Prelim Revision Questions

[Admin]

This is a topic just related to the up comming prelim, any past paper question querries or questions about the first 2 units them post them here.
Please feel free to answer each others questions and explain your answer. Studies have shown that this would actualy help both people concerned.
Oh and I gaffed this afternoon. Initial horizontal speed was V cos 0 and Initial vertical speed was V sin 0

[Scoobasteve]

i think ive developed insomnia.

[Admin]

I take it you watched the super 14 rugby it kicked off at 6.30 am.

[Suzy]

Just to say I got the email and password and stuff
thanks

[S.McGrath]

work & potential energy are they scalars or vectors?

do we need to know anything about electric fields and potential difference for the prelim ?

2005 past paper: Q27

2006 past paper: Q24

2007 past paper: Q24, 27(b)

2008 past paper: Q26

help please

[Admin]

Do you need to know about electric fields and potential difference, in a word yes.
2005 Q27.a. not covered yet we will cover it beginning of March.
2005 Q27.b.i. Vo = (P-Q) x Rf / R1. Vo = 2V, Rf/R1 = 2. so we get 2 = 2(P-Q); P-Q = 1V.
2005 Q27.b.ii. P.D. at Q = Vs x 20 / (100 + 20) = 2V, P.D. at P must be 3V. So 3 = 12 x Rt / (75 + Rt);
Rearrange to get 3 x (75 + Rt) = 12Rt, 225 = 12Rt - 3Rt, 225= 9Rt, Rt = 225/9 = 25Kohms.
2006 Q24.a. 200000 Joules of energy is given to each coulomb of charge passing through.
2006 Q24.b. Q is the negative plate so has negative charge. Protons have a positive charge. Opposite charges attract. Proton ex[eriences a force pulling it towards Q.
2006 Q24.c.i.W = QV = 1.6x10^-19 x 2 x 10^5 = 3.2 x 10^-14J.
2006 Q24.c.ii. Conservation of energy means Ek = W = 3.2x10^-14 = 0.5m(v^2) = 0.5 x 1.673x10^-27 x (v^2);
3.2 x 10^-14 = 0.8365 x 10^-27 x v^2
v^2 = (3.2 x 10^-14 / 0.8365 x 10^-27)
v = 6.2 x 10^6 m/s.
2006 Q24.d. No effect. v = Square root of (2Ek/m) = Square root (2W/m) = Square root (2QV/m). Q, V and m are constant.
2007 Q24.a. Wd = Change in Ek. Ek after = 3.05 X 10^-14, Ek before = 3.32 x 10^-27 x (2.6 x 10^6)^2
Wd = 3.05 x 10^-14 - 2.24 x 10^-14 = 8.1 x 10^-15J
2007 Q24.b. W = QV, V = W/Q = 8.1 x 10^-15 / 3.2 x 10^-19 = 25000 V
2007 Q24.c. Decreases. Ek = W = QV, Q decreases, V is canstant so Ek decreases.

[Admin]

2007 Q27.b.i. 2 inputs so differential.
Q27.b.ii. As Rt increases, P.d. Q decreases, inverting input decreases, Vout increases transistor switches L.E.D. on.
Q27.b.iii. Vo = (P-Q) x Rf/R1, 3.0 = (P-Q) x (100 / 40), 3.0 / 2.5 = (P-Q); (P-Q) = 1.2 V.
2008 Q26.a. if Bridge is balanced 1.2k / R = 4K / 6K; (1.2k / R) = (2 / 3); R = 1.8K from thr graph 1.8K = 0.48 Wm-2.
Q26.b.i. V = Vs x R / (1.2K + R), V = 12 x 2000 / (1200 + 2000) = 24000 / 3200 = 7.5V
Q26.b.ii. Vo = (7.2-7.5) x (140 / 20) = -0.3 x 7 = -2.1V.
Q26.b.iii. Yellow. Current flows from negative P.d. so from Vo to 0V line, yellow is forward biased and lights, blue is reverse biased and does not light.

[Suzy]

Sit, will you be free tomorrow period 2?
Cause I need help with vectors and tension still im not understanding it

[S.McGrath]

is work and potential energy scalers or vectors?

[Admin]

Suzy P2 tomorrow fine are you wnating me to dig out some problems before you arrive tomorrow on Tension?.
Steven sorry I had mis read your e-mail. Both work done and patential energy are Scalers

[S.McGrath]

okay cheers sir

[S.McGrath]

is it okay if i come in P2 2morrow aswel?

im still not 100% with the potential difference electric fields stuff

[Suzy]

Whatever you think'll help for prelim - i dont want to waste my time studying it if its not for tomorrow, i just still cant get my head around it

Thanks, see ya tomorrow

[Admin]

Steven, P2 tomorrow is fine.

[Scoobasteve]

ill be in tomorrow too, sir.