[Physics] Some questions
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Originally posted by davidche:
I think it is pretty evident. It seems that you are the last to have "guessed" that.
Paiseh lar.
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Originally posted by skythewood:
why is there a need for thermal equilibrium?
Otherwise heat flow won't occur. -
Doesn't increase in temperature leads to increase in resistance of cables?
Do you all realise why we have to cool cables?
And what's the deal with thermal equilibrium? A cable is not a warm-blooded mammal like us, such thing does not happen in practical situations.
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Originally posted by Garrick_3658:
Doesn't increase in temperature leads to increase in resistance of cables?
Do you all realise why we have to cool cables?
And what's the deal with thermal equilibrium? A cable is not a warm-blooded mammal like us, such thing does not happen in practical situations.
.......Yah hor.
Wait....confused already. There's such a effect, where the metal ions vibrate so much that it blocks the flow of the electrons. But I cannot remember if this effect is more or less than the freeing up of electrons when more energy is supplied to them.
And thermal equilibrium refers to the state where the temperature between 2 substances match. If one does not have the same as the other, heat will flow such that the temperature will be the same.
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Ok.
For metals, resistence is proportional to temperature.
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It's a culmulative effect if you cool the cables down, actually.
Less R --> Less power loss as heat
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Originally posted by SBS2601D:
Ok.
For metals, resistence is proportional to temperature.
Lol what, since when do we use graphite or semi-conductors as pylon cables anyway.
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seriously, how tall are your cables?
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A practical answer is this: Cost-effectiveness.
And because telephone lines are underground.
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Originally posted by SBS2601D:
A practical answer is this: Cost-effectiveness.
And because telephone lines are underground.
.......
we don't put power cable near the earth because the current might flow to the earth, which is bad.
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Originally posted by SBS2601D:
.......Yah hor.
Wait....confused already. There's such a effect, where the metal ions vibrate so much that it blocks the flow of the electrons. But I cannot remember if this effect is more or less than the freeing up of electrons when more energy is supplied to them.
And thermal equilibrium refers to the state where the temperature between 2 substances match. If one does not have the same as the other, heat will flow such that the temperature will be the same.
The ions that vibrate too much thingy till it impedes the flow of electrons is out of O level syllabus
Resistance do vary a little with temperature; higher temperature leads to higher resistance because of extra noise. Usually, for thicker wires, the effect is almost negligible. The effect is especially apparent in very thin and small circuits, for e.g. Integrated Circuits like DRAM cells => this is part of my job scope -.-"
There have also been theories that if you cool down wires to absolute zero temperature, you will get a super conductor.
But davidche, dun bother yourself with the details at the moment.
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Originally posted by SBS2601D:
.......Yah hor.
Wait....confused already. There's such a effect, where the metal ions vibrate so much that it blocks the flow of the electrons. But I cannot remember if this effect is more or less than the freeing up of electrons when more energy is supplied to them.
And thermal equilibrium refers to the state where the temperature between 2 substances match. If one does not have the same as the other, heat will flow such that the temperature will be the same.
I think your lvl of physics is actually higher than most of us.
When the temperature increases, it affects resistance in the following ways:
1) particles in wire vibrate more - increase resistance(this is out of syllabus btw)
2) expansion of wire?
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Originally posted by skythewood:
.......
we don't put power cable near the earth because the current might flow to the earth, which is bad.
Durnst know how near is near.But telephone lines have to be separated a fair distance from power lines and until recently, having cables overhead is cheaper than underground....
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Originally posted by davidche:
I think your lvl of physics is actually higher than most of us.
When the temperature increases, it affects resistance in the following ways:
1) particles in wire vibrate more - increase resistance(this is out of syllabus btw)
2) expansion of wire?
Um.Skythewood's is definitely higher than mine.
Not to mention mine is sibei rusty....I even thought initially the resistence is inversely proportional till you reminded me.
Ok. Expansion of wire will decrease resistence. But factor 1 outweighs factor 2.
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yeap!
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yeap more over the expansion of wire wld result in expansion in width and length so not much diff.
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One last qn.
a rectangle coil is in the process of moving towards the right, passing thru a large magnetic field.
The field's direction is into the paper.
My question is, when you use right hand rule to check direction of induced current, the current direction of either sides are the same-up, thus they opposes. So whats the direction of the current.
[ l----l ]
[ l l ]
[ l l ]
[ l----l ]
Key: the area covered by [ ] is the field. -
Originally posted by SBS2601D:
Um.Skythewood's is definitely higher than mine.
Not to mention mine is sibei rusty....I even thought initially the resistence is inversely proportional till you reminded me.
Ok. Expansion of wire will decrease resistence. But factor 1 outweighs factor 2.
erm
the expansion of wire is caused by increased vibrations of particles
So it would increase resistance
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Originally posted by SBS2601D:
Durnst know how near is near.But telephone lines have to be separated a fair distance from power lines and until recently, having cables overhead is cheaper than underground....
interesting :D
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Originally posted by davidche:
One last qn.
a rectangle coil is placed in a large magnetic field that covers the coil.
The field's direction is into the paper.
My question is, when you use right hand rule to check direction of induced current, the current direction of either sides are the same-up, thus they opposes. So whats the direction of the current.
[ l----l ]
[ l l ]
[ l l ]
[ l----l ]
Key: the area covered by [ ] is the field.btw the coil is moving to the right.
the direction of the current does not go in a circle. Instead, in this case, a potential difference will be formed across the coil. Just imagine where all the electrons will be moving.
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WHen a mass of 2kg is pushed along a horizontal surface, the friction force is 4N. When the mass is pushed with 10N, it moves with constant
acc of 3
acc of 5
speed of 3
speed of 5
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Originally posted by davidche:
WHen a mass of 2kg is pushed along a horizontal surface, the friction force is 4N. When the mass is pushed with 10N, it moves with constant
acc of 3
acc of 5
speed of 3
speed of 5
acc = Fnet / M = (10N - 4N) / 2 = 3
answer acc of 3
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Thansk for the various help esp from eagle. Today is the exam day! Good luck everyone.
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good luck & have fun !