# ~5 qestions

1. Jan 5, 2005

### 7bear

~5 qestions~

1. EM wave can be reflected by metal surface only. Right?
2. SONAR can be reflected by any materials. Right?
3. Diffraction only occur when parallel wavefront(not circular wavefront) passes through the gap. Right?
4. After we use cloth to rub some substances, some are positive charged or negative charged. Why?
5. electric force = electrostatic force?

2. Jan 5, 2005

### quasar987

We see the moon because the light from the sun reflects on its (non-metallic) surface. You see your surounding at night in your house because the light from the light bulb reflects on everything and enters your eye. Unless you meant something else by the question, here's your answer I guess.

3. Jan 6, 2005

### cepheid

Staff Emeritus
4. The rubbing causes electrons to be transferred from one material to another, leaving the material with fewer electrons with a net positive charge,and the material that gained electrons with a net negative charge. This buildup of static charge is possible only with insulators (dielectrics). In conductors, electrons would continue to flow around until everything was equalized. Whether a substance is more likely to lose or to gain electrons depends on its particular properties.

5. Talk about the fields that produce the forces: electrostatic fields are produced by stationary source charges, or by charge distributions whose charge density is constant with time. As I understand it, the term "electric field" is more general, referring to the field produce by any charge, whether in motion or not.

4. Jan 6, 2005

### 7bear

4. Whether a substance is more likely to lose or to gain electrons depends on its particular properties
Do u have any examples of those properties?

I have 2 more question.
1. Why do we use sound wave as SONAR, not radio wave? If we use radio wave, what will happen?
2. Why don't we put metal in oven? If we do so, what will happen?

5. Jan 6, 2005

### 7bear

u have explained that visible light can be reflected by non-metal substances.
How about others(i.e. radio wave, microwave, infra-red, ultraviolet, X-ray, gamma ray)? Can they be reflected by non-metal substances?

6. Jan 6, 2005

### DaveC426913

"Why don't we put metal in oven? If we do so, what will happen?"

I presume you mean microwave oven? The metal is very reactive to microwaves, and some of the metal will vapourize. You see this as a spark/flash. If you take the metal out and examine it, you will find pits where the metal has vapourized.

"How about others(i.e. radio wave, microwave, infra-red, ultraviolet, X-ray, gamma ray)? Can they be reflected by non-metal substances?"

Yes. Generally, the higher energy EM will get through the material more. What doesn't get through will be partially absorbed and converted to heat. Some will be reflected.

eg: you will sunburn faster when playing in the surf than when on the shore. Why? Because you are receiving twice as much sunlight (UV) from the surface of the water.

7. Jan 6, 2005

### Integral

Staff Emeritus
Sorry Dave,
This is not very accurate.

Microwaves induce currents in metal, this is seen by the microwave source as a large load (very low resistance) it is possible that these unexpected currents can damage the microwave source.

The sparks will form at any edges or points, as they tend to be concentrators of charge.

As for your other list of interactions, there are really simply to many variables to give a useful answer. The answer must be "It depends"

The reason we have sonar, is that sound waves are carried by water much better then radio waves, in addition, much of "sonar" is passive, that is they simply listen with out transmitting.

Radar is much longer ranged and is only useful in the atmosphere (or above).

8. Jan 6, 2005

### Gonzolo

3. Diffraction can happen with any wavefront.
5. An electrostatic force is an electric force that does not vary with time.

9. Jan 6, 2005

### Staff: Mentor

Because the "S" stands for sound and the "R" for radio. Otherwise, they are pretty similar concepts. If you mean how do you choose whether to use sonar or radar, that was hit already. One little expansion: you can use sonar out of the water though: a large number of industrial sensors are sonar (I bought my parents an ultrasonic stoplight for their garage - it measures how far your car is from the wall).
Some yes, some no. And its even frequency dependent within the different classes - ie, some frequencies of radio waves will be reflected by other materials.

Last edited: Jan 6, 2005
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