(a) A negatively charged iron ball (on the end of a plastic rod)exerts a strong

All matter contains both positively and negatively charged parti- cles. Why do most things have no net charge?

A particular solid is electrically charged after it is rubbed, but it is not known whether its charge is positive or negative. How could you determine which charge it has by using a piece of plastic and fur?

What is a positive ion? A negative ion?

What remains after a hydrogen atom is positively ionized?

Describe the similarities and the differences between the gravita- tional force between two objects and the electrostatic force between two charged objects.

When clothes are removed from a hot dryer, they often cling together, but when two identical articles of clothing, such as two matched socks, are taken out, they usually repel one another. Explain this difference in behavior

In otherwise empty space, what would happen if the size of the elec- trostatic force acting between two positively charged objects was exactly the same as that of the gravitational force acting between them? What would happen if they were moved closer together or farther apart?

(a) A negatively charged iron ball (on the end of a plastic rod) exerts a strong attractive force on a penny even though the penny is neutral. How is this possible? (b) The penny accelerates toward the ball, hits it, and then is imme- diately repelled. What do you think causes the sudden change from an attractive to a repulsive force on contact between the penny and the ball?

What is an electric field? Sketch the shape of the electric field around a single proton.

At one moment during a storm, the electric field between two clouds is directed toward the east. What is the direction of the force on any electron in this region? What is the direction of the force on a positive ion in this region?

Explain what an electrostatic precipitator is and how it works.

If electrons are flowing clockwise around an electric circuit, which way is the conventional current flowing in the circuit?

Saltwater contains an equal number of positive and negative ions. When saltwater is flowing through a pipe, does it constitute an electric current?

Materials can be classified into four categories based on the ease with which charges can flow through them. Give the names of these categories and describe each one.

A solid metal cylinder has a certain resistance. It is then heated and carefully stretched to form a longer, thinner cylinder. After it cools, will its resistance be the same as, greater than, or less than what it was before? Explain your choice.

A student using a sensitive meter that measures resistance finds that the resistance of a thin wire is changed slightly when it is picked up with a bare hand. What causes the change in the resistance, and does it increase or decrease?

If a new material is found that is a superconductor at all tempera- tures, what parts of some common electric devices would definitely not be made out of it? Explain.

Explain what current, resistance, and voltage are.

Make a sketch of a simple electric circuit and label the key ele- ments in the circuit.

Describe Ohms law.

A power supply is connected to two bare wires that are inserted into a glass of saltwater. The resistance of the water decreases as the volt- age is increased. Sketch a graph of the voltage versus the current in the water showing this type of behavior.

There are two basic methods for connecting more than one elec- trical device in a circuit. Name, describe, and give the advantages of each.

Make a sketch of an electric circuit that contains a switch and two lightbulbs connected in such a way that if either bulb burns out the other still functions, but if the switch is turned off, both bulbs go out.

Two 1.5-volt batteries are connected in series in an electric circuit. Use the concept of energy to explain why this combination is equiv- alent to a single 3-volt battery. When connected in parallel, what are two 1.5-volt batteries equivalent to?

An electrical supply company sells two models of 100-watt power supplies (the maximum power output is 100 W), one with an out- put of 12 V and the other 6 V. What can you conclude about the maximum current that the two power supplies can produce?

A simple electric circuit consists of a constant-voltage power supply and a variable resistor. What effect does reducing the resistance have on the current in the circuit and on the power output of the power supply?

What is the purpose of having fuses or circuit breakers in electric circuits? How should they be connected in circuits so they will be effective?

A 20-A fuse in a household electric circuit burns out. What catastro- phe could occur if it is replaced by a 30-A fuse?

Why is it economical to use extremely high voltages for the trans- mission of electrical power?

Explain what AC and DC are. Why is AC used by electric utilities? Why is DC used in flashlights?

If AC in a circuit can be thought of as a wave, which kind is it, lon- gitudinal or transverse?

If the electric utility company where you live suddenly changed the frequency of the AC to 20 Hz, what problems might this cause?

Two charged particles exert an electric force of 16 N on each other. What will the magnitude of the force be if the distance between the particles is reduced to one-half of the original separation? If the original distance between the particles is now restored, how much larger would the charge on either one of the particles have to become to yield the same force that was present when the particle separation was only one-half its original value?

What would the separation between two identical objects, one carry- ing 1 C of positive charge and the other 1 C of negative charge, have to be if the electrical force on each was precisely 1 N?

A particle having a positive charge of 2.0 106 C experiences an upward force of 8 N when placed at a certain location in space. What is the magnitude and direction of the electric field at that point? What would be different about your answer if the particle possessed a negative charge of 2.0 106 C?

During 30 seconds of use, 250 C of charge flow through a microwave oven. Compute the size of the electric current.

A lightning stroke lasts 0.05 s and involves a flow of 100 C. What is the current?

. A current of 0.7 A goes through an electric motor for 1 min. How many coulombs of charge flow through it during that time?

A calculator draws a current of 0.0001 A for 5 min. How much charge flows through it?

A current of 12 A flows through an electric heater operating on 120 V. What is the heaters resistance?

A 120-V circuit in a house is equipped with a 20-A fuse that will blow if the current exceeds 20 A. What is the smallest resistance that can be plugged into the circuit without causing the fuse to blow?

The resistance of each brake lightbulb on an automobile is 6.6 . Use the fact that cars have 12-V electrical systems to compute the current that flows in each bulb.

The lightbulb used in a computer projector has a resistance of 80 . What is the current through the bulb when it is operating on 120 V?

The resistance of the skin on a persons finger is typically about 20,000 . How much voltage would be needed to cause a current of 0.001 A to flow into the finger? (This is about enough current to just be felt by most people.)

A 150- resistor is connected to a variable-voltage power supply. (a) What voltage is necessary to cause a current of 0.3 A in the resistor? (b) What current flows in the resistor when the voltage is 18 V?

Compute the power consumption of the electric heater in Problem 8.

An electric eel can generate a 400-V, 0.5-A shock for stunning its prey. What is the eels power output?

An electric train operates on 750 V. What is its power consumption when the current flowing through the trains motor is 2,000 A?

All of the electrical outlets in a room are connected in a single paral- lel circuit ( Figure 7.37). The circuit is equipped with a 20-A fuse, and the voltage is 120 V. (a) What is the maximum total power that can be supplied by the outlets without blowing the fuse? (b) How many 1,200-W appliances can be plugged into the sockets without blowing the fuse?

18. Your cell phone typically consumes about 400 mW of power when you text a friend. If the phone is operated using a lithium-ion bat- tery with a voltage of 3.6 V, what is the current flowing through the cell-phone circuitry under these circumstances? Compare your result with the data given in Table 7.1. bulb, how long will the batteries be able to deliver power to the flashlight at this level?

A cars headlight consumes 40 W when on low beam and 50 W when on high beam. (a) Find the current that flows in each case (V 12 V). (b) Find the resistance in each case.

Find the current that flows in a 40-W bulb used in a house (V 120 V). Compare this with the answer to the first part of Problem 19.

An electric clothes dryer is rated at 4,000 W. How much energy does it use in 40 min?

A clock consumes 2 W of electrical power. How much energy does it use each day?

Which costs more, running a 1,200-W hair dryer for 5 min or leav- ing a 60-W lamp on overnight (10 h)?

A representative lightning strike is caused by a voltage of 200,000,000 V and consists of a current of 1,000 A that flows for a fraction of a second. Calculate the power.

A toaster operating on 120 V uses a current of 9 A. (a) What is the toasters power consumption? (b) How much energy does it use in 1 min? (c) What is the total resistance of the heating element wires?

A certain electric motor draws a current of 10 A when connected to 120 V. (a) What is the motors power consumption? (b) How much energy does it use during 4 h of operation? Express the answer in joules and in kilowatt-hours.

The generator at a large power plant has an output of 1,000,000 kW at 24,000 V. (a) If it were a DC generator, what would be the current in it? (b) What is its energy output each dayin joules and in kilowatt- hours? (c) If this energy is sold at a price of 10 cents per kilowatt-hour, how much revenue does the power plant generate each day?

The generator at a large power plant has an output of 1,000,000 kW at 24,000 V. (a) If it were a DC generator, what would be the current in it? (b) What is its energy output each dayin joules and in kilowatt- hours? (c) If this energy is sold at a price of 10 cents per kilowatt-hour, how much revenue does the power plant generate each day? 28. A lightbulb is rated at 60 W when connected to 120 V. (a) What current flows through the bulb in this case? (b) What is the bulbs resistance? (c) What would be the current in the bulb if it were connected to 60 V, assuming the resistance stays the same? (d) What would be its power consumption in this case?

29. About 40,000 J of energy is stored in a typical 1.5-V D-cell battery. If two such batteries are joined to produce a total of 3.0 V in a flash- light circuit and cause 2.0 A of current to flow through the flashlight bulb, how long will the batteries be able to deliver power to the flashlight at this level?

An electric car is being designed to have an average power output of 4,000 W for 2 h before needing to be recharged. (Assume there is no wasted energy.) (a) How much energy would be stored in the charged batteries? (b) The batteries operate on 30 V. What would the current be when they are operating at 4,000 W? (c) To be able to recharge the batteries in 1 h, how much power would have to be supplied to them?

Compute the electric force acting between the electron and the pro- ton in a hydrogen atom. The radius of the smallest orbit of the elec- tron around the proton is about 5.3 1011 m.

Use the result from Challenge 1 and the equation for centripetal force from Chapter 2 to compute the speed of the electron as it moves around the proton. The electrons mass is 9.1 1031 kg

Compute the number of electrons that flow through a wire each second when the current in the wire is 0.2 A.

Using your understanding of the nature of internal energy and temperature, explain why we might expect the resistance of a solid to increase if its temperature increases.

The current that flows through an incandescent lightbulb immedi- ately after it is turned on is higher than the current that flows moments later. Why?

An electrical device called a diode is designed to have very low resis- tance to current flowing through it in one direction but very large resistance to current flow in the other direction. Sketch a graph of the voltage versus current for such a device.

Imagine a company offering a line of hair dryers that operate on dif- ferent voltages, say, 12, 30, 60, and 120 V. Assume that all of the dryers are rated at 1,200 W and find the current that would flow in each as it operates. What would be different about the heating fila- ment wires and the motors in the various hair dryers?

Perform the calculation referred to in the last sentence of Example 7.6.

Combine Ohms law and the equation for power consumption to derive the equation that gives the power in terms of current and resistance. Use the result to answer the following question. A cable carrying electrical energy wastes 10 kWh of energy each day because of ohmic heating. If the current in the cable is doubled but the cables resistance remains the same, how much energy will it waste each day?

A defibrillator sends approximately 0.1 C of charge through a patients chest in about 2 ms. The average voltage during the dis- charge is approximately 3,000 V. Compute the average current that flows, the average power output, and the total energy consumed (Note: Only a small portion of this current actually flows through the heart in a typical defibrillating procedure. Much of the charge is diffused throughout the patients thoracic cavity.).