A2 Physics | Post your doubts here

[smzimran]

1. oh ok , now i get it! thank you
2. uhmm, the bandwidth of an FM wave , like an AM , max sidebands x 2 right? Another question : I dont understand how there are frequency sidebands for AM. I thought only amplitude changes?
But I meant for example, the bandwidth of an FM radio station is about 200kHz
and the maximum audio frequency broadcast is 15kHz. How can I find the audio frequency when only bandwidth is given?

I also have confusion why sidebands for AM exist!

I dont think you can find audio frequency using only bandwidth

 

[SkyPilotage]

http://www.google.com.pk/url?sa=t&rct=j&q=work done on system positive or negative&source=web&cd=4&ved=0CHkQFjAD&url=http://physics.bu.edu/~duffy/py105/Firstlaw.html&ei=07WsT8aIEsTN4QS39tDWAw&usg=AFQjCNF_O85WY6aHb-huo2PgboDfTPKwTw

see this also............this makes clear wht i said two sources cant be wrong?

Never said anything was wrong...It depends on what formula you are using.
Stick to the CIE formula and mentality :- INCREASE IN INTERNAL ENERGY = THERMAL ENERGY SUPPLIED TO SYSTEM +++++ WORK DONE ON THE SYSTEM.
The other formula is mentioned in sources is :- INCREASE IN INTERNAL ENERGY = THERMAL ENERGY SUPPLIED TO SYSTEM -------- WORK DONE BY THE SYSTEM.
P.S:- Capslock was used just to make it clearer, no feelings involved

 

[SkyPilotage]

I also have confusion why sidebands for AM exist!

I dont think you can find audio frequency using only bandwidth

We have to clear this solution cz its a main point in the syllabus.
(b) Candidates should be able to recall that a carrier wave, amplitude modulated by a single audio
frequency, is equivalent to the carrier wave frequency together with two sideband frequencies.

 

[SkyPilotage]

Never said anything was wrong...It depends on what formula you are using.
Stick to the CIE formula and mentality :- INCREASE IN INTERNAL ENERGY = THERMAL ENERGY SUPPLIED TO SYSTEM +++++ WORK DONE ON THE SYSTEM.
The other formula is mentioned in sources is :- INCREASE IN INTERNAL ENERGY = THERMAL ENERGY SUPPLIED TO SYSTEM -------- WORK DONE BY THE SYSTEM.
P.S:- Capslock was used just to make it clearer, no feelings involved

Please do not memorize formulas blindly. Understand that when a question asks for work done on gas then compression is positive. But when a question asks for work done by gas then compression is negative. So if you are asked to find the Increase in internal energy when there is compression, use thermal energy + work done on gas OR thermal energy - work done by gas.
Work done on gas = - (Work done by gas) See the difference? Its simple!

 

[smzimran]

We have to clear this solution cz its a main point in the syllabus.
(b) Candidates should be able to recall that a carrier wave, amplitude modulated by a single audio
frequency, is equivalent to the carrier wave frequency together with two sideband frequencies.

If you get this cleared, please do share !

 

[SkyPilotage]

If you get this cleared, please do share !

thts a not smart thing to say, course I will

 

[SkyPilotage]

functions of cellular exchange:
permits entry to PSTN
selects base station for any handset
allocates a carrier frequency/channel
monitors handset signal to re-allocate base station
allocates time slot for multiplexing
now cn anyone xplain first and last point.....and also tell what happens when a call is made from mobile to pstn telephone

First point, means it allow the mobile to call telephones.
Last point, TDM involves sequencing groups of a few bits or bytes from each individual input stream, one after the other, and in such a way that they can be associated with the appropriate receiver. If done sufficiently quickly, the receiving devices will not detect that some of the circuit time was used to serve another logical communication path.
http://en.wikipedia.org/wiki/Multiplexing

Hassam more hard practice questions!! dont stop!! maybe some more singapore past papers! Your doin an awesome job buddy!

 

[SkyPilotage]

Curious question:-
Why did the microphones in the 1950s-1990s kinda make your voice changed. Like if you watch a really old video, they nearly have this same kinda of tone which is not their real true voice.
Did they use different circuitry or signals were not well amplified or what?

 

[SkyPilotage]

Bandwidth of fibre optic cables transmissions??
In coaxial wires, the braid act as a return wire for the signal.
Where is the return pathway for the signal in fibre optic cables? Another cable?

 

[hassam]

Bandwidth of fibre optic cables transmissions??
In coaxial wires, the braid act as a return wire for the signal.

Where is the return pathway for the signal in fibre optic cables? Another cable?[/quote
no idea....bt one point on bandwidth,,.....coaxial cable has higher bandwidth cos it has electromagnetic sheilding.....changing currents inside cables produce em waves which radiate energy......so as frequency of signal increases attenuation increaSES cos electromagnetic waves are produced more often ......THATS WHY wire pairs have the samallest bandwidth nd optic fibre have largest cos signal is send as electromagnetic wave which is confined within the glass due to total internal reflection

 

[hassam]

name the two mechanism that cause attenuation in wire pairs?
The changing currents in the wires themselves
produce electromagnetic (EM) fields and this makes
the wires act as aerials, radiating EM waves. The
energy to emit these waves reduces the strength of
the signal sent along the wire. As the frequency of the
changing current increases, the emission of EM waves
increases and so the bandwidth of wire-pairs is law.
Energy is also lost in the wire due to electrical heating
in the resistance of the wires.

 

[omg]

http://www.xtremepapers.com/papers/...and AS Level/Physics (9702)/9702_s08_qp_4.pdf
q.4 part c

 

[angelgirl:)]

Please do not memorize formulas blindly. Understand that when a question asks for work done on gas then compression is positive. But when a question asks for work done by gas then compression is negative. So if you are asked to find the Increase in internal energy when there is compression, use thermal energy + work done on gas OR thermal energy - work done by gas.
Work done on gas = - (Work done by gas) See the difference? Its simple!

cud u plz relate the compression , the work done and the equation ...all together...coz i got more confused...

 

[hassam]

Bandwidth of fibre optic cables transmissions??
In coaxial wires, the braid act as a return wire for the signal.
Where is the return pathway for the signal in fibre optic cables? Another cable?

well.....m not sure....bt i belive that with coaxial cables and wire pairs u have signal in form of current...(an electrical signal)so u need to complete the circuit thats why there's return for the signal whereas optic fibre ahs signal in form of pulse of light....not changing CURRENT or P.D so no need for return

 

[SkyPilotage]

cud u plz relate the compression , the work done and the equation ...all together...coz i got more confused...

Never said anything was wrong...It depends on what formula you are using.
Stick to the CIE formula and mentality :- INCREASE IN INTERNAL ENERGY = THERMAL ENERGY SUPPLIED TO SYSTEM +++++ WORK DONE ON THE SYSTEM.
The other formula is mentioned in sources is :- INCREASE IN INTERNAL ENERGY = THERMAL ENERGY SUPPLIED TO SYSTEM -------- WORK DONE BY THE SYSTEM.
Compression = Work done on gas = - work done by gas
Expansion = Work done by gas = - work done on gas

You can just use the first formula mentioned in our syllabus, otherwise, press the reset button and forget everything I said to return to your original unconfused state

well.....m not sure....bt i belive that with coaxial cables and wire pairs u have signal in form of current...(an electrical signal)so u need to complete the circuit thats why there's return for the signal whereas optic fibre ahs signal in form of pulse of light....not changing CURRENT or P.D so no need for return

Informations needs to go and come back... So I really dont support that concept much.

 

[prannoy]

Can any one help me what is mean voltage exactly ? In Ac currents?

 

[smzimran]

June 08
P4
Q1 (c)

If centripetal force increases shouldn't the object be more likely to contnue its circular motion ??
http://www.xtremepapers.com/papers/...and AS Level/Physics (9702)/9702_s08_qp_4.pdf

http://www.xtremepapers.com/papers/...and AS Level/Physics (9702)/9702_s08_ms_4.pdf

 

[touchedbyHisNoodliness]

Can any one help me what is mean voltage exactly ? In Ac currents?

Do you mean the root mean squared(rms) voltage? It is simply the amount of direct voltage that gives the same Power to the resistor as the AC. That's how I've understood it..

 

[angelgirl:)]

You can just use the first formula mentioned in our syllabus, otherwise, press the reset button and forget everything I said to return to your original unconfused state

hmmm....

 

[shiningstar]

Please I need your help in this question.

A 850g of copper is being heated by a heater to determine its specific heat capacity.

The block is initially at 12*C. The heater is switched on and the time taken for the temperature to rise to 30*C is recorded. The Block is cooled to its original temperature, the potential difference across the heater is increased ad the experiment is repeated.

The results are as follows:

It took a 40 W power output of heater --> 190s

and a 60 W power output of heater --> 114s

Calculate the specific heat capacity of copper.

Thank you