Chapter Questions
What is electric current? What is the SI unit for electric currènt?
In a metal conductor, current is the result of moving electrons, Can charge carriers ever be positive?
What is meant by the term comentional current?
What is the difference between the drift speed of an electron in a metal wire and the average speed of the electron between collisions with the atoms of the metal wire?
There is a current in a metal wire due to the motion of electrons. Sketch a possible path for the motion of a single electron in this wire, the direction of the electric field vector, and the direction of conventional current.
What is an electrolyte?
What is the direction of conventional current in each case shown in Figure $19-18 ?$
Why must energy be continuously pumped into a circuit by a battery or a generator to maintain an electric current?
Name at least two differences between batteries and generators.
What is the difference between direct current and alternating current? Which type of current is supplied to the appliances in your home?
In an analogy between traffic flow and electric current, what would correspond to the charge, $Q$ ? What would correspond to the current, $I$ ?
Is current ever "used up"? Explain your answer.
Why do wires usually warm up when an electric current is in them?
A student in your class claims that batteries work by supplying the charges that move in a conductor, generating a current. What is wrong with this reasoning?
When a light bulb is connected to a battery, charges begin moving almost immediately, although each electron travels very slowly across the wire. Explain why the bulb lights up so quickly.
What is the net drift velocity of an electron in a wire that has alternating current in it?
How long does it take a total charge of $10.0 \mathrm{C}$ to pass through a cross-sectional area of a copper wire that carries a current of $5.0 \mathrm{A} ?$ (See Sample Problem 19A.)
A hair dryer draws a current of $9.1 \mathrm{A}$a. How long does it take for $1.9 \times 10^{3} \mathrm{C}$ of charge to pass through the hair dryer?b. How many electrons does this amount of charge represent?
How long does it take for $5.0 \mathrm{C}$ of charge to pass through a cross-sectional area of a copper wire if $I=5.0 \mathrm{A} ?$ (See Sample Problem 19A.)
What factors affect the resistance of a conductor?
Each of the wires shown in Figure $19-19$ is made of copper. Assuming each piece of wire is at the same temperature, which has the greatest resistance? Which has the least resistance?
Why are resistors used in circuit boards?
The critical temperature of aluminum is $1.19 \mathrm{K}$ What happens to the resistance of aluminum at temperatures lower than $1.19 \mathrm{K}$ ?
For a constant resistance, how are potential differ ence and current related?
If the potential difference across a conductor is con stant, how is current dependent on resistance?
Using the atomic theory of matter, explain why the resistance of a material should increase as its temperature increases.
Recent discoveries have led some scientists to hope that a material will be found that is superconducting at room tem perature. Why would such a material be useful?
A nichrome wire with a resistance of $15 \Omega$ is connected across the terminals of a $3.0 \mathrm{V}$ flashlight battery. How much current is in the wire? (See Sample Problem 19B.)
How much current is drawn by a television with a resistance of $35 \Omega$ that is connected across a potential difference of $120 \mathrm{V} ?$ (See Sample Problem 19B.)
Calculate the current that each resistor shown in Figure $19-20$ would draw when connected to a $9.0 \mathrm{V}$ battery
Compare and contrast mechanical power with electric power
What quantity is measured in kilowatt-hours? What quantity is measured in kilowatts?
If electrical energy is transmitted over long distances the resistance of the wires becomes significant, Why?
How many joules of energy are dissipated by a $50.0 \mathrm{W}$ light bulb in $1.00 \mathrm{s} ?$
How many joules are in a kilowatt-hour?
A $60 \mathrm{W}$ light bulb and a $75 \mathrm{W}$ light bulb operate from $120 \mathrm{V}$. Which bulb has a greater current in it?
Two conductors of the same length and radius are connected across the same potential difference. One conductor has twice as much resistance as the other. Which conductor dissipates more power?
It is estimated that in the United States (population 250 million) there is one electric clock per person, with each clock using energy at a rate of $2.5 \mathrm{W}$. Using this estimate, how much energy is consumed by all of the electric clocks in the United States in a year?
When a small lamp is connected to a battery, the filament becomes hot enough to emit electromagnetic radiation in the form of visible light, while the wires do not. What does this tell you about their rela tive resistances of the filament and the wires?
A computer is connected across a 110 V power supply. The computer dissipates $130 \mathrm{W}$ of power in the form of electromagnetic radiation and heat. Calculate the resistance of the computer.
The operating potential difference of a light bulb is $120 \mathrm{V} .$ The power rating of the bulb is $75 \mathrm{W}$. Find the current in the bulb and the bulb's resistance.
How mach would it cost to watch a football game for $3.0 \mathrm{h}$ on a $325 \mathrm{W}$ television described in item 43 if electrical energy costs $\$ 0.08 / \mathrm{kW} \cdot \mathrm{h} ?$ (See Sample Problem 19 D, )
Calculate the cost of operating a 75 W light bulb con tinuously for a 30 -day month when electrical energy $\cos t s 50.15 / k W \cdot h$ (See Sample Problem 19D.)
A net charge of $45 \mathrm{mC}$ passes through the crosssectional area of a wire in 15 s.a. What is the current in the wire?b. How many electrons pass the cross-sectional area in $1.0 \mathrm{min}$ ?
A potential difference of $12 \mathrm{V}$ produces a current of $0.40 \mathrm{A}$ in a piece of copper wire. What is the resistance of the wire?
The current in a lightning bolt is $2.0 \times 10^{5}$ A. How many coulombs of charge pass through a crosssectional area of the lightning bolt in 0.50 s?
A person notices a mild shock if the current along a path through the thumb and index finger exceeds $80.0 \mu$ A. Determine the maximum allowable potential difference without shock across the thumb and index finger for the following:a. a dry-skin resistance of $4.0 \times 10^{5} \Omega$b. a wet-skin resistance of $2.0 \times 10^{3} \Omega$
How much power is needed to operate a radio that draws $7.0 \mathrm{A}$ of current when a potential difference of $115 \mathrm{V}$ is applied across it?
A color television has a power rating of 325 W. How much current does this set draw from a potential difference of $120 \mathrm{V} ?$
An X-ray tube used for cancer therapy operates at $4.0 \mathrm{MV}$ with a beam current of $25 \mathrm{mA}$ striling a metal target. Calculate the power of this beam.
A steam iron draws $6.0 \mathrm{A}$ when connected to a potential difference of $120 \mathrm{V}$a. What is the power rating of this iron?b. How many joules of energy are produced in $20,0 \min ?$c. How much does it cost to run the iron for $20.0 \mathrm{min}$ at $\$ 0.010 / \mathrm{kW} \cdot \mathrm{h} ?$
An $11.0 \mathrm{W}$ energy-efficient fluorescent lamp is designed to produce the same illumination as a conventional $40.0 \mathrm{W}$ lampa. How much energy does this lamp save during $100.0 \mathrm{h}$ of $\mathrm{use} ?$b. If electrical energy costs $\$ 0.080 / \mathrm{kW} \cdot \mathrm{h}$, how much money is saved in $100.0 \mathrm{h} ?$
Use the electric bill shown in Figure $19-21$ to answer the following questions:a. How many joules of energy were consumed in this billing cycle?b. What is the average amount of energy consumed per day in joules and kilowatt-hours?c. If the cost of energy were increased to $\$ 0.15 / k W \cdot h,$ how much more would energy cost in this billing cycle? (Assume that the price of fuel remains constant.
The mass of a gold atom is $3.27 \times 10^{-25} \mathrm{kg}$. If $1.25 \mathrm{kg}$ of gold is deposited on the negative electrode of an electrolytic cell in a period of $2.78 \mathrm{h}$, what is the cur. rent in the cell in this period? Assume that each gold ion carries one elementary unit of positive charge.
The power supplied to a typical black-and-white television is $90.0 \mathrm{W}$ when the set is connected across a potential difference of $120 \mathrm{V}$. How much electrical energy does this set consume in $1.0 \mathrm{h}$ ?
A color television set draws about 2.5 A of current when connected to a potential difference of $120 \mathrm{V}$ How much time is required for it to consume the same energy that the black-and-white model described in item 57 consumes in $1.0 \mathrm{h}$ ?
The headlights on a car are rated at $80.0 \mathrm{W}$. If they are connected to a fully charged $90.0 \mathrm{A} \cdot \mathrm{h}, 12,0 \mathrm{V}$ battery, how long does it take the battery to completely discharge?
The current in a conductor varies over time as shown in Figure $19-22$a. How many coulombs of charge pass through a cross section of the conductor in the time interval $t=0$ to $t=5.0 \mathrm{s} ?$b. What constant current would transport the same total charge during the 5.0 s interval as does the actual current?
Birds resting on high-voltage power lines are a common sight. A certain copper power line carries a current of $50.0 \mathrm{A}$, and its resistance per unit length is $1.12 \times 10^{-5} \Omega / \mathrm{m}$. If a bird is standing on this line with its feet $4.0 \mathrm{cm}$ apart, what is the potential difference across the bird's feet?
An electric car is designed to run on a bank of batteries with a total potential difference of $12 \mathrm{V}$ and a total energy storage of $2.0 \times 10^{7}$ ).a. If the electric motor draws $8.0 \mathrm{kW}$, what is the current delivered to the motor?b. If the electric motor draws $8.0 \mathrm{kW}$ as the car moves at a steady speed of $20.0 \mathrm{m} / \mathrm{s}$, how far will the car travel before it is "out of juice"?