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# A2 physics watch

1. I think any problem i have now, i will post it all on this thread. This way it saves time from having to create a new thread each time.

Query 1(solved): Expansion of the universe. There are 3 possible scenarios.
(1) - open universe - this is where there is not enough matter hence the effect of gravity is not sufficient enough to stop the universe from expanding

(2) - critical universe- same as the open universe but there is slightly more matter but the universe will stop expanding from an infinite point in time i believe?

(3) - closed universe - there is enough matter and halt the expansion and cause the universe to contract. But what is it contracting to?

Is this right? if there is anything else to include that you deem important please do.

Query 2 (solved): "At present, estimates of the amount of ordinary matter in the universe suggest an open universe. However there is evidence for matter that cannot be detected using the emission or absorption of radiation. This DARK MATTER might be enough to produce a closed universe" Okay if it can't be detected how do they know it's present? are they looking at the gravitational effect of it? if so, how are they doing that?

Query 3 : Simple harmonic this is relating to. So the condition in order for SHM to work is that F = -kx.... which means that the force acting on the system needs to be in opposite direction to the displacement from the equilibrium position. Correct? Now does this only apply to "springs" cause in order for this work does it also have to obey Hooke's law? which is basically the above equation.

Query 4 : this is straight from the spec "explain the principles involved in continuous flow technique to measure thermal energy transfer" is this related with the formula E = mcDtheta?

Query 5 : what is difference between terminal potential difference and emf? (problem solved)

Query 6 : Lenses : if the focal length of an lens is 30 cm would it be right to say that image distance will be at 30cm too?
2. anyone?
3. anyone?
4. (Original post by boromir9111)
I think any problem i have now, i will post it all on this thread. This way it saves time from having to create a new thread each time.

Query 1: Expansion of the universe. There are 3 possible scenarios.
(1) - open universe - this is where there is not enough matter hence the effect of gravity is not sufficient enough to stop the universe from expanding

(2) - critical universe- same as the open universe but there is slightly more matter but the universe will stop expanding from an infinite point in time i believe?

(3) - closed universe - there is enough matter and halt the expansion and cause the universe to contract. But what is it contracting to?

Is this right? if there is anything else to include that you deem important please do.

Query 2 : "At present, estimates of the amount of ordinary matter in the universe suggest an open universe. However there is evidence for matter that cannot be detected using the emission or absorption of radiation. This DARK MATTER might be enough to produce a closed universe" Okay if it can't be detected how do they know it's present? are they looking at the gravitational effect of it? if so, how are they doing that?
ermm i am doing the wjec spec.
In closed universe there doesn't exist Dark energy i believe so no repulsive effect, as a result gravity stops the expansion of the universe, the universe contracts and there is no further progress to a point.
This is also known as Big Crunch.
5. (Original post by boromir9111)
I think any problem i have now, i will post it all on this thread. This way it saves time from having to create a new thread each time.

Query 1: Expansion of the universe. There are 3 possible scenarios.
(1) - open universe - this is where there is not enough matter hence the effect of gravity is not sufficient enough to stop the universe from expanding

(2) - critical universe- same as the open universe but there is slightly more matter but the universe will stop expanding from an infinite point in time i believe?

(3) - closed universe - there is enough matter and halt the expansion and cause the universe to contract. But what is it contracting to?

Is this right? if there is anything else to include that you deem important please do.

Query 2 : "At present, estimates of the amount of ordinary matter in the universe suggest an open universe. However there is evidence for matter that cannot be detected using the emission or absorption of radiation. This DARK MATTER might be enough to produce a closed universe" Okay if it can't be detected how do they know it's present? are they looking at the gravitational effect of it? if so, how are they doing that?
Dark Matter is a THEORITICAL MATTER which gives rise to various theories , according to the ones when we notice a galaxies rotation it seems to rotate at a greater speed than the mass i think and same goes for the light intensity it was noticed that a galaxy with certain amount of mass was giving out too less light which speculated that where is the rest mass being used up.Another relates that the rotational force of the galaxy is so strong that they should have torn themselves apart but due to a repulsive effect/ force its stable. Hope i made sense as last i read on DM was back in october .
6. (Original post by ibysaiyan)
Dark Matter is a THEORITICAL MATTER which gives rise to various theories , according to the ones when we notice a galaxies rotation it seems to rotate at a greater speed than the mass i think and same goes for the light intensity it was noticed that a galaxy with certain amount of mass was giving out too less light which speculated that where is the rest mass being used up.Another relates that the rotational force of the galaxy is so strong that they should have torn themselves apart but due to a repulsive effect/ force its stable. Hope i made sense as last i read on DM was back in october .
Thanks for your reply mate!!!! how can it be considered theoretical dark matter if you're saying that they have found reasons for it existing?
7. query 3
8. (Original post by boromir9111)
query 3
Query 3: Simple harmonic this is relating to. So the condition in order for SHM to work is that F = -kx.... which means that the force acting on the system needs to be in opposite direction to the displacement from the equilibrium position. Correct? Now does this only apply to "springs" cause in order for this work does it also have to obey Hooke's law? which is basically the above equation.
Correct.
At A Level it will also be applied to a simple pendulum.
The spring example is excellent because it follows it exactly, so long as you stay withing the spring's limit of proportionality. (If you pull it too far it doesn't work exactly.)
Regarding the pendulum, you will be told that it performs SHM so long as the amplitude of the oscillation is small*. Strictly speaking, it is an approximation. But a very close one, and one that provides useful results.
*small enough such that the sine of the angle of displacement of the pendulum to one side can be taken as equal to the angle itself in radians.
9. (Original post by Stonebridge)
Correct.
At A Level it will also be applied to a simple pendulum.
The spring example is excellent because it follows it exactly, so long as you stay withing the spring's limit of proportionality. (If you pull it too far it doesn't work exactly.)
Regarding the pendulum, you will be told that it performs SHM so long as the amplitude of the oscillation is small*. Strictly speaking, it is an approximation. But a very close one, and one that provides useful results.
*small enough such that the sine of the angle of displacement of the pendulum to one side can be taken as equal to the angle itself in radians.
Thanks for your reply if i have any further queries i'll just put them back at my OP
10. Thanks for your reply mate!!!! how can it be considered theoretical dark matter if you're saying that they have found reasons for it existing?
The evidence for dark matter is partly related to things to do with gravity, like galaxies rotating too fast to stay together given the amount of visible matter we see and inferring from this the gravitiational attraction holding the galaxy together. However, how do we know that this isn't from a problem of our understanding of gravity, not from a lack of matter? That is why it is still termed theoretical - plus the fact we haven't been able to actually find any directly! (not for lack of trying - LHC better deliver us something good!)

If you want to know more about on of the alternatives to dark matter, look up a proposal called MOND (Modified Newtonian Dynamics) which basically reckons that gravity is stronger than we think it is over distances of galactic scale (i.e. it departs from the normal inverse square law falloff that we see for the EM field and presume for the gravitational field)

(3) - closed universe - there is enough matter and halt the expansion and cause the universe to contract. But what is it contracting to?
Presumably a singularity, i.e. a black hole BUT this is difficult to say for reasons relating to entropy, which I must confess are WAY over my head :-p
11. (Original post by boromir9111)
I think any problem i have now, i will post it all on this thread. This way it saves time from having to create a new thread each time.

Query 1: Expansion of the universe. There are 3 possible scenarios.
(1) - open universe - this is where there is not enough matter hence the effect of gravity is not sufficient enough to stop the universe from expanding

(2) - critical universe- same as the open universe but there is slightly more matter but the universe will stop expanding from an infinite point in time i believe?

(3) - closed universe - there is enough matter and halt the expansion and cause the universe to contract. But what is it contracting to?

Is this right? if there is anything else to include that you deem important please do.

Query 2 : "At present, estimates of the amount of ordinary matter in the universe suggest an open universe. However there is evidence for matter that cannot be detected using the emission or absorption of radiation. This DARK MATTER might be enough to produce a closed universe" Okay if it can't be detected how do they know it's present? are they looking at the gravitational effect of it? if so, how are they doing that?

Query 3: Simple harmonic this is relating to. So the condition in order for SHM to work is that F = -kx.... which means that the force acting on the system needs to be in opposite direction to the displacement from the equilibrium position. Correct? Now does this only apply to "springs" cause in order for this work does it also have to obey Hooke's law? which is basically the above equation.
Hey sorry if i didnt cleared things up earlier LOL i was awake whole night ;p ..
no about query 3 , In SHM acceleration is always directed towards a particular point and as a result displacement being equal but in opposite direction.It applies to any object under forced oscillation i.e a sinusoidally varying force being applied on an oscillating system, one more thing to note an ideal SHM does not disppitate at all rather goes on forever, its the frictional force which damps it.hmm well it does apply to springs obeying hookes law. So in general a body might osicllating but not obeying SHM if hookes law is broken.This happens when the amplitude of motion is so large that the restoring force and displacement don't linearly go , simple put : they don't do f=-kx.
12. (Original post by ibysaiyan)
Hey sorry if i didnt cleared things up earlier LOL i was awake whole night ;p ..
no about query 3 , In SHM acceleration is always directed towards a particular point and as a result displacement being equal but in opposite direction.It applies to any object under forced oscillation i.e a sinusoidally varying force being applied on an oscillating system, one more thing to note an ideal SHM does not disppitate at all rather goes on forever, its the frictional force which damps it.hmm well it does apply to springs obeying hookes law. So in general a body might osicllating but not obeying SHM if hookes law is broken.This happens when the amplitude of motion is so large that the restoring force and displacement don't linearly go , simple put : they don't do f=-kx.
Thanks for your reply!!!! yeah, a = -w^2x but this is a quote from wikipedia itself

"In physics, simple harmonic motion (SHM) is the motion of a simple harmonic oscillator, a periodic motion that is neither driven nor damped. A body in simple harmonic motion experiences a single force which is given by Hooke's law; that is, the force is directly proportional to the displacement x and points in the opposite direction."

It quotes that the force is proportional to the displacement but in the opposite direction. So you can say that you need both conditions for it to be in SHM and it also needs to obey Hook'es law?
The evidence for dark matter is partly related to things to do with gravity, like galaxies rotating too fast to stay together given the amount of visible matter we see and inferring from this the gravitiational attraction holding the galaxy together. However, how do we know that this isn't from a problem of our understanding of gravity, not from a lack of matter? That is why it is still termed theoretical - plus the fact we haven't been able to actually find any directly! (not for lack of trying - LHC better deliver us something good!)

If you want to know more about on of the alternatives to dark matter, look up a proposal called MOND (Modified Newtonian Dynamics) which basically reckons that gravity is stronger than we think it is over distances of galactic scale (i.e. it departs from the normal inverse square law falloff that we see for the EM field and presume for the gravitational field)

Presumably a singularity, i.e. a black hole BUT this is difficult to say for reasons relating to entropy, which I must confess are WAY over my head :-p
Hey thanks for your reply mate, looking at the spec all i need to know is the definition of what dark matter is and how it can be related to the expansion of the universe. Anything else i included is just purely my curiosity and i need to stick to the point which i normally don't do
14. query 4
15. anyone?
16. query 4

as V = I R analogously

delta T = dQ/dt R where delta T is temp difference, dQ/dt = rate of flow of heat energy and R = "thermal resistance"

R = L/sigma A where L = length, A is cross sectional area and sigma = thermal conductivity

do a wiki on thermal conductivity to find out more
17. query 6
18. (Original post by boromir9111)
query 6
Query 6 : Lenses : if the focal length of an lens is 30 cm would it be right to say that image distance will be at 30cm too?
The image distance depends on where the object is, as well as the focal length. It also depends on the type of lens; convex or concave. (Converging or diverging)
19. (Original post by Stonebridge)
The image distance depends on where the object is, as well as the focal length. It also depends on the type of lens; convex or concave. (Converging or diverging)
are you talking about 1/u + 1/v = 1/f.... that's for a thin lens though. But is it for a converging for diverging?
20. (Original post by boromir9111)
Query 3 : Simple harmonic this is relating to. So the condition in order for SHM to work is that F = -kx.... which means that the force acting on the system needs to be in opposite direction to the displacement from the equilibrium position. Correct? Now does this only apply to "springs" cause in order for this work does it also have to obey Hooke's law? which is basically the above equation.
Yes.

Any system that undergoes simple harmonic motion exhibits two key features.

1. When the system is displaced from equilibrium there must exist a restoring force that tends to restore it to equilibrium.
2. The restoring force must be proportional to the displacement, or approximately so.

it does not have to be always a spring , it can be a string , a boat tied to harbour as long as they satisfy the above two conditions they will perform or can be modelled as performing SHM

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