Finding maximal total current for a system

[Phys student]

Homework Statement

a) There are two fuses connected in parallel: fuse A has resistance R_A = 1Ω and
maximal current (by which it melts) I_Amax = 1A; fuse B
has resistance R_B = 2Ω and maximal current (by which it
melts) IBmax = 1.2A. What is the maximal total current for
such a system of fuses?

b) What is the total current when the
fuse B is substituted with a fuse C which has R_C = 2Ω and
I_Cmax = 1.7A?

The Attempt at a Solution

for part (a) we have:

R_B/R_A = I_A/I_B = 2 because of that fuse A will melt first (the current through it will be 1 A) so we substitute it in the equation and solve for I_B, we get:

I_B = 0.5 A

So:

I = 0.5+1 = 1.5 A

I'm not sure how to do part B

Edit: Answers are: a) 1.5 A
b) 1.7 A

 

[haruspex]

Looks like a mistake in the question to me. Are you sure you've quoted it correctly?

 

[Phys student]

Looks like a mistake in the question to me. Are you sure you've quoted it correctly?

Yes, I'm sure.

 

[voko]

Imagine in part (B) current exceeds 1.5 A. What happens then?

 

[Phys student]

Imagine in part (B) current exceeds 1.5 A. What happens then?

Then fuse A would burn and the current would be redirected to fuse C only?

 

[voko]

Correct.

 

[Phys student]

Correct.

So the answer is 1.7A because after changing to fuse C a larger maximal current is possible where fuse A would burn and the current (1.7A) would be redirected to fuse C only.

 

[voko]

1.7 A is possible from the beginning, it is just that fuse A gets blown in the process. This is different from case (a), where the max current is only possible with the two fuses intact.

 

[haruspex]

1.7 A is possible from the beginning, it is just that fuse A gets blown in the process. This is different from case (a), where the max current is only possible with the two fuses intact.

Doh!