Online study session..Hsc success (1 Viewer)

[FcUk]

Hey guys these are the rules:
If you have a question you want answered you must first answer the question posed and then u may post yours, only one question may be posted at the time this way we all study the same stuff, ask anything even if you already know it so we can all help each other out and soon enough we will have our own little study notes...please keep comments that arnt physics related off the thread so its more concentrated
ill start..somethin easy

Q) what is the nature of cathode rays and what experiments are able to prove this?

 

[tennille]

Cathode rays are particles (electrons)- experiments used to improve this include the paddle wheel experiment. It proved the cathode rays had momentum and therefore a particle nature. They are deflected by a magnetic field and later by an electric field (by J.J. Thompson), suggesting they are negatively charged. The maltese cross experiment proved that it orignated from the cathode and that the particles travelled in straight lines. The lectric field experiment, however, is very difficult to perform at school (due to the fact that the pressures aren't low enough in the tubes).

 

[tennille]

If I did get the answer right, I was wondering if you could tell me how photcells and solar cells work because I'm really confused on that stuff.

 

[FcUk]

Photocells involve electrons initiating an electric current which is produced by the photoelectric effect. It consists of cathode n anode. When light falls on the cathode (coated with photosensitive material) it emits electrons. these electrons are accellerated to the anode resulting in a current which is proportional to the intensity of light falling on the cathode. this is attached to a measuring device. includes: electric eyes, radiation detectors and light meters

Solar cells are a little more confusing it involves joining both p and n type silicon (p-n junction) to make the silicon neutral. Diffusion takes place and a current is produced by p and n diffusing into each other causes an electric charge, and also allows electrons to drift'. when exposed to light 'photo-ionisation' takes place doing a number of things including setting up an emf. proportional to intensity of light. metal contacts take the electrons. to put it simply light energy is converted into electrical energy. PHEW!!! theres alot more that goes with this but it gets a bit complicated from there

i dont think we even needd to kno this for the test but anyway there it is.

Q) What contributions have the discovery of AC and DC had on our society. outline adv/disadv of each...

 

[tennille]

It's a secondary source investigation- thanks for that.

 

[aim54x]

shit u guys are serious.
Rise of use in electrical power. AC with transformers provide efficient transmitting of elec. talk bout negatives, pollution, resources.

AC - easy to transport, easy to change voltage, high voltages dangerous
DC - safer, no voltage issues, hard to transport efficiently, hard to change voltage

 

[FcUk]

shit u guys are serious.
Rise of use in electrical power. AC with transformers provide efficient transmitting of elec. talk bout negatives, pollution, resources.

AC - easy to transport, easy to change voltage, high voltages dangerous
DC - safer, no voltage issues, hard to transport efficiently, hard to change voltage

well if ur like me and dont like to 'officially study' i thought this would be a good way to go through stuff by jst reading it online..

oh BTW u have to ask a question

 

[t-i-m-m-y]

HSC succes.. by online study? haha

 

[CrashOveride]

im quite sure u dont need to know about photo cells or solar cells

 

[Xayma]

I'm pretty sure you do, I think its on the right hand side of the photoelectric part.

And btw. photocells also rely on the fact that resistance is inversly proportional to the amount of light falling on it, this is how breathalysers work.

 

[mr EaZy]

AC - easy to transport,
,

lol , u can't transport electricity

im sure u mean transmission
transporting would be like batteries and stuff

 

[speersy]

O.K. since that question is answered i will ask
Q) why if the smaller the radius of a satellites orbit the larger the velocity does a satellite spiral towards earth. Like if the resistance causes it to slow down wouldn't it want to go into a higher orbit?

 

[snoopwogg]

Net force is mass times acceleration. For an orbiting satellite, it has a net force in the downwards direction, towards teh centre of the earth. Hence, if there is an excess of resistance, the easier it is for the gravity to pull it down and hence the faster it spirals down towards the earth. Work has to be done to the object to move it from a point of less gravitational potential energy to one of greater gravitational potential energy. I might be blowassing here, but i thinik resistance loweres the momentum the satellite has, thus making it more vulnerable to the force of gravitation - DOWNWARDS. In order for the satellite to move up into a higher orbit, it has to be given kinetic energy, but with resistance, it loses it..?

 

[zeropoint]

Net force is mass times acceleration. For an orbiting satellite, it has a net force in the downwards direction, towards teh centre of the earth. Hence, if there is an excess of resistance, the easier it is for the gravity to pull it down and hence the faster it spirals down towards the earth. Work has to be done to the object to move it from a point of less gravitational potential energy to one of greater gravitational potential energy. I might be blowassing here, but i thinik resistance loweres the momentum the satellite has, thus making it more vulnerable to the force of gravitation - DOWNWARDS. In order for the satellite to move up into a higher orbit, it has to be given kinetic energy, but with resistance, it loses it..?

Sort of, but not really. As the satellite approaches the planet, the net force it experience increases in an inverse square fashion according to Newton's law of universal gravitation. This increases the orbital velocity as may be seen from the equality mv^2/r = GMm/r^2 which implies v = Sqrt[GM/r]. Orbital drag has the effect of performing negative work on the satellite, decreasing the total energy (not the momentum) of the system and thereby decreasing the radius of the orbit.

 

[snoopwogg]

Ahhh yeah thats it. THanks for clearing that up. Maths solves everyting! I hate using words, i can never communicate me idea properly. Gotta love that law, u can make it equal to anything you want.