Magnetic Flux passing through loop

[uzair_ha91]

Homework Statement

The magnetic field in a certain region is given by B=<40i-18k>T. How
much flux passes through a 5cm^2 area loop in this region if the loop
lies flat in the xy plane?

Homework Equations

Flux=B.A

The Attempt at a Solution

I got the answer (90 * 10^-4) in negative, and its in positive in the textbook at the back...
How can flux be negative and why can we drop the negative sign?

 

[Gib Z]

The amount of flux can be thought of as always a positive quantity, whilst the sign of the value dictates direction based on a previously decided convention. For example, if we define Up to be the positive direction, and we have 5 Wbs of flux downwards, we can express that as -5 Wbs. The sign tells you the direction the flux acts in, the sign tells you the direction.