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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Physics => Topic started by: HarveyD on September 27, 2011, 04:49:42 pm

Title: Quick Light Question
Post by: HarveyD on September 27, 2011, 04:49:42 pm
If "the minimum photon energy necessary to eject electrons from a silver surface is about half of that required to eject electrons from a sodium surface.

Does that mean the work function is half of silver or the threshold frequencyy?
Title: Re: Quick Light Question
Post by: Lasercookie on September 27, 2011, 05:39:01 pm
That sentence is saying that: The work function for silver is around half of the work function of sodium.
It is not talking about the threshold frequency because it talks about energy. Energy is not frequency alone (it's E=hf).
You can prove this to yourself by looking at the data: http://hyperphysics.phy-astr.gsu.edu/hbase/tables/photoelec.html
Ag is 4.73 eV, Na is 2.28 eV

That said, the work function and threshold frequency are related.

The work function is the minimum energy required to remove an electron.
Threshold frequency is the minimum frequency required remove an electron.
So that's how you get
Title: Re: Quick Light Question
Post by: HarveyD on September 27, 2011, 06:13:15 pm
which alternative would it be
Title: Re: Quick Light Question
Post by: Lasercookie on September 27, 2011, 06:40:48 pm
I would go for C. Work function is the y-int, and it Na's y-int to be approximately half of Ag's y-int.

Definitely, not A or D. Not B either, it shows that Sodium's work function is larger than Silver's.

Which exam is this from?

Edit: Wait, the answer is B.
the graph is ek = hf - W.
Using the values from that table from hyperphysics, Silver would have a y-int -4.73 (i.e. lower on the graph) and Sodium's y-int would be -2.28 (i.e. higher on the graph).
Title: Re: Quick Light Question
Post by: HarveyD on September 27, 2011, 06:43:42 pm
yeah i put that too,

this is lisachem 2008
Answer is:
Answer: B
The gradients of the graphs for each metal must be the same.
Silver has a higher threshold frequency
Title: Re: Quick Light Question
Post by: Lasercookie on September 27, 2011, 06:44:17 pm
Noticed my mistake, edited the post. I think B is the right answer.
Title: Re: Quick Light Question
Post by: HarveyD on September 27, 2011, 06:50:38 pm
but how would you tell that without knowing the data from hyperphysics?
Title: Re: Quick Light Question
Post by: Lasercookie on September 27, 2011, 07:11:06 pm
Quote from: HarveyD on September 27, 2011, 06:50:38 pm
but how would you tell that without knowing the data from hyperphysics?
Arbitrary values.
Silver has to be twice that of Sodium.
A larger work function will be lower on the graph. A small work function will be higher on the graph.
Title: Re: Quick Light Question
Post by: HarveyD on September 27, 2011, 07:22:35 pm
but why twice that of sodium? shouldnt it be half according to the question?
Title: Re: Quick Light Question
Post by: Lasercookie on September 27, 2011, 07:28:35 pm
Quote from: HarveyD on September 27, 2011, 07:22:35 pm
but why twice that of sodium? shouldnt it be half according to the question?
Oh yeah, forgot that the question said that. Sorry, probably shouldn't have confused myself with hyperphysics data.

In that case, C is the answer.

Interesting that the answer they give conflicts with the information they provide in the question. I wonder if they got tripped up by their own wording lol. Lisachem 2008 seems to be riddled with these kind of mistakes :/
Title: Re: Quick Light Question
Post by: HarveyD on September 27, 2011, 07:30:07 pm
lol yeah, i know its so annoying

anyway thanks for all the help (again)
really appreciate it