Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: volts, ohms, amps

Learn more physics!

Most recent answer: 06/03/2011
Q:
what would happen if voltmeter have low resistance? how it reduces the actual volt reading it has meant to record? how drift velocity decreases as area increases? why a lamp glows dimly if it has high resistance.please answer my questions analytically and discusing the depth of each topic
- pooja (age 19)
dhaka ,bangladesh
A:
Say you want to measure voltage  between two points in a circuit.  If the meter resistance is too low, it provides an alternative current path to those in the circuit itself. That changes the flow of current and changes the voltage pattern. In effect, it partly shorts out the circuit, reducing the voltage.

If you have a conductor with a fixed total current, then the more area the current flows through, the less current flows per unit area. That means that the charge carriers are flowing more slowly. For very small areas, in some circuits the voltage is nearly constant and the current just goes up as the area increases. For large enough areas, the circuit's ability to supply current will be too limited, and the voltage will drop, as described above.

Incandescent lamps are usually powered by a fixed-voltage source. If the resistance R is high, the power V2/R will be low. The light power out will always be less than the electrical power in, so high R means low light output.

Mike W.

(published on 06/03/2011)

Follow-up on this answer.