Last-minute Physics doubts, SOLVED!

Posted On: March 25, 2009 04:18 pm

Confused students posed their queries to Topper Science expert Leena Garg, during a live chat on March 25. And here's the transcript with all the answers!

Sanket Wani: Hi Maam. What is the resistance offered by a fuse, an ammeter and a voltmeter?
Leena Garg: Resistance of a fuse depends on the fuse wire. Resistance of an ammeter is ideally zero and that of a voltmeter is ideally infinity.

Neha: Hello Maam. Could u tell me that why the principal axis is always perpendicular to the mirror at its pole?

Leena Garg: That is how it is drawn, because by definition the principal axis is the line along which the ray of light passes through undeviated. Refer to What's HOTS?

Sanket Wani: What type of questions can be asked on the topic "Heating effects of electric current"?

Leena Garg: Practice numerical and learn the units well. Also learn the conversion of units. Practice by solving past exam papers.

Neha: Why does a convex mirror have a wider field of view?

Leena Garg: Because of its diverging nature.

Deepitapai: Can you please tell me as to how i should find the resistance in a circuit diagram when resistors are connected in different shapes & it becomes difficult to find out if they are connected in series or parallel.....many reference books give such questions, can such questions come in board exam?

Leena Garg: The only way out is to practice questions. You can refer to  What's HOTS?, Mock Papers, Solved board exam papers.

Sanket Wani : Ma'am, I am not able to apply the right hand thumb rule to a circular loop.Please help me.

Leena Garg: Right Hand Thumb Rule Imagine the conductor to be held in your right hand with the fingers curled around it. If the thumb points in the direction of the current, then the curled fingers show the direction of the magnetic field. Practice Mock Papers.

Gurman Bhatia: I basically face problems in questions regarding application of flemmings left hand and right hand rule. How can I overcome this problem?

Leena Garg: First of all remember, that you use the left hand rule when you have to find the direction of motion of the conductor. You use right hand rule to find direction of current. Just think of some memory mnemonic to remember this. Like say LM or left hand rule for motor.

Ahmed: What is 1 dioptre?

Leena Garg: 1 dioptre is the power of lens having 1 m focal length.

Monisha: Hi. Could you please tell me what will happen if the 3rd component from the left fails to work in a series circuit where the current is from left to right?

Leena Garg: I cannot understand your question without a diagram. If I have guessed correctly, you have 3 resistors connected in series. If the resistor fails the circuit will be discontinuous and current will stop flowing.

Creative: Is resistance provided by a fuse, an ammeter and a voltmeter?

Leena Garg: Resistance of ammeter and voltmeter is such that they do not affect the current in the circuit. Refer to Mock papers.

Aishwarya: Maam can u tell me why is an AC considered to be advantageous over DC for long range transmission of electric energy?

Leena Garg: AC is advantageous as the I^2R losses are less from it. You can think that while DC flows, full current flows for the entire duration of time. When AC flows, lesser current flows as the current varies with time. So, the I^2R losses are less.

Minhaja: Hi, can u tell me when i have 2 use negative signs while solving the lens problems....please answer this...?

Leena Garg: For all parameters which are on the left side of lens are generally taken to be negative.

Deepitapai: Name a device that helps to maintain a potential difference across a conductor.

Leena Garg: Battery or a cell. Watch and learn with the Hottest videos on Topper!

Nitish surana: Hi could u please tell me how will a graph be for non ohmic devices

Leena Garg: The graph will not be a straight line, such devices have non-linear graphs.

Deepitapai: A piece of wire of resistance R is cut into five equal parts. These parts are then connected in parallel. If the equivalent resistance of this combination is R’, then the find R/R’................can such questions come in the exam? I find them very difficult. Please explain as to how i should proceed with the question.

Leena Garg: Just practice whatever you can and don't bother making guesses. The resistance of each part is R/5, because resistance is proportional to length. When you join them in parallel, the total resistance is the combined resistance of 5 resistors each of R/5 in parallel. This will be R/25. Next find the ratio.

Meha: Hi Maam, please tell me how to study physics. It seems so hard to learn the formulae

Leena Garg: Practice the difficult formulae by writing them. Solve Mock papers, Refer to Solved board exam papers.

PRASANTHDJ: Why is the amount of gas collected in electrolysis of water double the amount collected in other? Name this gas.

Leena Garg: In the electrolysis of water, the gases obtained are hydrogen and oxygen 2H2O will give 2H2 and O2 so the amount of hydrogen liberated is twice the amount of oxygen.

Vaibhav Shoran: Why do stars appear to be twinkling to us whereas Planets don't?

Leena Garg: Stars are very small (point objects) as they are further away than planets. So, the effect of refraction is observable. Planets are comparatively closer and can be thought of as extended objects. So the effect of refraction is not perceptible.