Physics m/c help. (1 Viewer)

x.Exhaust.x · Sep 24, 2009

I need help with the multiple choice questions I got wrong attached. If you guys could do the following, that'd be nice:

1. Guide me as to which dotpoint each question relates to.
2. An explanation on why my answer is wrong or why is the boardofstudies answer correct.

I'm so bad at multiple choice

. Thanks in advance~!

Arowana21 · Sep 24, 2009

omg, that is so many wrong, and i cant be bothered doing it all.

kaz1 · Sep 24, 2009

For the first different appliances in the home need different voltages to operate so that is why a transformer is needed.

For the second one Cathode rays (thermoionic device) require large amounts of energy need so a large voltage is needed but Transistors are solid-state devices so need low voltages.

And OMFG at the amount of questions. You should really go to your teacher if you have that many questions. I will try to look at the other ones if I have time.

untouchablecuz · Sep 24, 2009

1. not B because transformers do not transform electricity from DC to AC (and vice versa), but rather act to alter the voltage of the electricity

so, A is the only option that fits in with this
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2. cathode ray tubes require very high voltages to emit electrons by thermionic emmision. the voltage required is much greater than 240V .'. CRT require step up transformers

transistor circuits can overheat and malfunction if the voltage is too high .'. step down transf required

so, A is the answer
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3. convert wavelength to metres

find F as a multiple of 10^15 (using f=1/wavelength)

use the graph to find which line the point (F, 2.5) lies on

this is your metal

4. D is the answer - related to orbital decay (loss of kinetic energy due to friction with atmospheric particles, gets worse as altitude decreases)

5. A -> bragg utilised x-ray diffraction. when EM radiation of similar wavelengths to the atomic spacing between atoms in the crystal (i.e. x rays) pass through the crystal, they are diffracted, producing certain patterns of constructive and destructive interference (which he captured on photographic film).

6. to explain better lets say these circles have equations x^2 + y^2 = 1

on diagram y, by the RHPR, at (0,1) the force is zero becuase the current direction and B field direction are paralell. @ (-1,0) their perpendicular and the force is maximum.

so we can conclude that the force on Q varies with time, this leaves C and D

on diagram X, we can see that if we resolve the vector P in the horizontal and vertical directions, that these component vectors are always perpendicular to the B field. so, the force is always maximum .'. always constant

leaving C

7. GPE = -GMm/r where M is the mass of the planet

so Mass of earth can be determined by comparing to values (using ratios)

.'. eliminate A

work done in moving between two points = change in GPE by definition

.'. elminate D

the escape velocity is the velocity such that final kinetic energy + GPE = 0

.'. eliminate C

so B is correct

8. D (look at the last major dotpoint of M&G)

not A because in DC motors the external B field does not rotate, the coil on the armature does

9. immediatly we cancel out A and B

D does not involve any "movement"

so we conclude C

what is happening is that eddy currents are induced in the wheel, but in the opposite direction, opposing the change that created them (in accordance with LOCOE). this is what is used in induction motors as the cage is dragged around

untouchablecuz · Sep 24, 2009

9. a p type semi conductor has a defiency of electrons i.e. holes that act as postive charge carriers

boron is in group 3 of the periodic table. silicin has 4 electrons in the valence shell

.'. doping silicon with boron means there will be one less electron for every boron atom, hence producing a p-type silicon conductor

hence C

10. if propellant is consumed at a constant rate, the force is constant

so eliminate B and D

as propellant is being consumed, the mass of the car is decreasing

from F=ma, if force is constant, a is inversly proportional to m

hence is m is decreasing, a is increasing

.'. C

11. A

not B because back emf is related to LOCOE

not C because voltage transformation relies on the fact that emf in = emf out which is a direct consequence of the LOCOE

not D because escape velocity is velocity required to leave the influence of a mutal graviational field. so when leaving the earth, energy is lost by earth and gained by the object .'. LOCOE (not too sure about this, someone correct me if i'm wrong)

Physics m/c help. (1 Viewer)

x.Exhaust.x

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Arowana21

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kaz1

et tu

untouchablecuz

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untouchablecuz

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