Here’s the solution sheet for the worksheet on Electric Current that I had posted earlier: worksheet – electric current – solutions
All the questions that you had posted recently and I have not responded to will be taken up once you are back in school.
So this is going to be the last post for this vacation. See you all now on 31st!
Hope you all would have finished reading the chapter on electric current by now.
Here’s the worksheet on electric current: Worksheet – Electric Current
I will post the answers for this worksheet on coming Tuesday.
Here’s a project for all of you. I would like you all to make a small electric motor or a small electric generator. You will find a lot of help on the Internet, including Youtube videos, on how to make small electric motors and generators of different kinds. I am not suggesting any one kind of design over another. Choose any design that you want to try making. Also feel free to experiment and tinker with the given design. You might come up with a simpler or more interesting design yourself.
Faraday’s Disk – the first electric generator
I would suggest Arvind Gupta’s website as one of the resources you might want to take a look at. See the short films under the title ‘ELECTRICITY EXPERIMENTS’ on this page: http://www.arvindguptatoys.com/films.html
We will use these models during our classes on electromagnetism. It will be easier for you to make these models right now during these holidays due to easier availability of resources at home. Do not forget to bring the models to school. It would also form a major part of the ‘internal assessment’ in your physics course. And before you go off on a tangent and spend your valuable time making a model which ‘looks’ neat and beautiful because I mentioned ‘internal assessment’, here’s something for you to think about:
“Design is not just what it looks like and feels like. Design is how it works.” — Steve Jobs
In case you are making a motor, try rotating something with your motor. If you are making a generator, try making it work using different sources of energy – water from the running tap, wind, etc. And yes, your generator should be able to at least light up an LED.
Here is the solution sheet for the optics worksheet I had posted earlier: worksheet – optics – solutions
I will be away from the Internet (again!) for another 10 days. Will be back to school on 15th May.
Read the first chapter of electricity – Electric Current (potential difference, current, resistance, Ohm’s law, series and parallel circuits)– from your textbook during this period.
I will give you a revision worksheet on electric current as soon as I’m back.
I will start with Mehak’s queries about physics notes, I would suggest that you focus on definitions, concepts, laws, relationships, formulae in your notes. Don’t solve numericals in your notes. Keep that separate. Maintaining a list of SI (and other) units and formulae will be useful.
Pratiksha
sir, if for the second question in loudness i wrote amplitude , would it still be correct?
PK
No. Amplitude is the property of the wave, loudness is the property of sound (as we hear it).
Shruti
Is there only one angle or one light ray that does the total internal reflection?
PK
No no no no no. Any light ray for which the angle of incidence is greater than the critical angle, undergoes total internal reflection.
Shruti
If there are two prisms and one of them has a greater angle of prism than the other and if a light ray falls on them with the same angle then what will be the difference?
PK
The angle of refraction (r1) on the first refracting surface will be same in both cases. But because the angle between the two refracting surfaces is different, the angle of incidence (i2) on the second refracting surface will be different and therefore the angles of emergence will also be different. At the end, the angle of deviation in the two cases will be different.
See the following diagrams:
Shruti
Why is the intensity of the refracted light less than the reflected one shouldn’t it be the same?
PK
When? What context you are talking about?
The intensity of refracted and reflected light will be different and which one is more intense depends on the two media. When light hits a clear transparent glass, the refracted light is more intense than the reflected light. But when it hits a polished metal surface, the reflected light is more intense.
Mehak
my q’s and conclusion on -(pls let me know if I am wrong) light-1- the same reversibility thing?
PK
What’s the conclusion? I had already replied to the question on reversibility in an earlier post.
Mehak
do we need to know the alternative method in the ‘experimental verification of laws of refraction and determination of refractive index of glass’?
PK
Not really. But is it difficult to understand? They just make two right angled triangles on both sides, with a hypotenuse 1 cm long.
That means sin i = (perpendicular of the triangle drawn in the first medium) / hypotenuse, which is 1. So, sin i = (that perpendicular) / 1 = (length of that perpendicular)
Similarly, sin r = (length of the other perpendicular)
So, refractive index = sin i / sin r = ratio of the length of the two perpendiculars
Mehak
is intensity the energy carried by the sound wave?
PK
Yes. But strictly defining it, intensity means the amount of energy measured for any wave at any place per unit time per unit area. So if a wave brings 300 J of energy per second on an area of 1 square metre, then at that place we will say the intensity of the wave was measured to be 300 W/m2.
Mehak
speed of sound is the same in the same medium in the same temp. no matter of its frequency?
PK
Generally the speed of sound in air is independent of its frequency.
Mehak
more frequency=longer the sound travels ?
PK
You mean it gets absorbed less?
No. ON the contrary, most of the materials absorb the high-frequency sounds better than low-frequency sounds. So lower frequency sounds generally will travel for longer distances. I guess that’s why whales, elephants and owls use low-frequency sounds for their calls. They travel very far in the oceans and the forests with very little absorption.
Mehak
when we use sound waves to detect things in water through high frequency waves doesn’t it effect the animals in the water?
PK
Must be affecting them. Just the way we get affected by (road and air) traffic noise.
Mehak
for a bat or a dolphin is there any such instance when the sound ranging may not work or give wrong result like refraction of sound due to temp. of the medium?( if the frequency is more wont in bend away more)
PK
I guess there must be (unconscious) ways to adjust to the change in the surrounding conditions and their navigation system should then guide them despite the change in the conditions. This readjustment, however, will happen through some kind of feedback loop and will therefore be a little behind in time. So for that period of time when the adjustment has not yet completed, they will get misguided a little bit.
Mehak
sonar- what if the sound is reflected of in some other directing ?
PK
If the sound gets reflected in a way that it does not reach the receiver, then nothing happens! You miss the stuff which sent the echo. That’s why you send the signal over a long distance so that you get something back, some time.
Mehak
how exactly does ultrasonography work? is it harmful?
PK
Ultrasonic waves are sent into the body and the echo is recorded. Now different layers inside the body (tissues, muscles, bones, cartilages, etc.) will send the echo at different times and also the intensity of the echo will differ. For example, the fatty layer under our skin may send the first echo which also will be a soft echo. Then will come a stronger echo from the bones. By analysing these echo (their timing and intensity), the computer generates an inside image of our organs. If you want to read more, check these two links: http://www.criticalecho.com/content/tutorial-1-basic-physics-ultrasound-and-doppler-phenomenon and http://www.dynamicultrasound.org/dugphysics.html
I don’t think ultrasonography is harmful for us.
Mehak
I will like to know the derivation. if u don’t mind
PK
Here’s the derivation for the relationship between the lateral displacement (d) and the factors that affect it, i.e. angle of incidence (i), thickness of the glass slab (t), and the refractive index (μ). Refer to the following diagram as you follow the derivation:
In ΔABN,
cos r = AN / AB = t / AB … … … … (I)
In ΔABM,
∠BAM = ∠i – ∠r
sin ∠BAM = sin (i – r) = BM / AB = d / AB sin (i – r) = d / AB … … … … (II)
On dividing both sides of equation (I) by respective sides of equation (II):
cos r / sin (i – r) = (t / AB) x (AB / d)
So, d = t x sin (i – r) / cos r [derived!]
Note: Though refractive index (μ) is not directly mentioned in the above relationship, remember angle r is dependent on angle i and refractive index μ.
Samarth
I was thinking about something related to electricity
And i want to ask if there can be a capacitor which gathers a small spark and then amplifies it
PK
Gather a spark? How? What does that mean? Tell me more.
Mehak
like any thing else light carries energy right -Q-in sunlight a rock get hot,and it only reflects the light does not let it pass through and say a window glass also reflects a part of it and lets the other part through so will the rock be hotter or the glass and y? does rock absorb more energy?(i think i know the answer but i am not sure )
PK
Rock absorbs more light than the glass. So yes, rock should (and it does) absorb more energy than glass. However, by how much this energy will raise the temperature of the rock (or the glass) depends on its thermal properties. So the temperature rise will depend both on the thermal property and the amount of energy absorbed.
Sonia
can u please explain the answer to the first part of the last question? how time is related to frequency…both the graphs look the same to me.
PK
See the images below:
For the sound from guitar (the first image), a wave cycle takes a little more than 3 units of time while for the sound from piano, the wave cycle takes a little more than 2 units of time. That means that the guitar is making more number of wave cycles per unit time and therefore is producing sound of higher frequency.
Sonia
how does a cassette or a cd or dvd produce sound and images???
PK
I do not know to what level of detail you want to understand this. Let me just give you a brief idea here. IF you have further questions, let me know.
A cassette has a magnetic tape, i.e. a tape which is coated with magnetic material. Now the direction of magnetization of the particles in this coating is ‘read’ by the tape player and converted into corresponding electrical signals, which in turn are converted into corresponding sound signals by the speakers.
A CD or DVD stores data (documents, pictures, songs or movies) in the form of very tiny (microscopic) notches on its surface. These notches are ‘read’ by the disc player and again converted into electrical signals, which are converted into sound signals (by the speakers) or visual signals (by the screen).
I saw the Google’s doodle today – two small kids running in tall grass.
Remember the train scene from Pather Panchali. It’s Satyajit Ray’s 92nd birthday today.
I also found Google had a special doodle on Euler’s birthday recently too on April 15th:
Want to see more of their doodles? http://www.google.com/doodles/finder/2013/All%20doodles
Here’s the first worksheet on optics (based on the first two chapters on refraction, from your textbook): worksheet – optics – I
Ignore the Submission Date mentioned on the worksheet. I would like you all to finish it by tomorrow evening. I will post the solutions on May 4th, Saturday.
