Q&A - PSLE Science

[chrisu]

I've seen question like brightness of bulb in series n parallel cct in my son physic last year when he was sec.4. Now I'm seeing similar question in my P6 daughter science worksheet recently. It's not taught by teacher nor in text book. You can see that such worksheet are simply compilation of questions from all other unknown sources where errors may occur.


So parents should check if possible what your kids learned are correct.

bestwishes:

andante:

[quote=\"chrisu\"]Hi, can help with this question? I don't think any one of the answers is correct. Pls see attached picture. Thank you.

http://img.photobucket.com/albums/v160/guppiesonli/Misc/P6-Sc.jpg\">

I thought that the answer is (2).

Hi parents,
Kids are not taught 'short circuit' will choose (2) as answers. Basically, i think the setter just want to test the simple concept learnt in syllabus 'currents flow thr closed circuits but not open circuits.'.
I think even O level students are taught to calculate the resistance, current & voltage for series & parallel circuits but not touch on if the bulbs can 'practically' light up or not.
Of course when parents get involved to tell kids that (2) is impossible due to short circuit, effective resistance, voltage etc (lots of higher level terms), then the qn will become a very complicated one!! Is it the fault of setters to have such a simple yet complicated qn? Is it necessary for parents to tell kids of more adv stuff in this level?
D:[/quote]

[atutor2001]

bestwishes:

means that there is current flows thr the bulb but not strong enough to light up the bulb? so if there is a higher voltage, the bulb can light up??

Hi bestwishes

Lets just discuss for the sake of better understanding (not for Pr students consumption).

Lets simplify the circuit : a bulb and a wire connected in parallel (short-circuit) to a variable electrical source. Lets assume that the resistance of the wire is 0.001 ohm as provided by chrisu.

For the bulb to light up, the normal voltage across the bulb needs to be 3V.

If we manage to have a electrical source that can maintain a 3V potential across the \"short-circuit\" wire, the current flowing through the short-circuit wire will be I = V/R = 3/0.001 = 3000 amp. This is a very high current and the wire will melt before the bulb can light up, which is why we cannot demonstrate a bulb being lighted up despite being short-circuited by increasing the power supply.

[chrisu]

Not forgetting a battery will have internal resistance and this will vary between a new and used battery. A used battery will have higher internal resistance if I didn't remember wrongly.


Thus the current may not be too high if the battery is shorted.

atutor2001:

bestwishes:

means that there is current flows thr the bulb but not strong enough to light up the bulb? so if there is a higher voltage, the bulb can light up??

Hi bestwishes

Lets just discuss for the sake of better understanding (not for Pr students consumption).

Lets simplify the circuit : a bulb and a wire connected in parallel (short-circuit) to a variable electrical source. Lets assume that the resistance of the wire is 0.001 ohm as provided by chrisu.

For the bulb to light up, the normal voltage across the bulb needs to be 3V.

If we manage to have a electrical source that can maintain a 3V potential across the \"short-circuit\" wire, the current flowing through the short-circuit wire will be I = V/R = 3/0.001 = 3000 amp. This is a very high current and the wire will melt before the bulb can light up, which is why we cannot demonstrate a bulb being lighted up despite being short-circuited by increasing the power supply.

[atutor2001]

chrisu:

Not forgetting a battery will have internal resistance and this will vary between a new and used battery. A used battery will have higher internal resistance if I didn't remember wrongly.


Thus the current may not be too high if the battery is shorted.

Yes, you are absolutely correct that the current won't be large if we use battery as battery has internal resistance.

The electrical source that I was having in mind was a DC transformer where a potential difference of 3V can be maintained. However, personally, I still find it hard to believe that a mere 3V potential difference will melt a wire but the computed current of 3000 amp is indeed very large and should be able to melt it (compared to the current needed for our normal household appliances which is less than 13 amp r.m.s)

[chrisu]

Hi atutor2001,

You are right to point out the short circuit part and the high current in the circuit. That's why it is important to incorporate a fuse in the appliances or circuit or device where it will open the circuit to prevent further damages to the components.

The teacher should not use short circuit to teach the difference between closed and open circuit. There is a great difference between short and closed circuit. Short circuit is to be avoided and the danger of it should not be disregarded. So even for primary school, the teacher should not confuse the students with short circuit and closed circuit.

atutor2001:

chrisu:

Not forgetting a battery will have internal resistance and this will vary between a new and used battery. A used battery will have higher internal resistance if I didn't remember wrongly.

Thus the current may not be too high if the battery is shorted.

Yes, you are absolutely correct that the current won't be large if we use battery as battery has internal resistance.

The electrical source that I was having in mind was a DC transformer where a potential difference of 3V can be maintained. However, personally, I still find it hard to believe that a mere 3V potential difference will melt a wire but the computed current of 3000 amp is indeed very large and should be able to melt it (compared to the current needed for our normal household appliances which is less than 13 amp r.m.s)

[tinasen]

A mountainer climbing to higher altitudes finds it difficult to breathe, as there is ----

a) less air b) less nitrogen c) more carbondioxide d) less water vapour

My answer is a) as density if air is less at higher altitudes whereas oxygen requirement by body remains the same.
But given answer is d) less water vapour. Reason being, composition of gases in atmosphere remains fairly constant at all altitudes. It is only the amount of water vapour that differs.
Can anyone help?

[Nebbermind]

tinasen:

A mountainer climbing to higher altitudes finds it difficult to breathe, as there is ----

a) less air b) less nitrogen c) more carbondioxide d) less water vapour

My answer is a) as density if air is less at higher altitudes whereas oxygen requirement by body remains the same.
But given answer is d) less water vapour. Reason being, composition of gases in atmosphere remains fairly constant at all altitudes. It is only the amount of water vapour that differs.
Can anyone help?

Is this taken from past yr paper or revision bk?

The answer shd be air. AT high altitude, the air pressure is lower, so the same volume of air will have less oxygen in it since the oxygen molecules (and also the rest of the gases) are packed less closely.

[tinasen]

From a revision book.

I also think the correct answer is a)
Thanks Nebbermind.

[Nebbermind]

for science, it’s always better to stick to the one the school prescribed. The qns in some of the easily available revision bks can be very confusing…not a good thing to attempt when so close to the PSLE.

[smartmummy]

Pls help me.

Is ceramic man-made?