OCR Physics A G482, Electrons, Waves and Photons, 25th May 2012

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conductivity is just 1/resistivity

Thank you!!! :biggrin:

Now just gotta hope it comes up tomorrow...

Also, question for whoever knows it, is an absorption spectrum mostly black with a few coloured lines or mostly coloured with a few black lines? o.O

Reply 241

karanrucks

Original post by jake9382

can someone plz help me understand this. If sound waves are longitudinal, then how come when you pluck a violin string a stationary transverse wave is produced? So the sound is transverse? i dont understand how sound is longitudinal then? Thanksss

Sound is created by the medium therefore the air vibrating not the string. The string vibrates the air which produces a longitudinal Sound Wave. Hope this helps your question

Reply 242

SamXi

Original post by Joseppea

Thank you!!! :biggrin:

Now just gotta hope it comes up tomorrow...

Also, question for whoever knows it, is an absorption spectrum mostly black with a few coloured lines or mostly coloured with a few black lines? o.O

Coloured with a few black lines

Think of it like this ... all ranges of Em radiation are been shone through but the electrons only absorb certain wavelengths/frequency which is why you get the black bands

Reply 243

sweetascandy

Original post by SamXi

conductivity is just 1/resistivity

Srsly? Are we meant to know this though? Well, even if we don't, I've learnt it now anyway.

Reply 244

jake9382

Original post by karanrucks

Sound is created by the medium therefore the air vibrating not the string. The string vibrates the air which produces a longitudinal Sound Wave. Hope this helps your question

thankss soo if vibrating air produces longitudinal wave then how does the whole transverse wave come into the situation? Is there a longitudinal and tranverse wave? :confused:

Reply 245

Joseppea

Original post by davie18

Hmm seems fine to me. What happens when you try to open it?

Massive implosion... followed by an explosion. :biggrin:

......it actually works fine for me :tongue:

Reply 246

shyro1000

oh dreading this exam tomorrow, anyone know anything that will almost definetly come up?

Reply 247

SamXi

Gonna dump all the experiments i know ... someone correct me if i miss something out pls !

Photoelectric effect
-em radiation shone on metal
-one photon acts with one electron
-if the photon is below the threshold frequency no electrons are displaced even if the intensity is altered
-if the photon is above the threshold frequency a surface electron is freed
-when the frequency is above the threshold the amount of electrons displaced is effected by intensity
-if the kinetic energy transfered from the photon to the electron is great enough it will travel through the vacuum to complete the circuit

Stopping potential
- The stopping potential can be found by passing a current the opposite direction to the freed electron
- To find the max KE of an electron we need to know find voltage is needed to cancel out the photoelectron
- We then multiply the voltage by e (electron charge / 1.6x10^-19) to find energy (Max KE)

Two slit - light
- Coherent Light shone at a screen through two slits
-where the light meets at the screen an odd number of half wavelengths apart (or out of phase) it creates min intensity which is destructive interference
-where the light meets and even number of half wavelengths apart (or in phase) it creates max intensity which is constructive interference

Two slit - microwave
Same as light but microwaves shone through 2 slits in a metal screen at a microwave detector

Find the speed of sound
- Vibrate a tuning fork of known frequency above a measuring cylinder
- Add water to the cylinder until resonance occurs (sound becomes loudest)
- Measure the coluomb of air above the water this is the fundemental frequency (1/4 a wavelength)
- times the length of air by 4 to find the wavelength then use V=FxLambda to find V

Finding planks constant, threshold frequency and the work function
-Alter the P.D. across LED's of different frequencies until they become visible
-Multiply the P.D. by e (Electron charge / 1.6x10^-19) to change it to energy
-Plot a graph of frequency against energy
-Gradient = planks constant
-x intercept = threshold frequency
-y intercept = work function

De Broglie diffraction
-An electron beam is shone through a sheet of material
-Where the electrons diffract the most is the point where the wavelength of the electron is aprox equal to the atomic spacing in the material
-by changing the amount of energy in the electron we can change the wavelength
-We can use this experiment to determine the atomic structure of materials

(edited 12 years ago)

Reply 248

sweetascandy

Original post by Joseppea

Thank you!!! :biggrin:

Now just gotta hope it comes up tomorrow...

Also, question for whoever knows it, is an absorption spectrum mostly black with a few coloured lines or mostly coloured with a few black lines? o.O

This may summarise it for you:
- A line spectrum = a series of bright light against a dark background.
- An absorption spectrum = a series of dark lines appear against bright backgrounds.

Reply 249

jake9382

Original post by SamXi

Hey you know the experiment where the circuit is used for measuring kintetic energy of photoelectrons (p174 in the book) could you please explain it to me? I dont get the whole p.d. thing :confused:

Reply 250

SamXi

Original post by jake9382

Hey you know the experiment where the circuit is used for measuring kintetic energy of photoelectrons (p174 in the book) could you please explain it to me? I dont get the whole p.d. thing :confused:

I dont have the book mate.. but i think thats stopping potential which ive explained above il try and go into more detail

- The stopping potential can be found by passing a current the opposite direction to the freed electron
- To find the max KE of an electron we need to know find voltage is needed to cancel out the photoelectron
- We then multiply the voltage by e (electron charge / 1.6x10^-19) to find energy

(edited 12 years ago)

Reply 251

jake9382

Original post by ebmaj7

When you pluck a violin string, it makes a sound. The sound is longitudinal, like any other sound wave.

However, if you ignore the sound, and look at the string, the string has a longitudinal wave going through it.

Like when you pluck an elastic band or something. The string goes up and down and makes a sorta stretched oval shape. There is a longitudinal wave going through the string.

•

Progressive wave is reflected at the edge of the violin.

•

Reflected wave superposes with incident wave.

•

The two waves interfere with each other.

•

Where destructive interference takes place, nodes are produced.

•

Where constructive interference takes place, antinodes are produced.

THANKKKS I get it noww :smile:

Reply 252

sweetascandy

Original post by Dale12

with 2 source interference, you move the probe along the 'screen' (where the screen would be in youngs double slit) and you'll get the interference pattern...

however to detect a stationary wave you move it between the transmitters and the metal sheet, sort of back and forth and you will be able to detect low signal (node) and high signal (antinode)

also the 2 source interference uses two transmitters connected to one source, for stationary waves only 1 transmitter is used to reflect off a metal sheet... that help?

Do you know an experiment for stretched strings forming stationary waves?

Reply 253

shyro1000

anyone know the experiments we need to know?

Reply 254

Joseppea

Original post by shyro1000

anyone know the experiments we need to know?

Check this thread a couple of pages back - lots of experiment comments :biggrin:

Reply 255

jake9382

Original post by SamXi

cool thanks a lot :smile:

Reply 256

shyro1000

Original post by Joseppea

Check this thread a couple of pages back - lots of experiment comments :biggrin:

ah ok thanks bud :smile:

Reply 257

SamXi

Original post by shyro1000

anyone know the experiments we need to know?

Look at my post on the previous page .. im adding all the ones i know

-two slit
-find planks constant
-speed of sound
-photoelectric effect
-gold leaf spectrometer photoelectric effect
-Voltage/current graphs
-de broglie diffraction

Reply 258

WarpStreak

So, to find the I-V characteristics of a resitor, lamp, LED, how do you set up the circuit? i know it's something to do with a potential resistor, and a voltmeter in parallel with component you're testing, but...

mark schemes seem to differ, one said that you couldn't just set up the PR in series with the component (and then the voltmeter in parallel with just the component), and i can't see why this is wrong or how i should draw it to make it correct.

Anyone know? :smile:

Reply 259