Simple Circuit Question

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  • #1
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Homework Statement



http://img125.imageshack.us/img125/5434/asdfjr7.jpg [Broken]

The Attempt at a Solution


Basically for this excercise I have to fill in the gaps with "increases", "decreases" or "stays the same".

Some of the parts I can do, others I am clueless in. It would be helpful if somebody could explain it to me in a very simple manner :smile:

a) If R7 increases, potential difference between points A and E increases. Assume no resistance in [PLAIN]http://img82.imageshack.us/img82/4954/26005668vm6.jpg [Broken] and έ (the battery)

b) The same as in a) but with resistance between [PLAIN]http://img82.imageshack.us/img82/4954/26005668vm6.jpg [Broken] and έ (the battery)
[I'm unsure what the answer is]

c) If R7 increases, voltage drop across R4 decreases

d) If R2 decreases, current through R1 _________ [unsure of answer]

There's a couple more, but I'll leave it at that for the time being so it's easier to "digest".

Homework Statement





Homework Equations





The Attempt at a Solution

 
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Answers and Replies

  • #2
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For a and b, you need to write Ohm's law on A-->E (the lower part which has the battery), you will see how the voltage drop over AE depends on the current.

Hint: your answer on a is wrong

For other parts, you base on the variation of the current to justify the drop on each part.
 
  • #3
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For a and b, you need to write Ohm's law on A-->E (the lower part which has the battery), you will see how the voltage drop over AE depends on the current.

Hint: your answer on a is wrong

For other parts, you base on the variation of the current to justify the drop on each part.
So in a) resistance increases, and we want to find V but how does current change?
 
  • #4
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Use ohms law to write an expression for the current -

current = voltage/total resistance. The voltage across any element is then easy to read off, V= current * resistance of element
 
  • #5
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Use ohms law to write an expression for the current -

current = voltage/total resistance. The voltage across any element is then easy to read off, V= current * resistance of element
You probably think I'm dyslexic, but I'm still struggling...

V=I*R, no worries I understand that.
R7 is increasing and we want to find V, but what about current? Does the current remain the same? How are we to find V if I is not known?
 
  • #6
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Imagine a circuit comprising 3 resistors and a battery with zero resistance, which has voltage V. So, I = V/(r1+r2+r3). The voltage across r1 is given by V1= I*r1 = V*r1/(r1+r2+r3)

It's obvious now that if you increase r2 or r3, the voltage across r1 goes down.
 
  • #7
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Imagine a circuit comprising 3 resistors and a battery with zero resistance, which has voltage V. So, I = V/(r1+r2+r3). The voltage across r1 is given by V1= I*r1 = V*r1/(r1+r2+r3)

It's obvious now that if you increase r2 or r3, the voltage across r1 goes down.
Ok yep, I understand that now.
So in relation to the example, r1=r7, r2=r5 and r3=r2?
 
  • #8
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I'd recommend writing out the equation for your circuit. You can combine the 2 parallel sets into one resistor which gives 4 in the current denominator.
 
  • #9
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Just thought I'd revisit this question.
After some thought, these are my rehashed answers

a) If R7 increases, potential difference between points A and E DECREASES. Assume no resistance in and έ (the battery)

b) The same as in a) but with resistance between and έ (the battery) DECREASES

c) If R7 increases, voltage drop across R4 DECREASES

d) If R2 decreases, current through R1 INCREASES
 

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