More about Resistanc 13.2 AQA Textbook Q3

Hi guys I am stuck on this question and I have been confused on it completely for ages. Its Question 3 13.2 AQA AS Level textbook page 219 but if you don't have a textbook the question is:

A 2 ohms resistor and a 4 ohms resistor are connected in series with each other. The series combination is connected in parallel with a 9 ohms resistor and a 3V battery of negligible internal resistance.

Calculate:

a) total resistance which i got right as being 3.6 ohms
b) battry curren which i got right as being 0.8333A
c) The power supplid to each resistor (this is the one i am stuck on)

the answer is resistor 2ohms = 0.5W, 4 ohms = 1.0W and 9 ohms = 1 W

Reply 1

Pangol

Original post by NajiAM17

You need to work out what the potential difference across each resistor is, and then use P = V^2 / R.

It should be straightforward to see what the pd across the 9 ohm resistor is, and what the total pd across the other two resistors in series is. You can then work out the pd across each of these resistors on their own using a potential divider argument.

Reply 2

Duck3E

Original post by Pangol

I tried V^2/R as in:3^2/ 2 = 4.5 W3^2/4 = 2.25 W3^2/9 = 1Wwhich isn't the right answer(also thanks for helping me)

Reply 3

Pangol

Original post by NajiAM17

I tried V^2/R as in:3^2/ 2 = 4.5 W3^2/4 = 2.25 W3^2/9 = 1Wwhich isn't the right answer(also thanks for helping me)

Notice that it is the right answer for the 9 ohm resistor. That is because the potential difference across that resistor is 3 V. But the potential difference across the other resistors is not 3 V. A potential divider argument will help you find what these two pds are.

Reply 4

Duck3E

Original post by Pangol

Sorry but i dont understand. I am pretty sure its not potential dividers as its the next chapter.

Reply 5

Pangol

Original post by NajiAM17

Sorry but i dont understand. I am pretty sure its not potential dividers as its the next chapter.

OK - you don't need to use the idea of potential dividers, but it does save a lot of time. We'll have to do this the long way round.

What is the total potential difference across the two resistors in series? What is the total resistance of the two resistors in series? If you can get those quantities, you can work out the current in the two resistors in series.

Now think about each resistor in turn. For each of them, you know their resistance and the current in them, so you can work out the potential difference across them.

Now, for each resistor, you know the pd across them and their resistance, so you can use P = V^2 / R.

Reply 6

uberteknik

Original post by NajiAM17

Sorry but i dont understand. I am pretty sure its not potential dividers as its the next chapter.

Find the current through the two series resistors by first summing them (6 ohms).

The current through any series path is the same at all points, so use I = V/R = 3/6 = 0.5A

Now all you need is P = I²R for each resistor.

Spoiler

(edited 6 years ago)

Reply 7

Duck3E