please help , physics doubts !

[irlzs95]

hey guyz i have some doubts in physics paper 3

first thing , when excatly to we use sin i / sin r nd when do we use sin angle in air/ sin angle in medium ???

second , pp qs

http://www.xtremepapers.me/CIE/Cambridg ... 2_qp_3.pdf Q9 part a

also
http://www.xtremepapers.me/CIE/Cambridg ... 6_qp_3.pdf
q2 , part c , my teacher told me to choose any angle between the resultant line nd the other line nd say , forexample here , 44 degrees toward 8N
but in the marking scheme they said , vertically upward , hw dd they noe nd its not correct my teachers way ?
also q 7 , part d ??

nd http://www.xtremepapers.me/CIE/Cambridg ... 7_qp_3.pdf
q7 , part bi , ?? its angle of incidence ? supposedly angle of refraction is the 90 !
nd q10 , part b




nd also wen they say speed in less or more than calculated or k.E is more or less than calculated , wen do we say lost as heat nd wen do we say used to increase in K.E ?

nd finally can someone helpt me with the radioactivity experiments , the one to say count background radiations nd then subtract the countrate , etc ,,,,,
is there a criterion ???



thaaank you very much in advance

 

[irlzs95]

ohh nd can someone explain these points in the syllabus ?
• Describe how electricity or other useful forms
of energy may be obtained from:
– chemical energy stored in fuel
– water, including the energy stored in
waves, in tides, and in water behind
hydroelectric dams
– geothermal resources
– nuclear fission
– heat and light from the Sun (solar cells
and panels)
• Give advantages and disadvantages of each
method in terms of cost, reliability, scale and
environmental impact


sorry :$

 

[medevil]

You use sini/sinr when the light ray travels from air into the glass where angle of refraction would be lesser than angle of incidence
the second formula u got it wrong. Its speed of light in air /speed of light in medium thats wats in our syllabus and u use it when they mention the speed in both mediums

i dont know the answer for year mayjune 2002 even i have a serious doubt in tht part

but for the question in 2006mayjune if u notice the pulleys are connected to pull T3 up. when u draw the vector diagram u get a resultant force of combined force of t1 and t2 so when both are combined the weight t3 is pulled up so its direction is upwards
For question 7d
Whenever they say measure distance in wave diagram they are referring to something related to the wavelength. If u notice the wavelength in that diagram is when a full wave is completed. That would be the point where the graph touches the x axis the second time. If u look carefully this distance is double of that of PX. Find wavelength using the given data using formula v=frequency x wavelength then divide by 2

for may/june 2007 i think u mean question 6bi
in that question angle of they already told u the angle of incidence is almost parallel to AE which is the edge of the block and is 90degrees to normal so a range of 88-90 should be the answer

question 10b instead of the usual resistor u use a variable resistor
Because using the variable resistor u can adjust the voltage needed by the current to pass through it
suppose you make resistance high then a higher voltage would be needed for the variable resistor so its total voltage share of the power supply increases and transistor works
to gain a greater share of the power supply the thermistors resistance has to be high and that can happen if the temperature is lower than usual even lower than that it would need if the normal resistor is used
in this way you can vary the temperature chage need by changing resistance
Hope this helped(H)

 

[irlzs95]

thank you alot
but r u sure sin angle air / sin angle in medium isnt included ?? but our teacher gave to us :S ?!

thx again
just left with other qs

 

[unplugged]

1. Remember it like this, the refractive index is equal to (Speed of light in air) / (Speed of light in medium) = sin (angle in air) / sin (angle in medium). It's always tied to the
refractive index. And remember that speed of light in air is greater than any medium so
the refractive index is always larger than one and therefore the upper part of the fraction
when doing sin i / sin r , should always be bigger.


2. Since the input is 1v DC, it should basically be a straight line using X amount of squares
in height to represent 1 volt. So you can basically connect it twice , switching the
terminals every time and taking the reading. Then Dividing the distance between the two
readings by two to get the scale for 1 volt.

3. i) What your teacher said isn't necessarily wrong , because it's in reference to something
else. If i get a horizontal line and said that there's another line perpendicular to it,
it's the same as saying that the line is vertical. But seeing as this is a case of
gravity , the force of gravity is pulling the object down , so the strings are
obviously pulling it up so the resultant is vertically up.

ii) Basically you know that speed = frequency x wave length and from observation you can
see that PX is half the wave length.

4. i) When trying to measure the critical angle , always imagine the ray originating from the
denser medium. So when the refracted ray(but we'll think of it as incidence) was at 43
, total internal reflection occurs or almost occurs as we can see from the statement
(almost parallel to AE) , so 43 is either the critical angle or just a little bit
smaller)

ii) By modifying the resistance of the bottom resistor. Since the transistor gets
a voltage = to the P.D. across the bottom resistor. So changing the resistance on the
bottom resistor will cause the lamp to turn on earlier at higher temperatures or turn
off later at colder temperature.

5. If K.E. decreases it's usually converted to heat or sound, saying speed is converted
to K.E. energy is meaningless.

6. In experiments to gauge radiation reduction. First you take count without source present,
then with source present and then with the aluminum sheets or whatever present.
You subtract the first reading from the others because the first was simply background
radiation and not related to the source. So subtraction gives you only the radiation from
the source.

7. i) In fossil fuel based energy plants , the heat acquired from burning fuel is used to
turn water into steam and then this water is used to turn turbines which in turn
activate generators that turn this kinetic energy into electricity.

ii) Hydroelectric and wave energy is the same , the kinetic energy from the waves or
the water stored behind a dam(making it seep out slowly) is used to turn turbines
which in turn activate the generators.

iii) Geothermal is steam stored underground , it's utilized in the same way that
steam in fossil fuel burning plants is used.

iv) Nuclear fission is the process of splitting atoms of uranium or plutonium and
this process causes a huge release of energy in the form of heat energy , which
then used to generate steam same as the fossil fuels.

v) Solar cells are actually pretty complicated in structures , but suffice to say that
heat is acquired from the sun rays and used to convert water into steam.

vi) 1. Fossil fuels are rare and hence expensive. We're running out of them so they're not
reliable. They pollute the environment due to the release of carbon dioxide.

2. Geothermal is expensive because of the drilling required and it's environment
friendly.

3. Hydro electric is expensive and causes environmental damage

4. Nuclear is expensive due to high safety standards. The waste from nuclear fuel is
very dangerous and stays radioactive for years.

5. solar cells are expensive and it's environmental friendly.