Maths Guru's Needed - Logarithms

The Funktopus

The Funktopus

The Funktopus

Soldato
Joined
14 Aug 2008
Posts
4,255
Location
North Sea
Hi guys,

As above really, I was hoping someone could give me a hand with dreaded Logarithms. The question I'm wrestling with is as follows.

"The temperature of steam in a valve is given by T=1.2^4P-18, where T is the temperature in Celsius and P is the pressure in bars.

Calculate the pressure, correct to 4 significant figures, when the temperature of the steam is 180 degrees C."

Now I understand that I'll have to take Logs of both sides, then transpose it to get P, but the examples we've covered in class only feature formulas along the lines of "S=Ra^t", nothing with multiple terms acting as a power. Any ideas how I can rearrange the above to get P? If I had any hair left, I'd be tearing it out by now. :o

Cheers, Mike
 
log(T) = (4P-18)log(1.2)

log(T) / log(1.2) = 4P-18

log(T) / log (1.2) + 18 = 4P

(log(T) / log(1.2) + 18) / 4 = P

Substitute T in and use a calculator to figure out the log values. Keeping it all in the calculator avoids rounding giving you a more accurate result :)
 
Use log with base 1.2

T = 1.2^(4P-18)
log(1.2)(T) = 4P - 18
(log(1.2)(T) + 18) / 4 = P

edit if it is actually T = 1.2^(4P) - 18, then:
log(1.2)(T + 18) = 4P
P = (log(1.2)(T + 18)) / 4
 
Last edited:
The Funktopus said:
Hi guys,

As above really, I was hoping someone could give me a hand with dreaded Logarithms. The question I'm wrestling with is as follows.

"The temperature of steam in a valve is given by T=1.2^4P-18, where T is the temperature in Celsius and P is the pressure in bars.

Calculate the pressure, correct to 4 significant figures, when the temperature of the steam is 180 degrees C."

Now I understand that I'll have to take Logs of both sides, then transpose it to get P, but the examples we've covered in class only feature formulas along the lines of "S=Ra^t", nothing with multiple terms acting as a power. Any ideas how I can rearrange the above to get P? If I had any hair left, I'd be tearing it out by now. :o

Cheers, Mike
Click to expand...

Hiya Mike.

Is that 1.2^(4p -18)

Or

(1.2^4p) - 18?

That'd help greatly. Cheers :)
 
In that case

94586831.gif


where log is the natural logarithm.

From http://www.wolframalpha.com/input/?i=solve(T=1.2^(4P)-18,P)

I'm not entirely sure why it flips the equation around :p
 
Last edited:
You must log in or register to reply here.
Back
Top Bottom