Physics: Post your doubts here!

[ahmedish]

the que states that red LED lights up when the door is opened
when the door is opened as stated in the first part Vout is positive...so current flows from ground to ouput
...and for V- ...the switch is closed ...door is closed so the current flows from switch to the ground

Don't we use conventional current here?

λN = dN/dt
dN / N = -λdt
bro, what?
t=0 when N=N and t=t when N= No
nw apply limits of N and No across dN/N = limits across -λ of o and t ...and solve

bro, what?

 

[ahmedish]

λN = dN/dt
dN / N = -λdt
t=0 when N=N and t=t when N= No
nw apply limits of N and No across dN/N = limits across -λ of o and t ...and solve

i know thats how you get the N=N0*e(lambda*t) but that doesnt explain why A= dN/N

 

[ashcull14]

Don't we use conventional current here?

bro, what?

the current flows from the right vertical bar carrying three resistors in a row as switch is only connected to the left bar with two resistors ...track the path of the current ...when v = + ...and ull understand tht it will pass through the red LED..i didnt mean to say tht it is generated frm the ground .. i was talking abput the pathway

 

[ashcull14]

i know thats how you get the N=N0*e(lambda*t) but that doesnt explain why A= dN/N

its not A = dN / N
as A= λN......a is the activity...as activity is the rate at which the isotope decays .....it is = dN / dt....so λN = dN / dt

 

[funky brat]

1) How did we get delta N/ N? I solved
A= delta N / delta T
decay constant = (From A= lambda*N) so lambda = delta N / (delta N* delta T)
is this wrong????
2) why would it have to decay by 8%? I get the percentage error is added, so its 10% - 2%, but I dont get why this 8% is the percentage of decay

I think the question mentioned sth like there's already an error of 2% in calibration and then it asked the maximum possible error which ruled out the possibility of using -2% as the error and the max error had to be 10% in total therefore only 8% could be the possible error in decay. Get it?
Also it is derived like this:
rate of decay is directly propotional to the no of nuclei
delta n/ delta t= N where N is no nuclei
delta n/ delta t= -lambda*N where lambda is the decay constant
and then make lambda the subject to get (delta n/ N) / T

 

[ahmedish]

I think the question mentioned sth like there's already an error of 2% in calibration and then it asked the maximum possible error which ruled out the possibility of using -2% as the error and the max error had to be 10% in total therefore only 8% could be the possible error in decay. Get it?
Also it is derived like this:
rate of decay is directly propotional to the no of nuclei
delta n/ delta t= N where N is no nuclei
delta n/ delta t= -lambda*N where lambda is the decay constant
and then make lambda the subject to get (delta n/ N) / T

Yeah man I agree with that! But in the mark scheme its written lambda = (delta N)/ (N)
whereas I think its (delta N)/( N* delta T)

 

[ahmedish]

the current flows from the right vertical bar carrying three resistors in a row as switch is only connected to the left bar with two resistors ...track the path of the current ...when v = + ...and ull understand tht it will pass through the red LED..i didnt mean to say tht it is generated frm the ground .. i was talking abput the pathway

The current flows from Vout. The resistors now are not considered. Switch open> Vout positive > then the Red diode should be forward, in order to light up.
I am at least 95% sure

 

[ashcull14]

Yeah man I agree with that! But in the mark scheme its written lambda = (delta N)/ (N)
whereas I think its (delta N)/( N* delta T)

send a snap of MS
might b deltaN/N = e*-λt

 

[ahmedish]

its not A = dN / N
as A= λN......a is the activity...as activity is the rate at which the isotope decays .....it is = dN / dt....so λN = dN / dt

How is A= dN/N in part ii? thats the first bit.

 

[ahmedish]

send a snap of MS
might b deltaN/N = e*-λt

In the mark scheme, it says A = dN/dT
this is a rule which I agree on
then it says lambda = dN/N
shouldnt it be A=dN/dT and A=Lambda * N so
Lambda * N = dN/dT
so lambda =dN/ (N*dT)
???
they removed the dT in the markscheme idk why

 

[ashcull14]

In the mark scheme, it says A = dN/dT
this is a rule which I agree on
then it says lambda = dN/N
shouldnt it be A=dN/dT and A=Lambda * N so
Lambda * N = dN/dT
so lambda =dN/ (N*dT)
???
they removed the dT in the markscheme idk why

which year?

 

[ahmedish]

which year?

June 2009 Q9

 

[ashcull14]

June 2009 Q9

λ = decay constant
means - probability of decay of a nucleus per unit time
as activity =λN
deltaN/ delta t = λN
delta N/ N = λ / delta t
delta N= decayed nuclei N = total no. of nuclei

 

[ahmedish]

λ = decay constant
means - probability of decay of a nucleus per unit time
as activity =λN
deltaN/ delta t = λN
delta N/ N = λ / delta t
delta N= decayed nuclei N = total no. of nuclei

Last part doesn't make any sense.
And about the diode btw, am I wrong about the direction or what?

 

[ashcull14]

Last part doesn't make any sense.
And about the diode btw, am I wrong about the direction or what?

last part?
part b?

 

[ashcull14]

Last part doesn't make any sense.
And about the diode btw, am I wrong about the direction or what?

if maximum possible error for decay is 10%
and the uncertainity in the calibration is 2%
then the element decays by 8 %
which means 92 % of element remains
as A/Ao = e*-λt
0.92 = e* (ln2 / 5.27 *365 ) t
231 days = t

 

[ahmedish]

if maximum possible error for decay is 10%
and the uncertainity in the calibration is 2%
then the element decays by 8 %
which means 92 % of element remains
as A/Ao = e*-λt
0.92 = e* (ln2 / 5.27 *365 ) t
231 days = t

Oh my god LOL
why is 9) a) ii) delta N / N INSTEAD OF delta N / (N* delta T) which I got from lambda* N = delta N/ delta T so if you make lambda subject, you get that.
2) Am I correct about the orientations of the diodes, here : The current flows from Vout. The resistors now are not considered. Switch open> Vout positive > then the Red diode should be forward, in order to light up.
I am at least 95% sure

 

[ashcull14]

Oh my god LOL
why is 9) a) ii) delta N / N INSTEAD OF delta N / (N* delta T) which I got from lambda* N = delta N/ delta T so if you make lambda subject, you get that.
2) Am I correct about the orientations of the diodes, here : The current flows from Vout. The resistors now are not considered. Switch open> Vout positive > then the Red diode should be forward, in order to light up.
I am at least 95% sure

omg the que asks us for the expression of the probability of decay not...λ,....λ= probability of decay per unit time
so the probability of decay would only b delta N/N
delta N = decayed nuclei
and N= total no. of nuclei

 

[ashcull14]

Oh my god LOL
why is 9) a) ii) delta N / N INSTEAD OF delta N / (N* delta T) which I got from lambda* N = delta N/ delta T so if you make lambda subject, you get that.
2) Am I correct about the orientations of the diodes, here : The current flows from Vout. The resistors now are not considered. Switch open> Vout positive > then the Red diode should be forward, in order to light up.
I am at least 95% sure

i really dont get wat concept u hav regarding diodes cn u express it through some diag

 

[ahmedish]

omg the que asks us for the expression of the probability of decay not...λ,....λ= probability of decay per unit time
so the probability of decay would only b delta N/N
delta N = decayed nuclei
and N= total no. of nuclei

Ahhhhhhhhhhhhhh!!!! That makes sense now! so probability of decay and decay constant are 2 different things

i really dont get wat concept u hav regarding diodes cn u express it through some diag

do you have the book? Page 399 it explains it