Devoir de Philosophie

electricity

Publié le 19/12/2014

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A positively charged glass rod is bought close to a suspended metal needle. What can we say about the charge on the needle given that the needle is a) attracted? b) repelled? Answer: The electrostatic force obeys the principle of linear super-position which basically states that force between two particles does not get affected by the charge of the other and hence if the rod is brought close to a suspended metal needle and is getting attracted then needle definitely should possess opposite charge relative to the rod or basically negative charge as opposite charges attract. (b) In case the needle which is brought near the sus[ended needle and gets repelled that shows that both of them possess same charge and hence positive charge on both objects and will start repelling each other. 2. How would you determine the sign of a charge on ...

« Answer: As per the direction of the electric field, it is always from +ve to -ve charge.

So, for Q2, the forces are from Q1 to Q2, and Q3 to Q2.

F12 = kQ1*Q2/r² = (9 * 10^9 * 8 * -5 * 10^-12)/(0.05)² = -360 * 10^-3/0.0025 = -144 N F32 = kQ3*Q2/r² = (9 * 10^9 * 30* -5 * 10^-12)/(0.1)² = - 1350 * 10^-2/0.01 = - 1350 N So, net force on Q2 is, F = F12 – F32 = -144 - (-1350) = -144 + 1350 = 1206 N 6.

Referring to the diagram below.

What is the magnitude and direction of the resultant force acting on Q2? Answer: Using the same concept, magnitude of the net force on Q2 is, F12 = kQ1*Q2/r² = (9 * 10^9 * 8 * -6 * 10^-12)/(0.03)² = (-48 * 10^-3 * 9 * 10^4)/9 = -480 N Similarly, F32 = kQ3*Q2/r² = (9 * 10^9 * 8 * -6 * 10^-12)/(0.04)² = -270 N. Since, both forces are directed perpendicular to each other, net force is addition of the two of them, i.e.

-480 – 270 = - 750 N. Direction of the force is, theta = tan^-1(-480/-270) = tan^-1(1.778) = 60.64 degrees.. »

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