Physics

anpanman SG

1. what happens when 2 materials are electrostatically charged by friction?

A one material gains a net +ve charge while the other gains a net -ve charge

B the net charges on both materials remain neutral

C the materials become either both -vely charge or both +vely charged

D the same charges are removed from both materials

Ans:A (for which I do not understand. Can someone provide a diagram for this if possible?)

anpanman SG

Which of the following regarding sound waves and electromagnetic waves is/are correct?

(i) The speed of sound is solower than the speed of all electromagnetic waves (agree)

(ii)The frequency of a sound wae is less than the frequency of all electromagnetic waves

(iii)The wavelength of a sound wave is longer than the wavelength of all electromagnetic waves

Ans: (i) only. Don't know why (ii) and (iii) are invalid. I can't prove their invalidity using any formulas.

ThunderFbolt SG

1. don't think diagram will explain. but when electrostatically charged, electrons from one material will "jump" over to the other material, causing one to have an overall positive charge and the other an overall negative charge lor.

said electrostatically charged so something should happen. (won't be B)

can't be both positive or both negative since electrons must come from somewhere and go somewhere (can't be C or D)

2. answer doesn't seem right. for one, i think the frequency of sound is limited only by the distance between 2 molecules, so can vary to be bigger or smaller than light's. same for wavelength. think best to ask your teacher for this.

yiha093 SG

formula for wave spd

v=f x wl

try this formula ba.

eagle SG

For a fact, wavelength of radio waves are in metres, and frequencies are in the MHz. And we know that radio waves have the longest wavelengths and shortest frequencies of EM waves.

Sound waves....
Human hearing range is 20 Hz~20 kHz, so super sonic sounds.... say 40 kHz (dogs hearing range)

330m/s / 40kHz gives < 1m for wavelength.... so already, (iii) is not correct

Also, sound wave doesn't necessary need to travel through air, they can travel through solids as well. So wavelengths can go even smaller :)

Frequency of sound wave depends on how fast the wave particle is moving. There are no limitations on it. Do check out what ultrasound is:

The above is from wiki. Note that sound wave can be 200MHz and above. FYI, radio waves can be in 93.8 MHz (that's why your radio station 93.8)... So seeing this, (ii) is also not correct.

eagle SG

Oh ya, for question 1, it's by conservation of charge.

Charge can't go anywhere... If one side loses electrons, the other side must gain the electrons. So D is wrong. C is also wrong. And if there's an exchange in charges, B is also wrong.

So left with A.