You are Here: Home >< Physics

A2 Physics A Paper A- HELP PLEASE!!!! watch

1. HELP!!!! I need urgent help.
Two issues:
One, when they ask for the initial number of nuclei, and you have worked out the decay constant, λ, already, you use the formula:

delta N/delta t = -λN

but in all the exam questions, when that comes up, they just use the initial activity for (delta N/delta t). Isn't that wrong? Cuz i usually use N at a single half life, so N is half of the original activity, and t is a half life. But thats wrong. WHHY?????????????????????

Two, does anyone know what questions came up on Paper A of Jan 05, cuz I was not in school for the mocks, and my class mates are all stupid, lol.

2. The intial activity is the rate of change of N with respect to t. (dN/dt). by putting it equal to -λN, where λ is known to calculate N

As the activity is constantly changing and obeys the laws of exponential decay you cannot simply calculate it as you suggest.

That is if i understand what you are saying...
3. activity = decay constant * number of nuclei

the decay constant is the chance that a nuclei will decay on one second, so this times the total number is, on average, the number of decays per second or the activity. A = λN
4. (Original post by mik1w)
activity = decay constant * number of nuclei

the decay constant is the chance that a nuclei will decay on one second, so this times the total number is, on average, the number of decays per second or the activity. A = λN
Ah ok, now i get it. So are you 110% sure about the "A = λN" formula, cuz i've never heard of it, but then again my teacher is as old as Newton himself, so probably forgot alot.
5. yeah 100%
6. ITs right A=lambda*N chilllll
7. Sometimes that formula isnt accurate enough unless delta is REALLY tiny, you have to use

N=Noe^(-λt)

where N is number No is initial number e is the mathematical e t is time and λis decay constant

It is the same formula for A (activity) if you replace N with A and No with Ao

Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: June 15, 2005
The home of Results and Clearing

1,012

people online now

1,567,000

students helped last year
Today on TSR

University open days

1. Keele University
Sun, 19 Aug '18
2. University of Melbourne
Sun, 19 Aug '18
3. Sheffield Hallam University