Lower Secondary Science

[Dr.033430Daniel]

Carbon mostly forms covalent bonds. The terms covalent and ionic really are relative terms. Most bonds are somewhere in between equal sharing of electrons between two atoms and electrons completely taken from one atom. Later on in chemistry they will introduce the concept of electronegativity which gets into the degree to which an atom or molecule attracts electrons to itself.


But in chemistry you do sometimes see carbon involved in ionic bonding. The compound Calcium Carbide or CaC2 is usually described as having ionic bonds within it.

But the main point about Carbon that is usually stressed in Lower Secondary is that it is very well suited to form covalent bonds because it has exactly half of the 8 valence electrons. Atoms that have one extra electron have a strong desire to give one away. Atoms that are one short have a strong desire to gain one. Carbon and Silicon, in the middle, are best suited for sharing electrons.

Happy NEW YEAR !! - Hope everyone has a good 2012 !
We can toast the new year with C2H6O

[Verysuperkiasu]

Dr.Daniel:

But the main point about Carbon that is usually stressed in Lower Secondary is that it is very well suited to form covalent bonds because it has exactly half of the 8 valence electrons. Atoms that have one extra electron have a strong desire to give one away. Atoms that are one short have a strong desire to gain one. Carbon and Silicon, in the middle, are best suited for sharing electrons.

Happy NEW YEAR !! - Hope everyone has a good 2012 !
We can toast the new year with C2H6O

:thankyou:
Happy a great year ahead too.. :rahrah:

[Koky462]

My DS has a physics question. It is on the topic of light.


Converging lens A & B have the same focal length, but B is only half the diameter of A. Both lenses are used to form the images of distant objects on a screen. Which of the statement is correct?

A. For both lenses, the distances from the lens to the screen is the same.
B. For both lenses, the images are of the same brightness.
C. For both lenses, the images are the same size as the object.
D. Lens B gives a smaller image than A.

The correct answer is C, but my DS still do not understand. May anyone enlighten us?

[mum_sugoku]

Koky462:

My DS has a physics question. It is on the topic of light.


Converging lens A & B have the same focal length, but B is only half the diameter of A. Both lenses are used to form the images of distant objects on a screen. Which of the statement is correct?

A. For both lenses, the distances from the lens to the screen is the same.
B. For both lenses, the images are of the same brightness.
C. For both lenses, the images are the same size as the object.
D. Lens B gives a smaller image than A.

The correct answer is C, but my DS still do not understand. May anyone enlighten us?

Think the correct answer is wrong. Should be A instead.

[mum_sugoku]

Dr.Daniel:

Yes, C is correct. It is the curvature of a lens that determines the magnification. Just take a magnifying glass and cover half of it. When you look through, you still see the same magnification. But not as much light makes it through. So the image from a full lens will be brighter than an image from half of the lens. This might not be very apparent at normal lighting conditions. But the magnification depends on how much the light bends – and that is due to the curvature of the lens, not how big the lens is.


With telescopes you can have two telescopes that have the same curvature to the lens, or the same magnification. But one with a big lens collects more light and you can generally see a sharper image. I was out looking at the lunar eclipse with the Singapore astronomers last month. One of the best views was a guy who had a huge set of binoculars. The magnification wasn’t that much but the image was very clear because he was collecting so much light due to the size of the lenses.

At O-Level there are lots of problems where you move an object closer to and further from a lens and the image focal point changes and so does the magnification. Doing those kind of problems (and some experiments with lenses) help students get a feel for it.

Agree that it is the curvature of a lens that determines the magnification.. However, did you notice that the question already specified that both A & B have the same focal length?

My understanding is that for image to have exactly the same size as the object, the object should be positioned at exactly 2x the focal length (2F) away from the (centre of) lens.. However, for this particular question, we are looking at images formed by distant objects (>>2F away), and based on what I know, image formed by distant object should be smaller (ie diminished) - instead of of the same size as object - and with an image distance (location where you can get a sharp image) at exactly 1F away from the lens..

Which is why I believe that C is incorrect and it should be A instead lor.

[Dr.033430Daniel]

Oh yes - good point. You are right - A looks like the right choice.


It says the images are the same size as the object, not necessarily true. Its true that the images are the same size as each other, but not the same size as the object.

Thanks

[mum_sugoku]

Dr.Daniel:

Oh yes - good point. You are right - A looks like the right choice.


It says the images are the same size as the object, not necessarily true. Its true that the images are the same size as each other, but not the same size as the object.

Thanks

:oops: actually, I had made exactly the same mistake as you - failed to notice that \"same focal length\" thingy and had also thought \"C\" was the right answers :slapshead: . (Alas nowadays, some questions can be so tricky that we'll have to read and re-read before we could correctly figure out what they are asking ya )

[Koky462]

Thanks everyone for contributing their replies!


My doubts are cleared for now!

[Koky462]

My DS still has another question on lens.


When light which is perpendicular to the principal axis passes through a converging lens, what happens to its path?

I would appreciate if anyone can enlighten me again.

[KS-Lim]

Hi everyone


Pls advise / recommend any good guide book for Sec 1 Science.

Thanks