Answer: Consider a circuit with a main wire that branches

Problem 1E What two things can be done to increase the amount of flow in a water pipe? Similarly, what two things can be clone to increase the current in an electrical circuit?

Problem 1P What is the effect on the current in a wire if both the voltage across it and its resistance are doubled? If both are halved?

Problem 1PC Calculate the current in a toaster that has a heating element of 15 ? when connected to a 120-V outlet.

Problem 1R Rank the circuits illustrated according to the brightness of the identical bulbs, from brightest to dimmest.

Problem 1RQ What condition is necessary for the flow of heat? What analogous condition is necessary for the flow of charge?

Problem 2E 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?

Problem 2P The wattage marked on a lightbulb is not an inherent property of the bulb, but depends on the voltage to which it is connected, usually 110 or 120 V. How many amperes flow through a 60-W bulb connected in a 120-V circuit?

Problem 2PC Calculate the current that moves through your fingers (resistance 1000 ?) when you touch them to the terminals of a 6-V battery.

Problem 2R The bulbs shown are identical. An ammeter is placed in different locations, as shown. Rank the current readings in the ammeter from greatest to least.

Problem 2RQ What condition is necessary for the sustained flow of water in a pipe? What analogous condition is necessary for the sustained flow of charge in a wire?

Problem 3E Consider a circuit with a main wire that branches into two other wires. If the current is 10 A in the main wire and 4 A in one of the branches, how much current is in the other branch?

Problem 3P Rearrange the equation current = voltage/resistance to express resistance in terms of current and voltage. Then solve the following: A certain device in a 120-V circuit has a current rating of 20 A. What is the resistance of the device (how many ohms)?

Problem 3PC Calculate the current in the 240-? filament of a bulb connected to a 120-V line.

Problem 3R All bulbs are identical in the circuits shown. An ammeter is connected next to the battery, as shown. Rank the current readings in the ammeter, from greatest to least.

Problem 3RQ Why are electrons, rather than protons, the principal charge carriers in metal wires?

Problem 4E 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 exhibits behavior more analogous to an electric circuit? Explain.

Problem 4P Using the formula power = current × voltage, find the current drawn by a 1200-W toaster connected to 120 V. Then, using the method from the previous problem, show that the resistance of the toaster is 12 ?.

Problem 4PC Calculate the power of a device that carries 0.5 A when impressed with 120 V.

Problem 4R All bulbs are identical in the following circuits. A voltmeter is connected across a single bulb to measure the voltage drop across it. Rank the voltage readings from greatest to least.

Problem 4RQ What exactly is an ampere?

Problem 5E What happens to the brightness of light emitted by a lightbulb when the current flowing through it increases?

Problem 5P The total charge that an automobile battery can supply without being recharged is given in terms of ampere-hours. A typical 12-V battery has a rating of 60 ampere-hours (60 A for 1 h, 30 A for 2 h, and so on). Suppose that you forget to turn the headlights off in your parked automobile. If each of the two headlights draws 3 A, how long will it be before your battery is “dead”?

Problem 5PC Calculate the power of a hair dryer that operates on 120 V and draws a current of 10 A.

Problem 5R Consider the three parts of the circuit: A, the top branch with two bulbs; B, the middle branch with one bulb; C, the battery. a. Rank the current through each, from greatest to least. ________________ b. Rank the voltage across each from greatest to least.

Problem 5RQ Name two kinds of practical “electric pumps.”

Problem 6E Your friend says that a battery supplies the electrons in an electric circuit. Do you agree or disagree? Defend your answer.

Problem 6PC Given that the power consumed by a device is 1200 W operating on a 120-V line, calculate the amount of current it draws.

Problem 6P Show that operating a 100-W lamp continuously for 1 week when the power utility rate is 15¢/kWh costs $2.52.

Problem 6RQ How much energy is supplied to each coulomb of charge that flows through a 12-V battery?

Problem 7E Is a current-carrying wire electrically charged because of the electrons moving in it?

Problem 7RQ Does charge flow through a circuit or into a circuit? Does voltage flow through a circuit, or is voltage established across a circuit?

Problem 7P A 4-W night-light is plugged into a 120-V circuit and operates continuously for 1 year. Find the following: (a) the current it draws, (b) the resistance of its filament, (c) the energy consumed in a year. (d) Then show that for a utility rate of 15¢/kWh the cost for a year’s operation is $5.25.

Problem 8E 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?

Problem 8P An electric iron connected to a 110-V source draws 9 A of current. Show that the amount of heat it generates in a minute is nearly 60,000 J.

Problem 8RQ Will water flow more easily through a wide pipe or a narrow pipe? Will current flow more easily through a thick wire or a thin wire?

Problem 9E In which of the circuits shown does a current exist to light the bulb?

Problem 9P Show in the previous problem that 540 C of charge flow through the iron in 1 minute.

Problem 9RQ Does heating a metal wire increase or decrease its electrical resistance?

Problem 10E Does more current flow out of a battery than into it? Does more current flow into a lightbulb than out of it? Explain.

Problem 10P In periods of peak demand, power companies lower their voltage. This saves them power (and saves you money!). To see the effect, consider a 1200-W coffeemaker that draws 10 A when connected to 120 V. Suppose the voltage is lowered by 10% to 108 V. By how much does the current decrease? By how much does the power decrease? (Caution: The 1200-W label is valid only when 120 V is applied. When the voltage is lowered, it is the resistance of the toaster, not its power, that remains constant.)

Problem 10RQ If the voltage impressed across a circuit is held constant while the resistance doubles, what change occurs in the current?

Something gets “used up” in a battery that eventually dies and goes flat. One friend says that current is used up. Another friend says that energy is used up. Who, if either, do you agree with, and why?

Problem 11RQ If the resistance of a circuit remains constant while the voltage across the circuit decreases to half its former value, what change occurs in the current?

Problem 12E Suppose you leave your car lights on while at 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?

Problem 12RQ How does wetness affect the resistance of your body?

Problem 13E Your friend says that, when jump-starting a dead battery, you should connect your live battery in parallel with the dead battery, which, in effect, replaces the dead one. Do you agree?

Problem 13RQ What is the function of the round third prong in a modern household electric plug?

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?

Problem 15E Why is the current in an incandescent bulb greater immediately after it is turned on than it is a few moments later? (That’s why bulbs usually burn out just as they are being turned on.)

Problem 14RQ Does a battery produce dc or ac? Does the generator at a power station produce dc or ac?

Problem 15RQ What does it mean to say that a certain current is 60 Hz?

Problem 16E Only a small percentage of the electric energy fed into a common lightbulb is transformed into light. What happens to the remaining energy?

Problem 16RQ What property of a diode enables it to convert ac to pulsed dc?

Problem 17E Why are all compact fluorescent lamps more efficient than incandescent lamps?

Problem 17RQ A diode converts ac to pulsed dc. What electrical device smoothes the pulsed dc to a smoother dc?

Problem 18E A simple lie detector consists of an electric circuit, often from one finger to another. A sensitive meter shows the current chat flows when a small voltage is applied. How does this technique indicate that a person is lying? (And when does this technique not indicate when someone is lying?)

Problem 18RQ What is the error in saying that electrons in a common battery-driven circuit travel at about the speed of light?

Problem 19E Why are thick wires rather than thin wires usually used to carry large currents?

Problem 19RQ Why does a wire that carries electric current become hot?

Problem 20E Why does the filament of a lightbulb glow while the connecting wires do not?

Problem 20RQ What is meant by drift velocity?

It is commonly said that a certain resistor draws a certain current. Does this mean that the resistor “attracts” the current? Defend your answer.

Problem 21RQ A tipped domino sends a pulse along a row of standing dominoes. Is this a good analogy for the way electric current, sound, or both travel?