This is best explained through the use of an optics diagram, this is a little too complicated to explain in a short answer, and as I can't draw an appropriate diagram in this answer, I will point you to this excellent resource which explains what you have asked very well!
Go onto the BBC website (you should have access to it even if you aren't in the UK) and paste this after the BBC url,
/bitesize/intermediate2/physics/waves_and_optics/image_formation_from_lens/revision/1/
Molecules, The movemnet of molecules
I'm giving you a general formula , its called ohm law it states that <span>electric current is proportional to voltage and inversely proportional to resistance
if V=voltage associated with the battery
R=resistance , I=current then
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Answer:
The rate of transfer of heat is 0.119 W
Solution:
As per the question:
Diameter of the fin, D = 0.5 cm = 0.005 m
Length of the fin, l =30 cm = 0.3 m
Base temperature, 
Air temperature, 
k = 388 W/mK
h = 
Now,
Perimeter of the fin, p = 
Cross-sectional area of the fin, A = 
A = 
To calculate the heat transfer rate:

where

Now,

Answer: 65000 seconds
Explanation:
Given that,
Current (I) = 2 mA
(Since 1 mA = 1 x 10^-3A
2 mA = 2 x 10^-3A)
Charge (Q) = 130 C
Time taken for a fully charged phone to die (T) = ?
Recall that the charge is the product of current and time taken.
i.e Q = I x T
130C = 2 x 10^-3A x T
T = 130C / (2 x 10^-3A)
T = 65000 seconds (time will be in seconds because seconds is the unit of time)
Thus, it will take a fully charged phone 65000 seconds to die