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physics
electricity and magnetism

Questions and Answers of Electricity and Magnetism

Explain why the electric flux through a closed surface with a given enclosed charge is independent of the size or shape of the surface.
Consider the electric field due to a non-conducting infinite plane having a uniform charge density. Explain why the electric field does not depend on the distance from the plane, in terms of the
Use Gauss’s law to explain why electric field lines must begin or end on electric charges. (Suggestion: Change the size of the Gaussian surface.)
On the basis of the repulsive nature of the force between like charges and the freedom of motion of charge within a conductor, explain why excess charge on an isolated conductor must reside on its
A person is placed in a large hollow metallic sphere that is insulated from ground. If a large charge is placed on the sphere, will the person be harmed upon touching the inside of the sphere?
Two solid spheres, both of radius R, carry identical total charges, Q. One sphere is a good conductor while the other is an insulator. If the charge on the insulating sphere is uniformly distributed
A common demonstration involves charging a rubber balloon, which is an insulator, by rubbing it on your hair, and touching the balloon to a ceiling or wall, which is also an insulator. The electrical
You may have heard that one of the safer places to be during a lightning storm is inside a car. Why?
Distinguish between electric potential and electric potential energy.
A negative charge moves in the direction of a uniform electric field. Does the potential energy of the charge–field system increase or decrease? Does the charge move to a position of higher or
Give a physical explanation of the fact that the potential energy of a pair of charges with the same sign is positive whereas the potential energy of a pair of charges with opposite signs is negative.
A uniform electric field is parallel to the x axis. In what direction can a charge be displaced in this field without any external work being done on the charge?
Explain why equipotential surfaces are always perpendicular to electric field lines.
Describe the equipotential surfaces for (a) An infinite line of charge and (b) A uniformly charged sphere.
Explain why, under static conditions, all points in a conductor must be at the same electric potential.
The electric field inside a hollow, uniformly charged sphere is zero. Does this imply that the potential is zero inside the sphere? Explain.
The potential of a point charge is defined to be zero at an infinite distance. Why can we not define the potential of an infinite line of charge to be zero at r = *?
Two charged conducting spheres of different radii are connected by a conducting wire as shown in Figure 25.25. Which sphere has the greater charge density?
What determines the maximum potential to which the dome of a Van de Graaff generator can be raised?
Explain the origin of the glow sometimes observed around the cables of a high-voltage power line.
Why is it important to avoid sharp edges or points on conductors used in high-voltage equipment?
How would you shield an electronic circuit or laboratory from stray electric fields? Why does this work?
Two concentric spherical conducting shells of radii a = 0.400 m and b = 0.500 m are connected by a thin wire as shown in Figure Q25.15. If a total charge Q = 10.0 +C is placed on the system, how much
Study Figure 23.4 and the accompanying text discussion of charging by induction. You may also compare to Figure 25.24. When the grounding wire is touched to the rightmost point on the sphere in
The plates of a capacitor are connected to a battery. What happens to the charge on the plates if the connecting wires are removed from the battery? What happens to the charge if the wires are
A farad is a very large unit of capacitance. Calculate the length of one side of a square, air-filled capacitor that has a capacitance of 1 F and a plate separation of 1 m.
A pair of capacitors is connected in parallel while an identical pair are connected in series. Which pair would be more dangerous to handle after being connected to the same battery? Explain.
If you are given three different capacitors C1, C2, C3, how many different combinations of capacitance can you produce?
What advantage might there be in using two identical capacitors in parallel connected in series with another identical parallel pair, rather than using a single capacitor?
Is it always possible to reduce a combination of capacitors to one equivalent capacitor with the rules we have developed? Explain.
The sum of the charges on both plates of a capacitor is zero. What does a capacitor store?
Because the charges on the plates of a parallel-plate capacitor are opposite in sign, they attract each other. Hence, it would take positive work to increase the plate separation. What type of energy
Why it is dangerous to touch the terminals of a high voltage capacitor even after the applied potential difference has been turned off? What can be done to make the capacitor safe to handle after the
Explain why the work needed to move a charge Q through a potential difference ∆V is W = Q V whereas the energy stored in a charged capacitor is U = ½ Q ∆V. Where does the factor come
If the potential difference across a capacitor is doubled, by what factor does the energy stored change?
It is possible to obtain large potential differences by first charging a group of capacitors connected in parallel and then activating a switch arrangement that in effect disconnects the capacitors
Assume you want to increase the maximum operating voltage of a parallel-plate capacitor. Describe how you can do this for a fixed plate separation.
An air-filled capacitor is charged, then disconnected from the power supply, and finally connected to a voltmeter. Explain how and why the potential difference changes when a dielectric is inserted
Using the polar molecule description of a dielectric, explain how a dielectric affects the electric field inside a capacitor.
Explain why a dielectric increases the maximum operating voltage of a capacitor although the physical size of the capacitor does not change.
What is the difference between dielectric strength and the dielectric constant?
Explain why a water molecule is permanently polarized. What type of molecule has no permanent polarization?
If a dielectric-filled capacitor is heated, how will its capacitance change? (Ignore thermal expansion and assume that the dipole orientations are temperature dependent.)
If you were asked to design a capacitor where small size and large capacitance were required, what factors would be important in your design?
In an analogy between electric current and automobile traffic flow, what would correspond to charge? What would correspond to current?
Newspaper articles often contain a statement such as “10 000 volts of electricity surged through the victim’s body.’’ What is wrong with this statement?
What factors affect the resistance of a conductor?
What is the difference between resistance and resistivity?
Two wires A and B of circular cross section are made of the same metal and have equal lengths, but the resistance of wire A is three times greater than that of wire B. What is the ratio of their
Do all conductors obey Ohm’s law? Give examples to justify your answer.
We have seen that an electric field must exist inside a conductor that carries a current. How is it possible in view of the fact that in electrostatics we concluded that the electric field must be
A very large potential difference is not necessarily required to produce long sparks in air. With a device called Jacob€™s ladder, a potential difference of about 10 kV produces an electric
When the voltage across a certain conductor is doubled, the current is observed to increase by a factor of three. What can you conclude about the conductor?
In the water analogy of an electric circuit, what corresponds to the power supply, resistor, charge, and potential difference?
Use the atomic theory of matter to explain why the resistance of a material should increase as its temperature increases.
Why might a “good” electrical conductor also be a “good” thermal conductor?
How does the resistance for copper and for silicon change with temperature? Why are the behaviors of these two materials different?
Explain how a current can persist in a superconductor without any applied voltage.
What single experimental requirement makes superconducting devices expensive to operate? In principle, can this limitation be overcome?
What would happen to the drift velocity of the electrons in a wire and to the current in the wire if the electrons could move freely without resistance through the wire?
If charges flow very slowly through a metal, why does it not require several hours for a light to come on when you throw a switch?
In a conductor, changes in the electric field that drives the electrons through the conductor propagate with a speed close to the speed of light, although the drift velocity of the electrons is very
Two conductors of the same length and radius are connected across the same potential difference. One conductor has twice the resistance of the other. To which conductor is more power delivered?
Two lightbulbs both operate from 120 V. One has a power of 25 W and the other 100 W. Which bulb has higher resistance? Which bulb carries more current?
Car batteries are often rated in ampere-hours. Does this designate the amount of current, power, energy, or charge that can be drawn from the battery?
If you were to design an electric heater using Nichrome wire as the heating element, what parameters of the wire could you vary to meet a specific power output, such as 1 000 W?
Explain the difference between load resistance in a circuit and internal resistance in a battery.
Under what condition does the potential difference across the terminals of a battery equal its emf? Can the terminal voltage ever exceed the emf? Explain.
Is the direction of current through a battery always from the negative terminal to the positive terminal? Explain.
How would you connect resistors so that the equivalent resistance is larger than the greatest individual resistance? Give an example involving three resistors.
How would you connect resistors so that the equivalent resistance is smaller than the least individual resistance? Give an example involving three resistors.
How would you connect resistors so that the equivalent resistance is smaller than the least individual resistance? Give an example involving three resistors.
Given three lightbulbs and a battery, sketch as many different electric circuits as you can.
When resistors are connected in series, which of the following would be the same for each resistor: potential difference, current, power?
What advantage might there be in using two identical resistors in parallel connected in series with another identical parallel pair, rather than just using a single resistor?
An incandescent lamp connected to a 120-V source with a short extension cord provides more illumination than the same lamp connected to the same source with a very long extension cord. Explain.
Why is it possible for a bird to sit on a high-voltage wire without being electrocuted?
When can the potential difference across a resistor be positive?
Referring to Figure Q28.13, describe what happens to the lightbulb after the switch is closed. Assume that the capacitor has a large capacitance and is initially uncharged, and assume that the light
What is the internal resistance of an ideal ammeter? Of an ideal voltmeter? Do real meters ever attain these ideals?
A “short circuit” is a path of very low resistance in a circuit in parallel with some other part of the circuit. Discuss the effect of the short circuit on the portion of the circuit it
If electric power is transmitted over long distances, the resistance of the wires becomes significant. Why? Which method of transmission would result in less energy wasted—high current and low
Are the two headlights of a car wired in series or in parallel? How can you tell?
Embodied in Kirchhoff’s rules are two conservation laws. What are they?
Figure Q28.19 shows a series combination of three lightbulbs, all rated at 120 V with power ratings of 60 W, 75 W, and 200 W. Why is the 60-W lamp the brightest and the 200-W lamp the dimmest? Which
A student claims that the second lightbulb in series is less bright than the first, because the first bulb uses up some of the current. How would you respond to this statement?
Is a circuit breaker wired in series or in parallel with the device it is protecting?
So that your grandmother can listen to A Prairie Home Companion, you take her bedside radio to the hospital where she is staying. You are required to have a maintenance worker test it for electrical
Suppose you fall from a building and on the way down grab a high-voltage wire. If the wire supports you as you hang from it, will you be electrocuted? If the wire then breaks, should you continue to
What advantage does 120-V operation offer over 240 V? What disadvantages?
When electricians work with potentially live wires, they often use the backs of their hands or fingers to move wires. Why do you suppose they use this technique?
What procedure would you use to try to save a person who is “frozen” to a live high-voltage wire without endangering your own life?
If it is the current through the body that determines how serious a shock will be, why do we see warnings of high voltage rather than high current near electrical equipment?
Suppose you are flying a kite when it strikes a high-voltage wire. What factors determine how great a shock you receive?
A series circuit consists of three identical lamps connected to a battery as shown in Figure Q28.29. When the switch S is closed, what happens?(a) To the intensities of lamps A and B;(b) To the
If your car’s headlights are on when you start the ignition, why do they dim while the car is starting?
A ski resort consists of a few chair lifts and several interconnected downhill runs on the side of a mountain, with a lodge at the bottom. The lifts are analogous to batteries, and the runs are
At a given instant, a proton moves in the positive x direction through a magnetic field in the negative z direction. What is the direction of the magnetic force? Does the proton continue to move in
Two charged particles are projected into a magnetic field perpendicular to their velocities. If the charges are deflected in opposite directions, what can you say about them?
If a charged particle moves in a straight line through some region of space, can you say that the magnetic field in that region is zero?

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