Chapter 23 Electric Current

What unit signifies (a) joule per coulomb (b) coulomb per second, (c) watt second?

(a) Volt, (b) ampere, (c) joule.

Does a battery produce dc or ac? Does the generator at a power station produce dc or ac?

A battery produces dc.

What property of a diode enables it to convert ac to pulsed dc?

A diode passes current in one direction only.

Figure 23.19 shows a fuse placed in a household circuit. In what other locations might a fuse be placed in this circuit to be useful, melting only if a problem arises?

A fuse in series with any one of the appliances might be useful because it would melt only if something went wrong with that particular appliance.

What is the function of fuses or circuit breakers in a circuit?

A fuse or a circuit breaker prevents overloading that may lead to fire.

If a glowing incandescent lightbulb is jarred and oxygen leaks inside, the bulb will momentarily brighten considerably before burning out. Putting excess current through a lightbulb will also burn it out. What physical change occurs when a lightbulb burns out?

A lightbulb burns out when a break occurs in the filament or when the filament disintegrates or falls apart.

it is commonly said that a certain resistor draws a certain current? Does this mean that the resistor "attracts" the current? Defend your answer.

A resistor doesn't "attract" or "draw" current, just as a pipe in a plumbing circuit doesn't "draw" water. Instead it "allows" or "provides for" the passage of current when an electrical pressure is established across it.

What condition is necessary for a sustained flow of electric charge through a conducting medium?

A sustained flow needs a sustained difference in potential across a conducting medium, due to a battery or generator.

Which of these is a unit of power and which is a unit of energy: a watt, a kilowatt, a kilowatt-hour?

A watt and a kilowatt are units of power. A kilowatt-hour is a unit of energy.

When a pair of identical resistors are connected in series, which of the following is the same for both resistors: (a) voltage across each, (b) power dissipated in each, (c) current through each? Do any of your answers change if the resistors are different from each other?

All are the same for identical resistors in series. If the resistors are not the same, then the one of greater resistance has less voltage across it and less power dissipated in it. Regardless of the resistances, however, the current through both is the same.

Which will do less damage: plugging a 110-V hairdryer into a 220-V circuit or plugging a 220-V hairdryer into a 110-V circuit? Defend your answer.

Damage generally occurs because of excess heating when too much current is driven through an appliance. Less damage will be done plugging the 220-V hairdryer into a 110-V circuit.

What is meant by drift velocity?

Drift velocity is the net velocity of the electrons that make up an electric current.

Does electric charge flow across a circuit or through a circuit? Does voltage flow across a circuit or is it impressed across a circuit?

Electric charge flows through a circuit. Voltage doesn't flow at all but is impressed across a circuit.

Why is the speed of an electrical signal so much greater than the speed of sound?

Electric energy is propagated through a circuit by electric fields moving at close to the speed of light, not by electron collisions. Sound, on the other hand, travels by molecular or atomic collisions----a much slower process.

Direct current (dc)

Electrically charged particles flowing in one direction only

Alternating current (ac)

Electrically charged particles that repeatedly reverse direction, vibrating about relatively fixed positions. In the United States, the vibrational rate is commonly 60 Hz.

Why are electrons, rather than protons, the principal charge carries in metal wires?

Electrons in metals are free to wander, whereas protons are imbedded in atomic nuclei, not free to roam.

What condition is necessary for the flow of heat? What analogous condition is necessary for the flow of charge?

Heat must have a difference in temperature. Charge must have a difference in electrical potential.

Does heating a metal wire increase or decrease its electrical resistance?

Heating a metal wire increases molecular motion and therefore its electrical resistance.

How much energy is given to each coulomb of charge passing through a 6-V battery?

Six joules of energy is given to each coulomb passing through a 6-volt battery.

Comment on the warning sign shown in the sketch (sign reads "DANGER! HIGH RESISTANCE 1,000000000 Ohms)

The sign is a joke. High voltage may be dangerous, but high resistance is a property of all nonconductors.

Ohm's law

The statement that the current in a circuit varies in direct proportion to the potential difference or voltage across the circuit and inversely with the circuit's resistance: A potential difference of 1 V across a resistance of 1 Ohm produces a current of 1 A

Will a lamp with a thick filament draw more current or less current than a lamp with a thin filament?

The thick filament has less resistance and will draw (carry) more current than a thin wire connected across the same potential difference.

Why are thick wires rather than thin wires usually used to carry large currents?

Thick wires have less resistance and carry currents more effectively without excessive heating.

What is the same for a 10 Ohms and a 20 Ohms resistor in parallel in a parallel circuit: current or voltage?

Voltage across parallel branches, whatever the resistance, remains the same.

How does wetness affect the resistance of your body?

Wetness lowers your body's electrical resistance.

Suppose you leave your car lights on while you see a movie. When you return, your battery is too "weak" to start your car. A friend gives you a jump-start with his battery and battery cables. What physics is occurring here?

Your friend is sharing voltage from his battery by connecting the two batteries in parallel.

Your tutor tells you that an ampere and a volt really measure the same thing and that the different terms only serve to make a simple concept seem confusing. Why should you consider getting a different tutor?

Your tutor is wrong. An ampere measures current, and a volt measures electric potential (electric "pressure"). They are entirely different concepts: Voltage produces amperes in a conductor.

Estimate the number of electrons that a power company delivers annually to the homes of a typical town of 40,000 people.

Zero. Power companies do not sell electrons; they sell energy. Whatever number of electrons flow into a home, the same number flows out.

An electron moving in a wire collides repeatedly with atoms and travels an average distance between collisions called the mean free path. If the mean free path is less in some metals, what can you say about the resistance of these metals? For a given conductor, what can be done to lengthen the mean free path?

All other things being equal, a material with a short mean free path offers more resistance to electron flow and has a higher electrical resistance. For all materials, the application of heat imposes more molecular chaos and shortens the path even more, increasing resistance in most materials. So, to lengthen the path, simply cool the material. Conductivities are greatly increased in most materials when they are cooled to low temperatures.

Series circuit

An electric circuit in which electrical devices are connected along a single loop of wire such that the same current is in each device.

Parallel circuit

An electric circuit in which electrical devices are connected in such a way that the same voltage acts across each one, and any single one completes the circuit independently of all the others.

Are automobile headlights wired in parallel or in a series? What is your evidence?

Auto headlights are wired in parallel. When one burns out, the other remains lit. If you've ever seen an automobile with one burned-out headlight, you have evidence that they're wired in parallel.

In a circuit of two lamps in parallel, if there is a voltage of 6 V across one lamp, what is the voltage across the other lamp?

Both lamps have 6 volts across them when connected in parallel.

In the circuit shown, how do the brightnesses of the identical lightbulbs compare? Which bulb draws the most current? What will happen if bulb A is unscrewed? If bulb C is unscrewed? (see figure, question 99-Chapter 23)

Bulb C is the brightest because the voltage across it equals that of the battery. Bulbs A and B share the voltage of the parallel branch of the circuit and have half the current of bulb C (assuming resistances are independent of voltages). If bulb A is unscrewed, the top branch is no longer part of the circuit and current ceases in both A and B. They no longer give light, while bulb C glows as before. If bulb C is instead unscrewed, then it goes out and bulbs A and B glow as before

If a current of one- or two-tenths of an ampere were to flow into one of your hands and out the other, you would probably be electrocuted. But if the same current were to flow into your hand and out the elbow above the same hand, you would survive even though the current might be large enough to burn your flesh. Explain.

In the first case the current passes through your chest: in the second case current passes only through your arm. You can cut off your arm and survive, but you cannot survive without your heart.

Only a small percentage of the electric energy fed into a common lightbulb is transformed into light. What happens to the remaining energy?

More than 90% of the electric energy in an incandescent lamp goes directly to heat. Thermal energy is the graveyard of electric energy.

Does more current flow out of a battery than into it? Does more current flow into a lightbulb than out of it? Explain.

No for both. Current flows through electrical devices, just as water flows through a plumbing circuit of pipes. If a water pump produces water pressure, water flows through both the pump and the circuit. Likewise with electric current in an electric circuit. For example, in a simple circuit consisting of a battery and a lamp, the electric current produced in the lamp is the same electric current in the wires that connect the lamp and the same electric current flowing through the battery. Electric charge flows through these devices (the flow of charge being current).

Name two kinds of practical "electric pumps."

One kind is a battery; another is a generator.

What happens to the brightness of bulb A when the switch is closed and bulb B lights up? (see figure, question 97-Chapter 23)

The brightness remains the same.

The circuit within an incandescent lamp is shown. Which of the battery arrangements will light the lamp? (see textbook question 63.-Chapter 23)

The circuit in the center is a complete circuit and will light the lamp.

One example of a water system is a garden hose that waters a garden. Another is the cooling system of an automobile. Which of these systems exhibits behavior more analogous to an electric circuit? Explain.

The cooling system of an automobile is more analogous to an electric circuit because it is a closed system and it contains a pump, like the battery or other voltage source in a circuit. The water hose does not recirculate the water as the auto cooling system does.

In a circuit of two lamps in series, if the current through one lamp is 1 A, what is the current through the other lamp? Defend your answer.

The current is 1 A everywhere in two lamps connected in series.

Potential difference

The difference in electric potential between two points, measured in volts (synonymous with voltage difference or simply voltage).

To connect a pair of resistors so that their combined (equivalent) resistance will be greater than the resistance of either one, should you connect them in series or in parallel?

The equivalent resistance of resistors in series is their sum, so connect a pair of resistors in series for greater resistance.

What is the error in saying that electrons in a common battery-driven circuit travel at about the speed of light?

The error is that no particle can travel at the speed of light.

What is the error in saying that the source of electrons in a circuit is the battery or generator?

The error is that the source is the conducting wires themselves, not the power source.

Consider a water pipe that branches into two smaller pipes. If the flow of water is 10 gallons per minute in the main pipe and 4 gallons per minute in one of the branches, how much water per minute flows in the other branch?

The flow is 6 gallons per minute (10-4 = 6).

Electric current

The flow of electric charge that transports energy from one place to another, measured in amperes, where 1 A is the flow of 6.25 x 10^18 electrons per second, or 1 coulomb per second.

What changes occur in the line current when more devices are introduced in a series circuit? In a parallel circuit? Why are your answer different?

The line current decreases as more devices are connected in series. But the line current increases as more devices are connected in parallel. This is because the circuit resistance is increased when devices are added in series, but decreased (more pathways) when devices are added in parallel.

Electrical resistance

The property of a material that resists electric current, measured in ohms.

Electric power

The rate of energy transfer, or the rate of doing work: the amount of energy per unit time, which can be computed as the product of current and voltage: Power = current x voltage Electric power is measured in watts (or kilowatts), where 1 W = 1 A x 1 V = 1 J/s.

If the voltage impressed across a circuit is held constant while the resistance doubles, what change occurs in the current?

When the resistance doubles, the current is halved.

From where do the electrons originate that produce an electric shock when you touch a charged conductor?

When you are shocked, your own body is the source of the electrons, but not the source of the energy imparted to them.

Batteries do have internal resistance, which is not always negligible. It shows when the current a battery supplies increases, whereupon the voltage it supplies decreases. Taking internal resistance of the battery into consideration, will the brightness of many bulbs diminish when connected in parallel? Defend your answer.

Yes. There will be a decrease in brightness when too many lamps are connected in parallel because of the increased current that flows through the battery. The internal voltage drop increases with current in the battery; which means reduced voltage is supplied at its terminals to the circuit it powers. (If the parallel circuit is powered by a stronger source, such as the power utility provides via common wall sockets, no dimming of bulbs will be seen as more and more parallel paths are added.)