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Original post by Branny101
Could anyone explain, please?

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Do you not understand how I explained it?
Anyone with Examiner's report for Jan 13 please? Urgent
Reply 522
Original post by Tomupcraft
Do you mean out of phase or 180 degrees out of phase? out of phase can be anything that is not in phase with a point, wheres a point in anti phase must be 180 degrees out of phase.

I meant 180 out of phase
Reply 523
Original post by Tomupcraft
Do you not understand how I explained it?


Sorry I just saw your reply, but doesn't frequency remain constant ?

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Original post by Branny101
Sorry I just saw your reply, but doesn't frequency remain constant ?

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Yes, If blue light goes from air into water its frequency will remain constant but its speed will decrease and so will its wavelength as when you draw diagrams the waves get closer together.

I wouldn't worry why shorter wavelengths refract more, just know that shorter wavelengths refract greater than larger wavelengths and therefore blue refracts more than red. So when you draw a diagram with a red and blue ray, the blue ray must refract closer to the normal.
Reply 525
Original post by Tomupcraft
One time period is between adjacent compressions. If the wave had traveled T the positions would basically look the same, the particles would have just moved up the line. However as 3/4T has passed then the compressions must be 3/4 along the line between two compressions.

If that does not make sense I will try to explain further.


Hi, is there any chance you could explain further, the bits that I don't understand is, if one wavelength (distance from 2 adjacent compressions ) is the time period as the time period = the amount of time taken for one complete oscillation, then how do we know where the particles would be at t +3/4T, I find it really confusing? Thanks :smile:
Original post by alygirl
I meant 180 out of phase


If you draw a diagram of a sine wave and label the axis 0, pi/2, pi, 3pi/2 and 2pi

Take the point 0, the point in phase with that is 2pi as it is a full wavelength ahead. Waves that are in phase are whole number of wavelengths ahead.

However the point at pi is not in phase with 0 as it is half a wavelength ahead, therefore it is out of phase. And because it is half a wavelength ahead this means it is in antiphase. Whenever two points are half a wavelength apart they are in antiphase.
Original post by LegendX
Hi, is there any chance you could explain further, the bits that I don't understand is, if one wavelength (distance from 2 adjacent compressions ) is the time period as the time period = the amount of time taken for one complete oscillation, then how do we know where the particles would be at t +3/4T, I find it really confusing? Thanks :smile:


The wave travels at a constant speed. So if we take the first compression, in one time period that compression will be in the same position as the second compression. As it travels at a constant speed if half a time period had passed then the first compression must now be half way between its original position and the original position of the second position.

If a tenth of a time period had passed then it would have been a tenth of the way along. Therefore if 3/4T has passed it must be three quarters between the original compression's.
Reply 528
Original post by Tomupcraft
If you draw a diagram of a sine wave and label the axis 0, pi/2, pi, 3pi/2 and 2pi

Take the point 0, the point in phase with that is 2pi as it is a full wavelength ahead. Waves that are in phase are whole number of wavelengths ahead.

However the point at pi is not in phase with 0 as it is half a wavelength ahead, therefore it is out of phase. And because it is half a wavelength ahead this means it is in antiphase. Whenever two points are half a wavelength apart they are in antiphase.


Thank youuuus :biggrin:, how is internal resistance measured?

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Reply 529
Anyone please tell me how to do this?
Original post by AT95
Anyone please tell me how to do this?


I did that a couple pages back :smile:


Original post by CharlieTT
This is a combination of resistance in parallel and potential dividers equation.

1/R1=1/R.1 + 1/R.2
=1/6000Ohms +1/6000Ohms
=1/3000Ohms
R1=3 kiloOhms

Vout=Vin x R2/(R1+R2)
=12V x 6000Ohms/9000Ohms
=2/3 x 12V
=8V
Reply 531
Original post by Tomupcraft
If you draw a diagram of a sine wave and label the axis 0, pi/2, pi, 3pi/2 and 2pi

Take the point 0, the point in phase with that is 2pi as it is a full wavelength ahead. Waves that are in phase are whole number of wavelengths ahead.

However the point at pi is not in phase with 0 as it is half a wavelength ahead, therefore it is out of phase. And because it is half a wavelength ahead this means it is in antiphase. Whenever two points are half a wavelength apart they are in antiphase.

That makes sense. Sorry to bother you again, but please if you get a chance can you help me on Jan 2009 qu 13 b?
Original post by Branny101
Thank young, how is internal resistance measured?

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If we take a solar cell as an example.
Have the solar cell in series with an ammeter and a variable resistor. Have a voltmeter in parallel with the solar cell.
Vary the resistance and record current and voltage.

as V = E - Ir rearrange to form V = -rI + E it is in the form y=mx+c
V on y axis
I on x axis
therefore E will be y intercept
so internal resistance, r will be the gradient.
Original post by alygirl
That makes sense. Sorry to bother you again, but please if you get a chance can you help me on Jan 2009 qu 13 b?


Current is split, if 30mA goes in then the current in both wires must = 30mA.
20mA in one wire so 10mA in the other wire so I1 = 10mA
15mA leave the first wire so 5mA must be remaining therefor I2 is 5mA
as current is conserved and 30mA went in, 30mA must also leave therefore I3 must be 30mA.
Reply 534
Original post by Tomupcraft
If we take a solar cell as an example.
Have the solar cell in series with an ammeter and a variable resistor. Have a voltmeter in parallel with the solar cell.
Vary the resistance and record current and voltage.

as V = E - Ir rearrange to form V = -rI + E it is in the form y=mx+c
V on y axis
I on x axis
therefore E will be y intercept
so internal resistance, r will be the gradient.


Thank you XD, what other experiments other than resistivity do we need to be able to understand and apply?

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Original post by Branny101
Thank you XD, what other experiments other than resistivity do we need to be able to understand and apply?

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Standing waves
Polarimetry
Finding resistivity of a metal

They are the main ones, I cannot remember any others off the top of my head.
Reply 536
Original post by Tomupcraft
Standing waves
Polarimetry
Finding resistivity of a metal

They are the main ones, I cannot remember any others off the top of my head.


The standing waves is just a tube with lycopodium powder right?

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Original post by Branny101
The standing waves is just a tube with lycopodium powder right?

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You mean Kundts tube? theres also the one with string attached to a mass hanging over a pulley.
Reply 538
Original post by Tomupcraft
You mean Kundts tube? theres also the one with string attached to a mass hanging over a pulley.


O__o... Let's stick with the glass tube :wink: ahaa

Thanks BTW, for your help.

EDIT: I just realised the experiment you meant

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(edited 10 years ago)
Reply 539
Can someone please tell me the equation to calculate path difference from phase difference? And the equations derived from the doppler effect, asap would be awesome :biggrin:

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