Calculating Full-Scale Deflection for Weakened Magnetic Field?

[pivoxa15]

Homework Statement

The needle of a moving coil meter gives a full-scale deflection for a current of 50uA. What current would give a full-scale deflection if the magnetic field weakened to 80% of its original value?

Homework Equations

The Attempt at a Solution

Decrease magnetic field => needle not deflected as much for a given current such as 50uA. A full-scale deflection now would result in 20% more current than before because of the 20% decrease in magnetic field. I assume the powers of magnetic strength and current are equal and unity. So now 50uA*1.2=60uA would result. However the answers suggested 63uA. Why?

 

[AlephZero]

Because "a 20% decrease" and a "20% increase" are not the inverse of each other.

0.8 times 1.2 = 0.96, not 1.0

Your basic idea about how to solve the problem is OK.

 

[pivoxa15]

That is good. It's liitle things like this that tests your mathematical intuition. For me, it's obviously not very good but I'm working on it.