Year 12s: what will be the first way you research your future options? >>

Electricity [Help Please]

I'm very stuck on some questions that I've attempted. I've answered the questions though but I don't understand why I got my answers wrong and why the mark scheme's answer is right. I would be very grateful if someone could explain these concepts to me.

Question 1

What I don't understand with this question is that why is it when X is closed that buzzer P doesn't have charge flowing through?

Question 2

I know this is just a 2 mark question, but I'm quite skeptical on what I should write down for this. Is it right to say in order to answer this question that, at room temperature the metal lattice ions have some vibrational energy, making collisions between the delocalised electrons and the metal lattice ions more likely and as a result the temperature increases?

Question 3

I'm particularly stuck on iii). I wrote down that the temperature will increase but in the mark scheme it says that it will cause a short-circuit and I don't understand why.

Reply 1

uberteknik

Original post by minibuttons

What I don't understand with this question is that why is it when X is closed that buzzer P doesn't have charge flowing through?

Closing the switch X short circuits the current to P. That is the resistance of the short circuit is very low compared with the resistance of the buzzer path.

The great majority of current will now flow in the short circuit path and NOT through the buzzer since I = V/R and R becomes very small for the short circuit where the buzzer resistance remains large by comparison.

Hence very little power (I²R) can be developed in the buzzer and it will not sound.

Original post by minibuttons

Question 2

Yes. Vibrational kinetic motion = resistance because of the increased probability of collisions/capturing free electrons causing then to give up their kinetic energy to the receptor atoms/lattice ions.

In doing so, the lattice ions vibrate even more. i.e. the lattice ion vibrational energy increases and hence resistance has also increased. (+ve temperature coefficient).

Reply 2

uberteknik

Question 3

I'm particularly stuck on iii). I wrote down that the temperature will increase but in the mark scheme it says that it will cause a short-circuit and I don't understand why.

The question states that the ammeter resistance is zero.

With the ammeter placed across the cell, there is no resistance to limit the current flow other than the internal resistance of the battery. i.e this is the definition of a short circuit.

The 6 amps the ammeter measured must also flow through the internal resistance and all of the current now heats up the internal resistance.

The battery will become exhausted very quickly and will dissipate:

r = 2.4/6 = 0.4 ohms

P = I²R = 6²x 0.4 = 14.4 watts in the internal resistance.

(edited 9 years ago)

Reply 3

minibuttons

Original post by uberteknik

Question 3

I'm particularly stuck on iii). I wrote down that the temperature will increase but in the mark scheme it says that it will cause a short-circuit and I don't understand why.

The question states that the ammeter resistance is zero.

With the ammeter placed across the cell, there is no resistance to limit the current flow other than the internal resistance of the battery. i.e this is the definition of a short circuit.

The 6 amps the ammeter measured must also flow through the internal resistance and all of the current now heats up the internal resistance.

The battery will become exhausted very quickly and will dissipate:

r = 2.4/6 = 0.4 ohms

P = I²R = 6²x 0.4 = 14.4 watts in the internal resistance.

Wow your explanations are very clear :smile: