Test 2

Question 1

Why do we need to be careful about work done on the system versus work done by the system in calculations?

Answer 1

They are equal in magnitude, but differ by a minus sign

Question 2

Does the order in which we assemble a system of point charges affect the total work done?

Answer 2

No, though certain orderings may be simpler to compute

Question 3

Discuss how potential difference and electric field strength are related. Give an example

Answer 3

The greater the potential difference between two points, the stronger the electric field in that region. The classic example is two parallel plates, held at different potentials.

Question 4

What is the strength of the electric field in a region where the electric potential is constant?

Answer 4

The electric field strength is zero because electric potential differences are directly related to the field strength. If the potential difference is zero, then the field strength must also be zero.

Question 5

If a proton is released from rest in an electric field, will it move in the direction of increasing or decreasing potential? Also answer this question for an electron and a neutron. Explain why.

Answer 5

proton, decreasing; electron, increasing; neutron, stays still

Question 6

Voltage is the common word for potential difference. Which term is more descriptive, voltage or potential difference?

Answer 6

Potential difference is more descriptive because it indicates that it is the difference between the electric potential of two points.

Question 7

If the voltage between two points is zero, can a test charge be moved between them with zero net work being done? Can this necessarily be done without exerting a force? Explain

Answer 7

If a test charge is moved from one point to the other, the net work will be zero. It will be necessary to apply a force if the electric field between the two points is not zero, but positive work will be done over some portion of the path and an (equal magnitude) negative work will be done over the remainder of the path

Question 8

What is the relationship between voltage and energy? More precisely, what is the relationship between potential difference and electric potential energy?

Answer 8

They are very similar, but potential difference is a feature of the system; when a charge is introduced to the system, it will have a potential energy which may be calculated by multiplying the magnitude of the charge by the potential difference.

Question 9

Voltages are always measured between two points. Why?

Answer 9

Voltage, like potential energy, is specified with respect to an arbitrary baseline. It is only differences in potential (and potential energy) that are physically relevant.

Question 10

How are units of volts and electron-volts related? How do they differ?

Answer 10

An electron-volt is a volt multiplied by the charge of an electron. Volts measure potential difference, electron-volts are a unit of energy.

Question 11

Can a particle move in a direction of increasing electric potential, yet have its electric potential energy decrease? Explain

Answer 11

Yes, if it has a negative charge

Question 12

Would Gauss’s law be helpful for determining the electric field of a dipole? Why?

Answer 12

No, there is not a useable symmetry.

Question 13

In what region of space is the potential due to a uniformly charged sphere the same as that of a point charge? In what region does it differ from that of a point charge?

Answer 13

The region outside of the sphere will have a potential indistinguishable from a point charge; the interior of the sphere will have a different potential.

Question 15

Can a wire carry a current and still be neutral—that is, have a total charge of zero? Explain.

Answer 15

If a wire is carrying a current, charges enter the wire from the voltage source’s positive terminal and leave at the negative terminal, so the total charge remains zero while the current flows through it.

Question 16

Car batteries are rated in ampere-hours (). To what physical quantity do ampere-hours correspond (voltage, current, charge, energy, power,…)?

Answer 16

AH is a measure of charge.

Question 17

When working with high-power electric circuits, it is advised that whenever possible, you work “one-handed” or “keep one hand in your pocket.” Why is this a sensible suggestion?

Answer 17

Using one hand will reduce the possibility of “completing the circuit” and having current run through your body, especially current running through your heart.

Question 18

Does the resistance of an object depend on the path current takes through it?

Answer 18

R=p(L/A)

Question 19

If aluminum and copper wires of the same length have the same resistance, which has the larger diameter? Why?

Answer 19

Copper has a lower resistivity than aluminum, so if length is the same, copper must have the smaller diameter.

Question 20

Common household appliances are rated at 110 V, but power companies deliver voltage in the kilovolt range and then step the voltage down using transformers to 110 V to be used in homes. You will learn in later chapters that transformers consist of many turns of wire, which warm up as current flows through them, wasting some of the energy that is given off as heat. This sounds inefficient. Why do the power companies transport electric power using this method?

Answer 20

Although the conductors have a low resistance, the lines from the power company can be kilometers long. Using a high voltage reduces the current that is required to supply the power demand and that reduces line losses

Question 21

Your electric bill gives your consumption in units of kilowatt-hour (kWh). Does this unit represent the amount of charge, current, voltage, power, or energy you buy?

Answer 21

The unit is a unit of energy, power times time

Question 22

What effect will the internal resistance of a rechargeable battery have on the energy being used to recharge the battery?

Answer 22

Some of the energy being used to recharge the battery will be dissipated as heat by the internal resistance.

Question 23

A battery with an internal resistance of r and an emf of 10.00 V is connected to a load resistor R=r. As the battery ages, the internal resistance triples. How much is the current through the load resistor reduced?

Answer 23

The current is cut in half.

Question 24

A voltage occurs across an open switch. What is the power dissipated by the open switch?

Answer 24

No power is dissipated by the switch because zero current flows through the circuit when the switch is open

Question 25

The severity of a shock depends on the magnitude of the current through your body. Would you prefer to be in series or in parallel with a resistance, such as the heating element of a toaster, if you were shocked by it? Explain.

Answer 25

It would probably be better to be in series because the current will be less than if it were in parallel.

Question 26

Suppose you are doing a physics lab that asks you to put a resistor into a circuit, but all the resistors supplied have a larger resistance than the requested value. How would you connect the available resistances to attempt to get the smaller value asked for?

Answer 26

connect two or more in parallel

Question 27

Some light bulbs have three power settings (not including zero), obtained from multiple filaments that are individually switched and wired in parallel. What is the minimum number of filaments needed for three power settings?

Answer 27

two filaments, a low resistance and a high resistance, connected in parallel

Question 28

FYI All charges cannot move into a junction. At least one current must leave the junction and at least one current must enter the junction.

Answer 28

ok

Question 29

Do batteries in a circuit always supply power to a circuit, or can they absorb power in a circuit? Give an example.

Answer 29

A battery can absorb power in a circuit. An example is a circuit used to charge a battery.

Question 30

What are the advantages and disadvantages of connecting batteries in series? In parallel?

Answer 30

In series the voltages add, but so do the internal resistances, because the internal resistances are in series. In parallel, the terminal voltage is the same, but the equivalent internal resistance is smaller than the smallest individual internal resistance and a higher current can be provided.

Question 31

Semi-tractor trucks use four large 12-V batteries. The starter system requires 24 V, while normal operation of the truck’s other electrical components utilizes 12 V. How could the four batteries be connected to produce 24 V? To produce 12 V? Why is 24 V better than 12 V for starting the truck’s engine (a very heavy load)?

Answer 31

To obtain 24 V, two of the batteries connected in series connected in parallel with the other two batteries in series. To provide 12 V, connect all four in parallel. For a given resistance, a higher voltage will provide a greater current.

Question 32

A battery, switch, capacitor, and lamp are connected in series. Describe what happens to the lamp when the switch is closed.

Answer 32

When the switch is first closed, the current through the lamp will be high and the lamp will burn brightly. As the capacitor charges, the current decreases to zero, and the brightness of the lamp will decrease until the bulb goes dark.

Question 33

hen making an ECG measurement, it is important to measure voltage variations over small time intervals. The time is limited by the RC constant of the circuit—it is not possible to measure time variations shorter than RC. How would you manipulate R and C in the circuit to allow the necessary measurements?

Answer 33

The time constant can be shortened by using a smaller resistor and/or a smaller capacitor. Care should be taken when reducing the resistance because the initial current will increase as the resistance decreases.