Moving Charges And Magnetism

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Common Misconceptions

4 min read

- What you are getting from a statement might not be necessarily true. Let's bust some of the common misconceptions

Does magnetic force do work?

Whenever a charged particle with charge q and velocity

v

enters a region with field

B

it experiences a force given by

F = q [v \times B]

Thus, The resultant magnetic force is always perpendicular to the velocity of motion of the charge.

Or we can say,

F . v = 0

The magnetic force acts as a centripetal force and results in a circular motion of the charge in the field. Hence, magnetic force does no work.

Biot Savart's Law or Ampere's Law: Which one to use?

Both Biot Savart's Law and Ampere's Law are brilliant tools to find the magnetic field around a current carrying conductor. But what confuses most students, is deciding where to apply them. Let us find a way to clear the misconceptions around the laws.

Do voltmeter/ammeter alter circuit measurements?

Ideal voltmeter has INFINITE resistance and is connected in parallel to a circuit, So it cannot draw any current from the circuit and alter circuit
Ideal ammeter has ZERO resistance and is connected in series with the circuit. Hence it does not alter the circuit

But is this true for a real circuit?

Real voltmeter, should have a resistance few orders higher than the device across which potential has to be measured.

Ammeters on the other hand should have a resistance several orders smaller than the circuit resistance so as not to alter the current in the circuit while taking the measurement. If the resistance is comparable to the circuit resistance, connecting an ammeter reduces circuit current leading to wrong readings.

Thus, ammeter and voltmeter can alter the measurements if the ammeter/voltmeter specifications aren't decided correctly.