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March 25, 2009

Your Science board exam is on March 28. So, we got Topper Physics expert Leena Garg to clarify your doubts! Here is the transcript for those who missed the chat:

Gurman Bhatia
: Can you please tell the differences between a/c current and d/c current
Leena Garg: DC current flows in only one direction. AC current keeps reversing the direction. But in the household supply this happens at an extremely fast rate, so the bulb seems lit at all times.

NITISH AGARWAL: What all we need to know for ac/dc generators/motors as most of it is not in syllabus?
Leena Garg: Learn the principle of both.

dazzling neeraja: name any two elements that are used in fabricating solar cells
Leena Garg: Silicon and germanium are used in solar cells.

swati: while drawing the ray diagrams for concave mirror or convex lens when the object is at C i do not get the image at C or the size of the image is not the same as the object.why?
Leena Garg: Draw a more accurate ray diagram. Use a pencil and scale to draw the lines. Don't forget to sharpen your pencil as that will give you a more accurate image. Use a compass to draw a lens and take a straight line in the middle of the lens. Draw rays up to this line.

Pranav Gupta: What is presbyopia? Is it a combination of Myopia and Hypermetropia?
Leena Garg: No, presbyopia is age-related hypermetropia. When your grandfather reads the paper by holding it at a distance, you know that he is suffering from hypermetropia which in his case is called presbyopia.

deepitapai: what does deliquescent nature of NaOH & KOH mean?
Leena Garg: Deliquescent nature of NaOH and KOH means that when these compounds are left open in air. They absorb moisture from air and become wet.

minhaja: can u pls..tell me the brief explanation hou 2 find the position of elements in periodic table.......??
Leena Garg: You need to learn the modern periodic table. First 20 elements should be learnt very well. The modern periodic table consists of periods and groups. A particular element belongs to a specific group and period.

Vatsal Agarwal: In a question where they ask u 2 complete a diagram,where a prism is given & a light ray falls on it,do we show the spectrum formation or the other diagram(the one showing the normal, refracted & emergent ray, etc) ?
Leena Garg: If the question asks you to show dispersion of light, draw the spectrum. If it asks you to mark the angle of deviation of the ray draw the angle between the incident ray and emergent ray.

aakanksha ag: Mam please tabulate diff between an electromagnet and permanent magnet. Please give def of electromagnet also.
Leena Garg: Stretch the fingers of your right hand so that they are perpendicular to each other. Then the first Finger points in the direction of the field and thuMb is in the direction of motion then second finger gives the current. Note the cap letters are to remember the quantity that the finger points to. Normally you are asked to state the direction of the current say north, south, east ,west or up and down.