# Full-scale deflection?

## 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?

## 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?

## Answers and Replies

AlephZero
Science Advisor
Homework Helper
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.

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.