Year 11 Physics - Help

[nbalakers24]

1 a. In a solution of salt water a total positive charge of + 15C was seen to move past a point to the right in 5 s and in the same time a total negative chrage -30C was seen to move to the left. What was the current through the solution during this time?

b. Some time later it was found that in 5 seconds a total of +5C had moved to the right while - 15C had moved to the right as well. What was the current this time?

2. The dome on a fully charged Van de Graaff machines may carry something of the order of 50 million million extra electrons. When running well it may take about 3 s to charge up. If we assume no loss of charge in this time, what is the current flowing up the belt to the dome?

spent like 30 minutes on them, couldnt get the right answer. lol.
help appreciated

[Potter]

1a. What you have to understand for this question is the direction of current. If a negative charge moves in one direction, a positive is moving in the opposite.

+15c moving to the right and -30 moving to the left. Is the same as

+15c moving to the right and +30c moving to the right. 

Therefore we have 45c of charge over 5 seconds.
A= Coulomb per second.

45/5 = 9

Therefore the current is 9A

1b)
For this one, since both charges are moving in the same direction we find the difference between the two charges.

15c-5c = 10c

Therefore we have a 10c charge over 5 seconds
10/5 =2

Current = 2A

2.

For this one you need to read in carefully. It's 50 million million electrons.
In other words 50 x 10^12 for 3 seconds.

So - if we work it out to per second we have
(50 x 10^12)/3
=16.67 x 10^12

Since we want to find the current, we need work out how many coulombs is in 16.67 x 10^12
So we divide 16.67 by a coulomb.
Therefore-

(16.67 x 10^12)/ (6.24 x 10^18)

= 2.67 x 10^-6 C
= 2.67 µC
= 2.67 µA
The book may round it up to 3 µA, but I'm pretty sure thats right..

Hope this helps.