A2 Physics | Post your doubts here

[princesskt]

Exactly! you need to push and exert a force to compress a gas! Therefore you have to make some work to compress it i.e work is +ve!

no...........
when work is done by gas means expansion work 1s +ive
compression taken as -ive work
check it out in any book..........plz make it clear..

 

[SkyPilotage]

no...........
when work is done by gas means expansion work 1s +ive
compression taken as -ive work
check it out in any book..........plz make it clear..

There is a difference between work done on the gas and work done by the gas.
Compression = work done on gas = +ve.
Expansion = work done by gas = -work done on gas = -ve

 

[smzimran]

Can someone please explain me O/N 2010 43 Q 8(b) ??
I saw the MS but still dont understand.. Help needed....

The threshold wavelength is 340 nm as calculated in the ms

Now, we know that any wavelength smaller than or equal to threshold will give rise to emmision
So, the ultraviolet light of wavelenth 120nm (smaller than threshold) will give rise to emission

Whereas, the second lamp has wavelength 450nm (greater than threshold wavelength) so it will not cause emission, the number of incident photons per unit area remain constant and therefore, the photoelectric current is constant.

 

[kagome]

Help please... Specimen Paper P4 Q 11 (c) ?? Which formula to apply & why?
Thx.

 

[SkyPilotage]

b) 63 db = 10 log ( P out / P in )
c) attenuation dB = 10 log ( Pout / Pnoise )
length = attenuation / length x attenuation

 

[SkyPilotage]

Q1-How is the anode in an x-ray tube held at earth potential when electrons are accelerated through a voltage?
Q2-How do we get the frequency of the transmission when we have the bandwidth provided?
I know for AM waves, we divide bandwidth by 2. How about for FM and other means of communications?

 

[princesskt]

There is a difference between work done on the gas and work done by the gas.
Compression = work done on gas = +ve.
Expansion = work done by gas = -work done on gas = -ve

http://books.google.com.pk/books?id=WN6TuD8gTYEC&printsec=frontcover#v=onepage&q&f=false
see page 472 plzzzzzzzzzzzzzzzzzzzzzzzzzzz

 

[kagome]

b) 63 db = 10 log ( P out / P in )
c) attenuation dB = 10 log ( Pout / Pnoise )
length = attenuation / length x attenuation

Can you please list the formula that is available in this topic please..?
I'm sort of confusd. Thanks

 

[SkyPilotage]

http://books.google.com.pk/books?id=WN6TuD8gTYEC&printsec=frontcover#v=onepage&q&f=false
see page 472 plzzzzzzzzzzzzzzzzzzzzzzzzzzz

I'm afraid the book has the "First Law of Thermodynamics" stated rather unclearly.
It states that Thermal Energy = Internal Energy + Work Done BY system (gas)
Rearrange to give Internal Energy = Thermal Energy - Work Done BY system. Nothing fancy here. They are assuming work done by system as postive which will be subtracted from thermal energy.
But as stated in our CIE syllabus, the examiners would rather prefer to use work done ON system
Internal Energy = Thermal Energy + Work Done ON System.
Therefore, if the gas is compressed, then work is done on the system hence +VE!
My advice to you, stick to the CIE mentality to avoid confusions.

 

[kagome]

b) 63 db = 10 log ( P out / P in )
c) attenuation dB = 10 log ( Pout / Pnoise )
length = attenuation / length x attenuation

I would like to ask. Isn't the output power calculated in b is the input poeer to the wire in c?
Then why ur formula is Pout / Pnoise
Sorry i'm a bit confused here

 

[SkyPilotage]

I would like to ask. Isn't the output power calculated in b is the input poeer to the wire in c?
Then why ur formula is Pout / Pnoise
Sorry i'm a bit confused here

I'm sorry you kinda lost me there. They wanna reduce the signal power to noise level.
The signal power is the output power of the microphone after it is amplified. I used Pout= after amplification and Pin = before amplification.

 

[hassam]

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

 

[smzimran]

Q1-How is the anode in an x-ray tube held at earth potential when electrons are accelerated through a voltage?
Q2-How do we get the frequency of the transmission when we have the bandwidth provided?
I know for AM waves, we divide bandwidth by 2. How about for FM and other means of communications?

1. The cathode is at a high potential which repels electrons so they are accelerated!

2. The frequency deviation is given you use it to calculate frequency

 

[SkyPilotage]

WELL this question suggest we must memorise that table in booklet

which year is this? if you have the answer, do we shade the two boxes at the right for satellite communiations because the cover from 10 cm to 1 mm.
My advice to you, just memorize the frequencies. You can use 3x10^8 = Wavelength x frequency to find the wavelength.

 

[SkyPilotage]

1. The cathode is at a high potential which repels electrons so they are accelerated!

2. The frequency deviation is given you use it to calculate frequency

1.according to the diagram, the anode is at a higher potential in the app booklet.
2.didnt quite get what you mean, example plz?

 

[smzimran]

1.Sorry the cathode is at a lower potential ( very negative ) so it repels electrons and a potential difference is created (anode at earth while cathode at negative potential)

2. see jun2011 p41 q11 (b)
http://www.xtremepapers.com/papers/...nd AS Level/Physics (9702)/9702_s11_qp_41.pdf

 

[angelgirl:)]

There is a difference between work done on the gas and work done by the gas.
Compression = work done on gas = +ve.
Expansion = work done by gas = -work done on gas = -ve

plz confirm it, coz iam confused now ...as i read that work done by the gas which is expansion is +ve and vice versa...

 

[SkyPilotage]

1.Sorry the cathode is at a lower potential ( very negative ) so it repels electrons and a potential difference is created (anode at earth while cathode at negative potential)

2. see jun2011 p41 q11 (b)
http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Physics (9702)/9702_s11_qp_41.pdf

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?

 

[princesskt]

http://www.google.com.pk/url?sa=t&r...9tDWAw&usg=AFQjCNF_O85WY6aHb-huo2PgboDfTPKwTw

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

 

[SkyPilotage]

plz confirm it, coz iam confused now ...as i read that work done by the gas which is expansion is +ve and vice versa...

Correct ! Work done by gas is +ve with expansion!
Work done by gas is +ve if you are using the formula Delta U = Thermal energy - Work done by gas
But Work done by gas is -ve if you are using the formula U = Thermal energy + Work done on gas
Get the difference? Pleasee, press the reset button! I didnt mean to confuse anyone :S