Edexcel Unit 2: Physics at Work ~9th June 2014

Original post by AdilMalikN

can someone please clarify how exactly the pulse width (duration) and the actual Wavelength of the pulse effects resolution?

Shorter wavelength= greater resolution, and shorter pulses = greater resolution

Please correct me if im wrong

Reply 82

Original post by jtbteddy

Shorter wavelength= greater resolution, and shorter pulses = greater resolution

Please correct me if im wrong

Thank god i thought i was going mad. what about range now? is the maximum distance traveled by a wave = Speed X PulseTime ?

How does Wavelength and frequency effect range?

Frequency increases absorption so it reduces range as far as i can remember?

Reply 83

Original post by AdilMalikN

Thank god i thought i was going mad. what about range now? is the maximum distance traveled by a wave = Speed X PulseTime ?

How does Wavelength and frequency effect range?

Frequency increases absorption so it reduces range as far as i can remember?

Duration of a pulse = 2 x the distance/ speed

Speed = frequency/wavelength.. Insert those into the first equation and youll know

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Reply 84

Original post by jtbteddy

Duration of a pulse = 2 x the distance/ speed

Speed = frequency/wavelength.. Insert those into the first equation and youll know

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Thanks a lot! but im still confused about the first equation. This is a question in the june 2013 Resit paper about the dolphin. it says that the time between pulses is 0.0625 seconds, and the speed is 1530ms-1, calculate the maximum distance it can determine when finding prey.

here wouldn't the maximum distance be the pulse duration X speed rather than dividing it by 2 at the end? because the pulse can go (1530x0.0625)m and then reflect back to the dolphin?

Reply 85

Original post by AdilMalikN

Thanks a lot! but im still confused about the first equation. This is a question in the june 2013 Resit paper about the dolphin. it says that the time between pulses is 0.0625 seconds, and the speed is 1530ms-1, calculate the maximum distance it can determine when finding prey.

here wouldn't the maximum distance be the pulse duration X speed rather than dividing it by 2 at the end? because the pulse can go (1530x0.0625)m and then reflect back to the dolphin?

ImageUploadedByStudent Room1401046542.260485.jpg

Nope, youve got to divide by two because the "pulse duration x speed" would give you the total distance the PULSE travels, i.e. to the prey and back, in other words, twice the distance to the prey. So if the maximum distance the pulse can travel is given by that equation, dividing it by two will give the maximum distance from the prey. I hope that makes sense, im not really good at explaining

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Reply 86

Original post by jtbteddy

Nope, youve got to divide by two because the "pulse duration x speed" would give you the total distance the PULSE travels, i.e. to the prey and back, in other words, twice the distance to the prey. So if the maximum distance the pulse can travel is given by that equation, dividing it by two will give the maximum distance from the prey. I hope that makes sense, im not really good at explaining

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Oh i understand that but, the question is weird, it says that the time each pulse the dolphin makes is 0.0625 seconds long, so that would imply that it should travel speed x time long? if it said that this was the time it took for pulse to return then it would make sense dividing it. what do you think, im confused as hell all of a sudden.

Reply 87

BP_Tranquility

OP

Original post by jtbteddy

Shorter wavelength= greater resolution, and shorter pulses = greater resolution

Please correct me if im wrong

Why is that? Is it because a shorter wavelength/higher frequency has more energy so less diffraction (so the wave's energy doesn't dissipate and reflect with significantly less energy) and shorter pulses reduces the chances of the incident wave interfering with the reflected wave?

Reply 88

foofighters96

Finding it hard to remember all the topics so i'm looking for some advice. Do you guys make notes on all the topics first then do past papers or do you just read over the textbook, memorize, then do past papers? For me I think notes would take way too long but memorizing from the textbook is nigh on impossible. So which method do you find works the best(as i really must start revising seriously now).

Reply 89

Original post by AdilMalikN

Oh i understand that but, the question is weird, it says that the time each pulse the dolphin makes is 0.0625 seconds long, so that would imply that it should travel speed x time long? if it said that this was the time it took for pulse to return then it would make sense dividing it. what do you think, im confused as hell all of a sudden.

I'm not really sure, when it comes to pulses, I always take the time they give me as the time to travel to and fro unless they say otherwise. Anyways, these definitions might help:

By definition, "a pulse doesn't contain any repeated up and down motion", and the time period is the time taken to complete one whole oscillation, so when the pulse returns, the time it takes is actually its time period.

Someone please further elaborate on this.

Original post by BP_Tranquility

Why is that? Is it because a shorter wavelength/higher frequency has more energy so less diffraction (so the wave's energy doesn't dissipate and reflect with significantly less energy) and shorter pulses reduces the chances of the incident wave interfering with the reflected wave?

Shorter wavelengths diffract much less than longer wavelengths, this means that the shorter the wavelength, the less the waves spread out as they travel, and the location of the interfaces between tissues can be mapped more precisely.

The pulses of ultrasound must be very short so that the reflections from nearby interfaces don't reach the transducer before the pulse has ended. The gap between pulses must be long so that all the reflectes waves from one pulse return to the transducer before the next pulse is transmitted.

Reply 90

Original post by jtbteddy

I'm not really sure, when it comes to pulses, I always take the time they give me as the time to travel to and fro unless they say otherwise. Anyways, these definitions might help:

By definition, "a pulse doesn't contain any repeated up and down motion", and the time period is the time taken to complete one whole oscillation, so when the pulse returns, the time it takes is actually its time period.

Someone please further elaborate on this.

Shorter wavelengths diffract much less than longer wavelengths, this means that the shorter the wavelength, the less the waves spread out as they travel, and the location of the interfaces between tissues can be mapped more precisely.

The pulses of ultrasound must be very short so that the reflections from nearby interfaces don't reach the transducer before the pulse has ended. The gap between pulses must be long so that all the reflectes waves from one pulse return to the transducer before the next pulse is transmitted.

I hope someone clarify this, because i know the complete opposite to what you said. the pulse can contain more than one wave of a wavelength.

see this image: http://www.sprawls.org/ppmi2/USPRO/25USPROD03.png

(edited 9 years ago)

Reply 91

Original post by AdilMalikN

I hope someone clarify this, because i know the complete opposite to what you said. the pulse can contain more than one wave of a wavelength.

see this image: http://www.sprawls.org/ppmi2/USPRO/25USPROD03.png

ImageUploadedByStudent Room1401111287.010136.jpg

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Reply 92

Original post by jtbteddy

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Im so confused now..

Wait so in pulse echo, its a short pulse travelling, through the length of the air, or is it just one wave (one wavelength) that moves and reflects?

Reply 93

Original post by AdilMalikN

Im so confused now..

Wait so in pulse echo, its a short pulse travelling, through the length of the air, or is it just one wave (one wavelength) that moves and reflects?

that's what i thought, but now i'm confused too.. someone explain?

Reply 94

Original post by jtbteddy

that's what i thought, but now i'm confused too.. someone explain?

Maybe that resit paper had a typo? maybe it was the time the dolphin received it back?

Reply 95

Original post by AdilMalikN

Maybe that resit paper had a typo? maybe it was the time the dolphin received it back?

No, I definitely agree with the answer.. The confusion is over what a pulse actually means.
Btw, can you please help me read what this says?

Sin tÃtulo.png16.3KB

Reply 96

Darnell LD97

This video is very good for a summary on waves.

http://www.youtube.com/watch?v=SS3WRGu8wGg

Reply 97

jay_em

2

Getting really confused with refractive index stuff, mainly with the difference between a refractive index for an interface, and the index for a material itself. Also, this question:
Diamond has a refractive index of 2.42. Show that the critical angle for light passing from diamond into air is about 24°.
Any help?

Reply 98

Original post by jay_em

Getting really confused with refractive index stuff, mainly with the difference between a refractive index for an interface, and the index for a material itself. Also, this question:
Diamond has a refractive index of 2.42. Show that the critical angle for light passing from diamond into air is about 24°.
Any help?

oh its the same thing, just remember this formula:

Refractive index FROM X TO Y = Speed in X / Speed in Y = Sin (angle in X) / Sin (angle in Y). so if they want you to find at the air - glass interface use the formula above.

For the second one use SinC = 1/n(refractive index of denser material)
plug values and you get 24.

Reply 99