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

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Original post by wibletg
These taken from the specification?


Yh.
Original post by rafcan
Can someone write some quick answers to these?



Electricity

Describe the uses and benefits of using light-emitting diodes.

Uses: Mainly as an indicator to show something is on or off but can no be used as a replacement to filament lamps. Benefits: Cheaper to run as they are more energy efficient. This is because they have a threshold voltage: no current will flow above this voltage whilst filament lamps will let a current flow and any potential difference. Plus they only conduct on way round. (long leg of LED is positive terminal).

Describe an experiment to obtain the I-V characteristics of a resistor at constant temperature, filament lamp and light emitting diode.

Ammetre, Resistor/Filament Lamp/LED and Potentiometre in series. Place voltmetre in parallel with the component being tested. Limit the current flowing by varying the potentiometre accordingly, taking current and potential difference readings respectively. Plots these I on V graph. V (x axis) I (y axis). Graph trends: Resistor follows ohms law so straight line through origin. Filament Lamp is non-ohmic as its temperature varies so the line should go up and then curve so the gradient= 0. LED is non-ohmic as well, so no current should flow until a certain threshold voltage, so flat line to indicate no current, then a upwards line to show current is now flowing.

Define the Coulomb

The SI unit of electrical charge. A charge of 1C passes through a point when a current of 1A flows for 1s. Therefore 1C = 1A s

Explain that electric current is a net flow of charged particles

Wires are made from metal. The metal contains a sea of delocalised electrons which move in random directions. When a cell is connected to the wire and electrical force is applied to the electrons making them 'drift'. They still move in random directions however they have an overall velocity or movement (for electrons negitive---->positive terminal) This can happen with ions too. This overall velocity is defined as the drift velocity: v= I/nAe

Describe the difference betwen conductors, semiconductors and insulators in terms of the number density n.

Conductors have lots of charge carriers and therefore can easily let current flow, semi conductors have free few delocalised charge carriers so less current flows, it is important to note that semi conductors conduct better at higher temperatures as more electrons break free from the atoms. Insulators have very few/no charge carriers and therefore don't let any current flow.

Explain the meaning of the term Terminal P.d.

The potential difference across a source. Not too sure on this one any suggestions??

Waves

Explain what is meant by reflection and refraction.

Reflection: the bouncing back of a wave from an impenetrable surface or barrier, their wavelengths remain the same. Refraction: When a wave crosses a different medium interface causing its direction to change and well as its velocity and wavelength

State typical values for the wavelengths of the different regions of the EM spectrum.

Radio: 10^-1 -------- 10^6 Radio transmissions
Microwaves: 10^-3 -------- 10^-1 Radar, microwave cookery, TV transmissions
Infrared: 7x10-7 -------- 10^-3 Night-visions, optical fibres, remote controls
Ultraviolet:10^-8 ------- 4x10^-7 Sunbeds, Security Markings
X-Rays: 10^-13 ------- 10^8 See damage to bones, Kill cancer cells
Gamma Rays: 10^-16 -------- 10^-1 Irradiation of food, Sterilisation, Chemotheraphy

Describe some practical uses of EM waves.

Above^

Recall and use Malus law

Need to revise this.

State principle of superposition

When two or move waves meet at a point and interfere, the sum of their individual displacements is equal to the resultant displacement.

Apply graphical methods to illustrate the principle of superposition

Ripple tank... two dippers creates circular waves that interfere with one another. Where two wave fronts meet in phase= constructive interference.

Describe Experiments that demonstrate two-source interference using sound, light and microwaves.

Sound: Two loudspeakers, x distance apart plugged into a signal generator with varying frequencies. Have a microphone parallel to the loudspeaker and a certain distance away from the speakers. Plug microphone is an oscilloscope to show wave. (Coherent source)
Light: Diffract it using a slide with a double slit that has comparable slit distance to the wavelength of the source of light you are using. Place a screen behind the double slit- a series of bright and dark fringes will appear. (double slit makes sure wave are in phase with one another)
Microwave: Same idea ^^^^ metal plated double slit, microwave transmitter, microwave probe, meter.

Couldn't be arsed to do the rest, hope this helps in some way. Badly written soz.
how these answers are 'quick' enough
Hi, if anyone could explain this one question to me, I will be very grateful :smile:

(Sorry if I explain it poorly, but I can't find the actually question paper online [Jan 2011 - Qu7 c.i.])

The light from a filament is veiwed through a diffraction grating, having 300 lines per mm. The continous first order spectrum appears between angles 7 and 12 (degrees) to the direction of the incident light;

State the colour of the light the is seen at an angle of;

7:
12:


The used: The angle = arcsin(wavelength/d)

And used typical values for red (~350nm) and violet (~700nm) and from this you get roughly 7 for red and 12 for violet, but in the mark scheme it has it the other way round :confused:

Probably very simple, but I really don't get how they got the two that was around :colondollar:
Reply 64
Avatar for hanlem
hanlem
^^ i think that's because red has a longer wavelength so is diffracted more? so is at the larger angle

we did that paper for our mock and i did the same as you so that might be completely wrong!

also for stating the wavelengths, do you think it's okay to write
radio ~ 1m
microwaves ~ 1cm
etc

or do you need to give the range of wavelengths?

thank you!
Original post by A for Andromeda
Hi, if anyone could explain this one question to me, I will be very grateful :smile:

(Sorry if I explain it poorly, but I can't find the actually question paper online [Jan 2011 - Qu7 c.i.])

The light from a filament is veiwed through a diffraction grating, having 300 lines per mm. The continous first order spectrum appears between angles 7 and 12 (degrees) to the direction of the incident light;

State the colour of the light the is seen at an angle of;

7:
12:


The used: The angle = arcsin(wavelength/d)

And used typical values for red (~350nm) and violet (~700nm) and from this you get roughly 7 for red and 12 for violet, but in the mark scheme it has it the other way round :confused:

Probably very simple, but I really don't get how they got the two that was around :colondollar:


You got those mixed up.

I've just done the question and for

7 degrees: 3.3x10^-6 x sin(7) = 4.02..x10^-7m (about 402nm) which is the wavelength for U.V

12 degrees: 3.3x10^-6 x sin(12) = 6.86...x10^-7m (686nm) which is the wavelength for Red.

From Red to Ultra violet the wavelength decreases.
(edited 12 years ago)
Reply 66
Avatar for demesto
demesto
Original post by Jukeboxing
You got those mixed up.

I've just done the question and for

7 degrees: 3.3x10^-6 x sin(7) = 4.02..x10^-7m (about 402nm) which is the wavelength for U.V

12 degrees: 3.3x10^-6 x sin(12) = 6.86...x10^-7m (686nm) which is the wavelength for Red.

From Red to Ultra violet the wavelength decreases.


How are we supposed to know what colour the wavelength corresponds to?
Original post by davie18
For anyone interested I found a link to some well set out notes for this unit: http://milanmehta.net/ocrphysics/OCRPhysicsAG482RevisionNotes.pdf


Omg you're amazing, Thankyouu so much! (+rep) :biggrin:
Original post by demesto
How are we supposed to know what colour the wavelength corresponds to?


By knowing the wavelengths.

Violet Light
The visible violet light has a wavelength of about 400 nm.

The visible indigo light has a wavelength of about 445 nm.

Blue Light
The visible blue light has a wavelength of about 475 nm.

Green Light

The visible green light has a wavelength of about 510 nm.

Yellow Light
The visible yellow light has a wavelength of about 570 nm.

Orange Light
The visible orange light has a wavelength of about 590 nm.

Red Light
The visible red light has a wavelength of about 650 nm.
Reply 69
Avatar for Wilko94
Wilko94
6
Quick question -

How do you determine the power lost by a wire, as opposed to the power a wire carries?
Reply 70
Avatar for The medjai
The medjai
Original post by Jukeboxing
By knowing the wavelengths.

Violet Light
The visible violet light has a wavelength of about 400 nm.

The visible indigo light has a wavelength of about 445 nm.

Blue Light
The visible blue light has a wavelength of about 475 nm.

Green Light

The visible green light has a wavelength of about 510 nm.

Yellow Light
The visible yellow light has a wavelength of about 570 nm.

Orange Light
The visible orange light has a wavelength of about 590 nm.

Red Light
The visible red light has a wavelength of about 650 nm.

This is not included in the specification. The maximum we would need to know is that Red has the the Highest wavelength and Blue has the lowest wavelength although this is OCR and they like to put questions in their paper which isn't even covered in their main book. :s-smilie:
Original post by The medjai
This is not included in the specification. The maximum we would need to know is that Red has the the Highest wavelength and Blue has the lowest wavelength although this is OCR and they like to put questions in their paper which isn't even covered in their main book. :s-smilie:


I know its annoying. Thats why i'm just trying to gain a little extra knowlege from elsewhere.
Original post by Wilko94
Quick question -

How do you determine the power lost by a wire, as opposed to the power a wire carries?


Well, i guess you would have to know how much energy reaches to whatever your giving the energy too and calculate the difference.
Original post by The medjai
This is not included in the specification. The maximum we would need to know is that Red has the the Highest wavelength and Blue has the lowest wavelength although this is OCR and they like to put questions in their paper which isn't even covered in their main book. :s-smilie:


How did you find the mechanics paper? i thought it was very easy.
Reply 74
Avatar for The medjai
The medjai
Original post by Jukeboxing
How did you find the mechanics paper? i thought it was very easy.


Yeah it was although I messed up the Vector question which was worth 4 marks. Everything else was fine.:smile:
Original post by The medjai
Yeah it was although I messed up the Vector question which was worth 4 marks. Everything else was fine.:smile:


lol, when i saw it i was thinking im screwed so i thought let me just do pythgoras and luckily for me it turned to be the correct method. After doing so many past papers of not getting how to solve vectors, when it came to the real thing i got it correct, how ironic.

Edit: lol, why neg?:s-smilie:
(edited 12 years ago)
Reply 76
Avatar for The medjai
The medjai
Original post by Jukeboxing
lol, when i saw it i was thinking im screwed so i thought let me just do pythgoras and luckily for me it turned to be the correct method. After doing so many past papers of not getting how to solve vectors, when it came to the real thing i got it correct, how ironic.


Yeah, the main thing is you get it right in the exam so well done dude.:smile:
Reply 77
Avatar for choc_1209
choc_1209
Original post by davie18
Also you can download it as a word document here if you want to make your own little adjustments. :smile:


i cant open this document?!
Reply 78
Avatar for davie18
davie18
10
Original post by choc_1209
i cant open this document?!

Hmm seems fine to me. What happens when you try to open it?
Original post by The medjai

Original post by the real viper


Explain the meaning of the term Terminal P.d.

The potential difference across a source. Not too sure on this one any suggestions??.


Terminal p.d. is the potential difference across the external resistor connected to an EMF source. This is the definition which I've learnt :smile:

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