15) entropy is often called times arrow because it tells us in which direction natural processes occur. If a movie were run backward, name some processes that you might see that would tell you that time was running backward.

In an action movie, seeing a building or car go from an exploded state to an un-exploded state. In a movie with vehicle crashes, seeing two collided vehicles separate from each other, becoming un- wrecked as they separate. Watching someone "un-write" something on a piece of paper - moving a pen over paper, taking away written marks as the pen moves.

(Chapter 15) can you warm a kitchen in winter by leaving the oven door open? Can you cool the kitchen on a hot summer day by leaving the fridge door open?

It is possible to warm the kitchen in the winter by having the oven door open. The oven heating elements radiate heat energy into the oven cavity, and if the oven door is open, the oven is just heating a bigger volume than usual. However, you cannot cool the kitchen by having the refrigerator door open. The refrigerator exhausts more heat than it removes from the refrigerated volume, so the room actually gets warmer with the refrigerator door open. If you could have the refrigerator exhaust into some other room, then the refrigerator would be similar to an air conditioner, and it could cool the kitchen, while heating up some other space.

(Chapter 18) can a copper wire and an aluminum wire of the same length have the same resistance?

Resistance is given by the relationship R L A . If the ratio of resistivity to area is the same for both the copper wire and the aluminum wire, then the resistances will be the same. Thus if A A or A A , the resistances will be the same.

(Chapter 13) Alcohol evaporates more quickly than water at room temperature. What can you infer about the molecular properties of one relative to the other?

Since the alcohol evaporates more quickly, the alcohol molecules escape "easier" than the water molecules. One explanation could be that the intermolecular forces (bonds) for alcohol are smaller than those for water. Another explanation could be that the alcohol molecules are moving more rapidly than the water molecules, indicating that alcohol molecules are less massive than water molecules. However, the simplest alcohol (CH3OH) has a molecular mass higher than that of water, so mass is probably not the explanation.

(Chapter 20) in what direction are the magnetic field lines surrounding a straight wire carrying a current that is moving directly away from you?

The magnetic field lines form CLOCKWISE circles centered on the wire. // \\ x \\ //.

(Chapter 14) why does water in a metal canteen stay cooler if the cloth jacket surrounding the canteen is kept moist?

The mechanism of evaporation of the water from the moist cloth jacket requires energy (the latent heat of vaporization), some of which will come from the interior of the canteen. This removal of energy from the interior helps to keep the interior of the canteen cool. Also, the metal canteen is a good thermal conductor, and so heat can transfer from the water to the cloth jacket to cool the water.

(Chapter 16) contrast the net charge on a conductor to the free charges in the conductor.

The net charge on the conductor is the unbalanced charge, or excess charge after neutrality has been established. The net charge is the sum of all of the positive and negative charges in the conductor. If a neutral conductor has extra electrons added to it, then the net charge is negative. If a neutral conductor has electrons removed from it, then the net charge is positive. If a neutral conductor has the same amount of positive and negative charge, then the net charge is zero. Free charges in a conductor refer to those electrons (usually 1 or 2 per atom) that are so loosely attracted to the nucleus that they are "free" to be moved around in the conductor by an external electric force. Neutral conductors have these free electrons.

(Chapter 19) if two identical resistors are connected in series to a battery does the battery have to supply more power or less power than when only one of the resistors is connected?

The power supplied by the battery is the product of the battery voltage times the total current flowing from the battery. With the two resistors in series, the current is half that with a single resistor. Thus the battery has to supply half the power for the two series resistors than for the single resistor.

(Chapter 13) Long steam pipes that are fixed at the ends often have a section in the shape of a U. Why?

The steam pipe can have a large temperature change as the steam enters or leaves the pipe. If the pipe is fixed at both ends and the temperature changes significantly, there will be large thermal stresses which might break joints. The "U" in the pipe allows for expansion and contraction which is not possible at the fixed ends. This is similar to the joints placed in concrete roadway surfaces to allow expansion and contraction.

(Chapter 17) can two equipotential lines cross?

Two equipotential lines cannot cross. That would indicate that a region in space had two different values for the potential. For example, if a 40-V line and a 50-V line crossed, then the potential at the point of crossing would be both 40 V and 50 V, which is impossible. Likewise, the electric field is perpendicular to the equipotential lines. If two lines crossed, the electric field at that point would point in two different directions simultaneously, which is not possible.

(Chapter 17) we have seen that the capacitance C depends on the size shape and position of the two conductors as well as on the dielectric constant K. What then did we mean when we said that C is a constant in Eq 17-7?

We meant that the capacitance did not depend on the amount of charge stored or on the potential difference between the capacitor plates. Changing the amount of charge stored or the potential difference will not change the capacitance.