Q&A - PSLE Science

[Vanilla Cake]

kittycat01:

2) picture shows a pond with frogs outside pond and tadpoles inside pond, butterflies flying and caterpillars on the leaves of some aquatic plants (there are 3 different types of plants), fishes.


How many communities in the picture above? Some argue 2 because caterpillars & butterflies belong to garden community. But by definition of community - it means DIFFERENT populations together that means at least 2 or more right? Cater. & butt belong to 1 pop right? So if there is only 1 pop, then cannot form a community. So is it 1 or 2 comm?

atutor2001:

2) I think the most important word we need to consider for \"community\" is \"interaction\". A person standing at a pond community is not part of that community because he/she does not depend/interact on/with the other pollutions in the pond community for survival and vice versa.

With regard to the \"size/limit\" of a community, there is no set rule on the \"size/limit\" of a community. It is up to the person who is doing the study to decide. A pond in a garden can be broken into 2 communities i.e. pond & garden or it can also be considered as 1 single community. The important factor is that there must be interaction among the populations in the \"defined community.\"

Therefore, in your example, I will take it as 1 community (garden with pond community) because there is interation between frog and butterfly (frog eats butterfly/caterpillar)

The diagram below shows a habitat with some living things.
http://i40.tinypic.com/ivlb7r.png\">
Based on the diagram shown, which one of the following statements is correct?

(1) There is one community with six populations.
(2) There is one community with eight populations.
(3) There are two communities with six populations.
(4) There are two communities with eight populations.

[atutor2001]

Vanilla Cake:

Hi atutor2001,
Here's the question (placed under a lamp and not under the sun):

Study the set-up as shown below:
http://i41.tinypic.com/2aa0t3a.png\">
After some time, the rate of photosynthesis of the plant slowed down. This was most likely caused by a lack of ________________.

(1) water
(2) oxygen
(3) chlorophyll
(4) carbon dioxide

Thanks Vanilla Cake for posting the picture. With the picture, I agree with choco05 that (4) should be the answer because carbon dioxide will be depleted before water as the roots are immersed in a flask of water.

[atutor2001]

Vanilla Cake:

The diagram below shows a habitat with some living things.
http://i40.tinypic.com/ivlb7r.png\">
Based on the diagram shown, which one of the following statements is correct?

(1) There is one community with six populations.
(2) There is one community with eight populations.
(3) There are two communities with six populations.
(4) There are two communities with eight populations.

I will pick (1) as my answer.

[Verysuperkiasu]

http://i41.tinypic.com/2602yac.jpg\">

There are 2 different books explaining the same diagram above. Their explanations are contradictory.

book 1 :
In the example below, the two bottom batteries cancel each other out. The overall effect is thus the same as using only one battery. Bulb is still able to light up.

book 2 :
bulb will not light up as the bottom 2 batteries are not connected properly.

Which is correct? IMO, i go with book 2 because if the batteries connection are wrong, then there will be a broken circuit. Want to confirm this is correct be4 telling my dd.

[small]

I will pick the answer from Book 2…

[atutor2001]

Verysuperkiasu:

http://i41.tinypic.com/2602yac.jpg\">

There are 2 different books explaining the same diagram above. Their explanations are contradictory.

book 1 :
In the example below, the two bottom batteries cancel each other out. The overall effect is thus the same as using only one battery. Bulb is still able to light up.

book 2 :
bulb will not light up as the bottom 2 batteries are not connected properly.

Which is correct? IMO, i go with book 2 because if the batteries connection are wrong, then there will be a broken circuit. Want to confirm this is correct be4 telling my dd.

Book 1 is correct. A battery is a conductor of electricity regardless whether it is flat, connected in the wrong direction...unless the battery has \"exploded\" i.e. the chemical leak out. When the polarity are connected in the same direction in series, the voltages will add up. When connected in opposite directions, the resultant voltage is the net voltage i.e. 2 batteries of the same voltage but in opposite directions will cancel each others voltage.

[Verysuperkiasu]

atutor2001:

Verysuperkiasu:

http://i41.tinypic.com/2602yac.jpg\">

There are 2 different books explaining the same diagram above. Their explanations are contradictory.

book 1 :
In the example below, the two bottom batteries cancel each other out. The overall effect is thus the same as using only one battery. Bulb is still able to light up.

book 2 :
bulb will not light up as the bottom 2 batteries are not connected properly.

Which is correct? IMO, i go with book 2 because if the batteries connection are wrong, then there will be a broken circuit. Want to confirm this is correct be4 telling my dd.

Book 1 is correct. A battery is a conductor of electricity regardless whether it is flat, connected in the wrong direction...unless the battery has \"exploded\" i.e. the chemical leak out. When the polarity are connected in the same direction in series, the voltages will add up. When connected in opposite directions, the resultant voltage is the net voltage i.e. 2 batteries of the same voltage but in opposite directions will cancel each others voltage.

Oh, I see. So u mean even if the batteries are not connected properly, the current is still able to flow through? Just that the voltages is zero?....TIA

[atutor2001]

Verysuperkiasu:

Oh, I see. So u mean even if the batteries are not connected properly, the current is still able to flow through? Just that the voltages is zero?....TIA

Your statement is not correct. The current can only flow if the voltage in one direction is higher than the other direction i.e. there is a net voltage in the circuit.

For example, if there are 5 batteries in one direction and 3 batteries in the opposite direction then there is a \"net voltage\" of 2 batteries (5 - 3 = 2). The brightness of the bulb is the same as a circuit with 2 batteries connected in series in the same direction.

If there are 5 batteries in one direction and 5 batteries in the opposite direction then there is a net voltage of 0 (5 - 5 = 0). The bulb will NOT light up because the net voltage is 0. The batteries are still conductors of electricity but NO current will flow because current can only flow if there is a \"net voltage\" from the batteries in the circuit.

[Verysuperkiasu]

atutor2001:

Verysuperkiasu:

Oh, I see. So u mean even if the batteries are not connected properly, the current is still able to flow through? Just that the voltages is zero?....TIA

Your statement is not correct. The current can only flow if the voltage in one direction is higher than the other direction i.e. there is a net voltage in the circuit.

For example, if there are 5 batteries in one direction and 3 batteries in the opposite direction then there is a \"net voltage\" of 2 batteries (5 - 3 = 2). The brightness of the bulb is the same as a circuit with 2 batteries connected in series in the same direction.

If there are 5 batteries in one direction and 5 batteries in the opposite direction then there is a net voltage of 0 (5 - 5 = 0). The bulb will NOT light up because the net voltage is 0. The batteries are still conductors of electricity but NO current will flow because current can only flow if there is a \"net voltage\" from the batteries in the circuit.

sorry, I'm quite bad in understanding electricity. If batteries are conductors of electricity (then shouldn't electricity still flows) isn't flow of electricity = flow of current? that is, if batteries still can conduct electricity, shouldn't current flow as well?

[Sun_2010]

Verysuperkiasu:

atutor2001:

[quote=\"Verysuperkiasu\"]
Oh, I see. So u mean even if the batteries are not connected properly, the current is still able to flow through? Just that the voltages is zero?....TIA

Your statement is not correct. The current can only flow if the voltage in one direction is higher than the other direction i.e. there is a net voltage in the circuit.

For example, if there are 5 batteries in one direction and 3 batteries in the opposite direction then there is a \"net voltage\" of 2 batteries (5 - 3 = 2). The brightness of the bulb is the same as a circuit with 2 batteries connected in series in the same direction.

If there are 5 batteries in one direction and 5 batteries in the opposite direction then there is a net voltage of 0 (5 - 5 = 0). The bulb will NOT light up because the net voltage is 0. The batteries are still conductors of electricity but NO current will flow because current can only flow if there is a \"net voltage\" from the batteries in the circuit.

sorry, I'm quite bad in understanding electricity. If batteries are conductors of electricity (then shouldn't electricity still flows) isn't flow of electricity = flow of current? that is, if batteries still can conduct electricity, shouldn't current flow as well?[/quote]Let me try to explain-

Current will flow only of there is a potential difference ie. voltage.

as atutor put it \"If there are 5 batteries in one direction and 5 batteries in the opposite direction then there is a net voltage of 0 (5 - 5 = 0). The bulb will NOT light up because the net voltage is 0. \"
In this case the net voltage is 0 so there is no current. Just like if you take a wire and connect both ends there will not be any current in it even though it is a conductor as there is no voltage.
For case 2 \"if there are 5 batteries in one direction and 3 batteries in the opposite direction then there is a \"net voltage\" of 2 batteries (5 - 3 = 2). \"
As there is a \"net voltage\" and the circuit is complete, there will be a flow of current .