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In a certain region of space, the electric field is uniform. It can be proved that the volume charge density must be zero based on the Gauss's law.

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更多“In a certain region of space, …”相关的问题

第1题

In which of the following contexts can Gauss’s law be easily applied to find the electric field?

A、A point at a distance L from a uniformly charged wire with length L.

B、Near the surface of a large uniformly charged plane sheet.

C、Beside a charged disk with radius R.

D、Outside a uniformly charged cube of length L.

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第2题

The most popular of all powered personal care appliances is the electric razor. And with refinements in design and performance, its popularity continues to grow.

Three kinds of motors are used in electric shavers. Some contain a tiny universal motor, others contain a vibrator type motor. Cordless razors contain NiCad cells, miniature DC motor, and charging components all within the shaver unit. A scraper or series of scrapers attached to the motor moves in reciprocating fashion—that is, in a back-and-forth, straight line way.

Reciprocating motion is established by means of a small pinion. Essentially, the pinion and scraper are connected by a stud set in an offset position in the pinion. Thus, the scraper is prevented from making a full-circle turn as the motor revolves; its travel is limited to the size of the razor head.

There are variations in the way different makes of electric razors are built, but the principle just described applies to all, with the exception of those that employ rotary heads.

Electric razors generally use very thin perforated shields to enclose the whiskers as skin is pressed against the shields. The rapidly moving scraper shears off the hairs extending through the surface of the shield.

The power source to operate an electric shaver's motor may come from a wall outlet by means of a line cord or through a tiny nickel-cadmium battery in cordless electric razors. Batteries of cordless electric razors have to be recharged periodically by being connected to a wall outlet through use of a line cord.

PROBLEMS, CAUSES, REPAIRS

The electric razor is one appliance that owners—even those with technical ability--cannot repair. Replacement parts are not usually available, not even to professional appliance technicians. The shaver must be returned to the factory when a problem occurs.

The key word to keep in mind regarding caring for your electric razor is "clean." The razor should be brushed clean of hair after every use to maintain its effectiveness and extend its life.

How many types of motors are generally used in the shaver?

A．Two.

B．Three.

C．Four.

D．Not known.

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第3题

It can be inferred from the passage that by the eleventh century the Byzantine militaryforces

A．had eliminated the Bulgarian army.

B．were comparable in size to the army of Rome under Augustus.

C．were strong enough to withstand the Abbasid Caliphate's military forces.

D．had achieved control of Byzantine governmental structures

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第4题

It can be inferred that by the eleventh century the Byzantine military forces ______.

A．had reached their peak and begun to decline

B．had eliminated the Bulgarian army

C．were comparable in size to the army of Rome under Augustus

D．were strong enough to withstand the Abbasid Caliphate's military forces

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第5题

It can be inferred that by the eleventh century the Byzantine military forces______.

A．had reached their peak and begun to decline

B．had eliminated the Bulgarian army

C．were comparable in size to the army of Rome under Augustus

D．were strong enough to withstand the Abbasid Caliphate's military forces

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第6题

It can be inferred from the passage that [A] the "code of silence" often helps protect guards from violence. [B] the governor's office has decided to give a new life to the prisons. [C] the solution to overpopulation in prisons lies in softening sentences. [D] the prison population calls for more prison-building programs.

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第7题

It can be concluded form. the passage that rubbish is a potential remedy for the shortage of raw material.

A．Right

B．Wrong

C．Not mentioned

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第8题

It can be learned from the last paragraph that______.

A．genetic tests for most of the diseases can be produced now

B．gene tests are only suitable for those who have a strong family history of disease

C．almost all diseases can be cured with the development of the gene testing