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A2 Unit 4 section A questions

I was wondering what the best/quickest way to solve these types of questions are? I've seen them come up a few times and the two charges question took me a while to solve

(Ignore all the workings for the second question, I was mainly testing to see if the method I tried would work)

Thanks!

With the 2nd one the quick way to do it to get the correct MCQ answer is to remember that the force (field strength) is prop to 1/x²

Forget the nC units and the mm as they cancel.
8/y² = 2/x² at the point where the forces are equal but opposite. y is the distance to the larger charge, x to the smaller.

This means
y²/x² = 8/2 = 4

y/x = 2

You then simple divide up that 60mm into two sections in the ratio 2 to 1

That would give 40mm and 20mm

These MCQ questions will always have numbers where this ratio works out simply.
eg
If the ratio of the charges was 9, the ratio of the distances would be 3 and you divide the length into two sections with ratio 3 to 1. (In this case the total length would probably have been something like 80mm which could be divided into 60mm and 20mm.)

For the 1st one it's just plugging numbers into the standard formula

E = BLv

where B is the vertical component of the Earth's field
L the lenght of the wing span and
v the speed of the plane.
All in appropriate SI units, of course. (Which they are, here.)

(edited 9 years ago)

Reply 2

dannyoboy007

Thanks, seemed to clear it up! I was also wondering about the hose type of questions, I did it once but I can't find my answers and I can't remember how I did it, so if you could help for this one too that would be great :smile:

Reply 3

Stonebridge

Original post by dannyoboy007

The momentum per second will be found from the mass of water leaving the tube per second multiplied by its velocity.

Imagine the water that has left the tube in one second is a cylinder of water of length v. It's length is v, the water velocity, because a length equal to the velocity leaves every second.

This cylinder has volume v times the cross section area, A. (Volume of cylinder formula)

So volume per second = Av --- (1) (you are given A)
You are given the mass per second, and volume = mass x density
As you know density you can find the volume per second.
Sub the volume into (1) above to find v, the velocity of the water.

Then finally, momentum = mass x velocity.