Understanding Simple Circuits: Answering Common Homework Questions

[t_n_p]

Homework Statement

http://img125.imageshack.us/img125/5434/asdfjr7.jpg

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

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 and έ (the battery)

b) The same as in a) but with resistance between [PLAIN]http://img82.imageshack.us/img82/4954/26005668vm6.jpg 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".

 

[Weimin]

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.

 

[t_n_p]

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?

 

[Mentz114]

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

 

[t_n_p]

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?

 

[Mentz114]

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.

 

[t_n_p]

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?

 

[Mentz114]

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.

 

[t_n_p]

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