Physics marathon 2011 (3 Viewers)

InVinsanity · Oct 9, 2011

Describe the principle of relativity?

The principle of relativity states that it would be impossible to determine wether an object is stationary or traveling at constant velocity without the reference of an external point. Think of einsteins train thought experiment where the train is traveling at c and he is looking at the mirror, if he can't see his image then that would tell him he is traveling at c, which would violate the principle Of relativity.

Describe the main features of a dc motor and the role of each feature

taeyang · Oct 9, 2011

DC Motor -

Rotor - The rotor of a DC motor is a coil with 'n' turns to act as a current carrying conductor, providing the current and therefore torque on the coil
Stator - The stator of a DC motor consists of either a pair of permanent or electromagnets to provide the magnetic field that cuts the plane of the coil, in turn producing the torque
Split Ring Commutator - It's job is to change the direction of the flow of current through the rotor every half rotation so as to keep a one-directional flow
Brushes - The brushes of a DC motor deliver the current from the external power-source and circuit by "brushing" against the split ring commutator, providing a complete circuit
External Power Source or External EMF - This is the blood and bones of the operation, the power source provides the EMF to the coil via batteries or a cell.

The transistor is used in communication and entertainment devices across the world, explain what device the transistor replaced and why by refering to the effects on both society and the environment - From something similar to 2009

Uzername · Oct 9, 2011

taeyang said:
The transistor is used in communication and entertainment devices across the world, explain what device the transistor replaced and why by refering to the effects on both society and the environment - From something similar to 2009

- The transistor replaced the thermionic device. Although both are used for the same purpose (amplifying a signal or 'electrical switching'), the transistor replaced the thermionic device for many reasons: it is cheaper, smaller & lightweight, requires less power, has a longer life and is more durable.

Effects on society:
- transistors are much faster than their thermionic counterparts. They require no 'warm-up' time at all and operate efficiently.
- Transistors have been implemented in microchips and microprocessors, offering untold advantages for society, hence replacing the thermionic device. Transistors have allowed for the building of small, compact and lightweight computers that could never be achieved with thermionic devices. Computers are now part of our everyday life, they have enormous applications: at home (e.g. computer), in education (e.g. calculator), etc. A higher quality of life is a result from the invention of the transistor. Tasks that were once done manually are now simply automated with microchips/processors.

Effects on environment
- Large-scale thermionic devices can heat up to very high temperatures, leading to a small degree of thermal pollution. However, the use of transistors in microchips/processors, leading to the huge applications we have today (computers & electronic devices) obviously draw a lot of power (fossil fuels) leading to negative environmental impacts.

Next question:
Saturn's equatorial rotation period is 10.23 Earth hours. Its mass is $5.96\times 10^{26}$ kg and its equatorial radius is 60268 km.

What would be the orbital radius of a satellite that moved in a 'geostationary' orbit around Saturn?

InVinsanity · Oct 10, 2011

Workings on paper but is this correct?

50681km above the surface.

elbatiolpxeho · Oct 10, 2011

Fizzy_Cyst said:
NEW QUESTION:
A piece of space junk of Mass M drops from rest from a position of 30,000km from the Earth's centre. Calculate the final speed it attains when it reaches an altitude of 1000km above the earth's surface. Assume air resistance is negligible. The radius of the earth is 6.37x10^6m

cant remember where i got this question from

$E_p\text{ at 30000 km}=-6.67\times10^{-11}\times\frac{6\times10^{24}\times M}{6.37\times10^6+30000\times10^3}$

$E_p\text{ at 1000 km}=-6.67\times10^{-11}\times\frac{6\times10^{24}\times M}{6.37\times10^6+1000\times10^3}$

$\Delta E_p=E_p\text{ at 1000 km}-E_p\text{ at 30000 km}$
$\Delta E_p=-M\times4.3298\times10^7\text{ J}$

This loss in gravitational potential energy is transformed into kinetic energy.

$E_k+\Delta E_p=0$
$E_k=-\Delta E_p$

$\frac12 Mv^2=M\times4.3298\times10^7$
$v^2=8.6595\times10^7$
$v=9305.6592\text{ ms}^{-1}$

nazfiz · Oct 10, 2011

With what velocity must a metal ball be projected vertically to reach a maximum height of 100m? How long will it take to reach this height?

Can someone help me with this question?
thanks

jamesfirst · Oct 10, 2011

nazfiz said:
With what velocity must a metal ball be projected vertically to reach a maximum height of 100m? How long will it take to reach this height?

Can someone help me with this question?
thanks

v^2=u^2 +2as

v = 0 (object comes at rest for a while before coming back down)

0 = u^2 - 2 x 9.8 x 100

u = 44.27 m/s

-----------------

v = u + at

0 = 44.27 - 9.8t

t = 4.52 s

Fizzy_Cyst · Oct 11, 2011

elbatiolpxeho said:
$E_p\text{ at 30000 km}=-6.67\times10^{-11}\times\frac{6\times10^{24}\times M}{6.37\times10^6+30000\times10^3}$

$E_p\text{ at 1000 km}=-6.67\times10^{-11}\times\frac{6\times10^{24}\times M}{6.37\times10^6+1000\times10^3}$

$\Delta E_p=E_p\text{ at 1000 km}-E_p\text{ at 30000 km}$
$\Delta E_p=-M\times4.3298\times10^7\text{ J}$

This loss in gravitational potential energy is transformed into kinetic energy.

$E_k+\Delta E_p=0$
$E_k=-\Delta E_p$

$\frac12 Mv^2=M\times4.3298\times10^7$
$v^2=8.6595\times10^7$
$v=9305.6592\text{ ms}^{-1}$

Correct!

I don't see any unanswered questions, so...

Name a metal, metal alloy and a ceramic superconductor and identify their critical temperatures

megabakedpotato · Oct 11, 2011

Some of these answers are fucking horrible, jus' sayin'

jamesfirst · Oct 11, 2011

Fizzy_Cyst said:
Name a metal, metal alloy and a ceramic superconductor and identify their critical temperatures

what the ....

Uzername · Oct 11, 2011

InVinsanity said:
Workings on paper but is this correct?

50681km above the surface.

Nah sorry lol, the answer is 109,000km. I don't get how to do the question either, if someone could explain that'd help.

Fizzy_Cyst · Oct 12, 2011

jamesfirst said:
what the ....

Not familiar with the syllabus?

 process information to identify some of the metals, metal alloys and compounds that have been identified as exhibiting the property of superconductivity and their critical temperatures

Straight from the syllabus mate

taeyang · Oct 12, 2011

Fizzy_Cyst said:
Not familiar with the syllabus?

 process information to identify some of the metals, metal alloys and compounds that have been identified as exhibiting the property of superconductivity and their critical temperatures

Straight from the syllabus mate

I think it's the bit from your last post that says "identify some of the metal, metal alloys and compounds", I mean the 's' was off the end of metal but it really doesn't matter we should've picked it up

jamesfirst · Oct 12, 2011

Fizzy_Cyst said:
Not familiar with the syllabus?

 process information to identify some of the metals, metal alloys and compounds that have been identified as exhibiting the property of superconductivity and their critical temperatures

Straight from the syllabus mate

I have it in my notes but I never memorised them cos they're too long.

I have never seen it appear in the exam. lol

DNETTZ · Oct 12, 2011

Name a metal, metal alloy and a ceramic superconductor and identify their critical temperatures

Metal - Lead - superconducting at 7K
Alloy - Nickel-titanium - Superconducting below 23K
Ceramic - YBaCuO - Superconducting at temperatures below 77k.

I pose a physics history question for 6 marks.

'Einstein laid the groundwork for solar cells as a viable possible energy source'

With reference to the photoelectric effect and the physical properties of metals, assess the validity of this statement.

Good luck.

Physics marathon 2011 (3 Viewers)

InVinsanity

Member

taeyang

Member

Uzername

B.S., M.S., M.A., Ph.D.

InVinsanity

Member

elbatiolpxeho

Member

nazfiz

Member

jamesfirst

Active Member

Fizzy_Cyst

Well-Known Member

megabakedpotato

Member

jamesfirst

Active Member

Uzername

B.S., M.S., M.A., Ph.D.

Fizzy_Cyst

Well-Known Member

taeyang

Member

jamesfirst

Active Member

DNETTZ

Camp-italist Fatcat

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