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# PHYSICS - Last question(s) on Electricity watch

1. Are the answers I have filled in correct? Also not sure on (c)?

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2. (Original post by ohnanailikenanas)
Are the answers I have filled in correct? Also not sure on (c)?

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what happens to the voltage in a parallel circuit? This should help you with d and I think b is right not sure tbh
3. Does it not stay the same? So is it just 9V? Is (a) right?

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4. (Original post by ohnanailikenanas)
Does it not stay the same? So is it just 9V? Is (a) right?

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(a) is right and yes it does so yup
5. (Original post by ohnanailikenanas)
Are the answers I have filled in correct? Also not sure on (c)?

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The answer to parts a) and b) are correct.

The answers to part d) are all incorrect.

You need to find the answers to part c) before you can answer part d).

Think of the current paths between the battery terminals.

R2 is in parallel with R3.

R1 is in series with that parallel combination.

The 9V battery is across R1 in series with the parallel combination.

The p.d. developed across R1 is not 9V

The p.d. developed across the parallel combination of R2 and R3 is not 9V

You also need to use Kirchoff's voltage and current rules.
6. Part A and B I got the same answers. For part D I think you would have to do part C first in my opinion because, in that circuit the total resistance is 4.5 ohms with 9 volt battery. The 3 ohm resistor is in series with the parallel combination of total resistance 1.5 ohms. What I would have done was used a ratio technique there by dividing the battery voltage by the total resistance and multiplying it by the 3 ohm resistor to calculate the voltage across V1. However, correct me If I'm wrong because I haven't done a question like this in a while....
7. (Original post by MSB47)
What I would have done was used a ratio technique there by dividing the battery voltage by the total resistance and multiplying it by the 3 ohm resistor to calculate the voltage across V1. However, correct me If I'm wrong because I haven't done a question like this in a while....
That's an OK method.

(Original post by ohnanailikenanas)
Does it not stay the same? So is it just 9V? Is (a) right?

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This is mainly aimed at the OP:

Part c)

1)Find the total resistance of the circuit between the battery terminals using the series-parallel technique you learned in your previous questions (XY and XZ).

2)Use Ohms law to calculate the total current flowing out of the battery terminals: i.e. the battery voltage and the total circuit resistance calculated in 1) above.

3)Use Ohms law to calculate the voltage across the 3 ohm series resistor. i.e. the total current calculated in 2) above and the value of 3 ohms.

4)Use Kirchoff's Voltage Law (KVL) to calculate the potential difference (p.d.) across the parallel combination of resistors. KVL states: the sum of the potentials dropped around a circuit is equal to the supply voltage. You will need the battery voltage and and the p.d. calculated in 3) above to do this.

The answer to parts cii and ciii are identical because there can only be one p.d. developed between the two common junctions of a parallel combination.

Part d)

5)The current flowing through R1 is already calculated in part 2) above.

6)Use Ohms law to calculate the current in each parallel branch. The voltage across each parallel resistor is calculated in 4) above and each resistance value is known.

OP: what answers did you get?

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