question about ammeter and voltmeter?

Question

An ammeter and a voltmeter of suitable ranges are to be used to measure the current and voltage of an electric lamp. If mistakes were made and meters interchanged, what will happen?

Answer 1

While the voltmeter has a very high impedance, the ammeter has a very low series resistance, so if that gets placed across the AC line, very bad things will happen. The ammeter may even burn up catastrophically (i.e. mini-explosion).

If the voltmeter is placed in series *before* the ammeter then it will be protected, but if the voltmeter is placed in series with the line *after* the ammeter, then the ammeter will receive the brunt of the energy.

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Answer 2

The voltmeter has a high impedance and hence if connected in series with the lamp will read the voltage of the power supply, since there is next to no current and hence negligible voltage drop across the lamp (Ohm's law: E = IR, hence no I (current) means no E (voltage)). The lamp would not light.

Since accidentally connecting an ammeter in parallel with a voltage source is a very common error, and even experienced people do it occasionally (e.g. by forgetting to change a multimeter's settings from the last time you measured a current) the input is fused. So all that happens is that the fuse blows (the lamp would light, however) and you have to replace it before you can read currents again. Multimeters commonly have a fuse testing position on the range selector switch, so that a blown fuse can be identified without having to open up the meter.

Answer 3

As the resistance of ideal voltmeter is infinite and, by mitake has been put in series inthe circuit , the current in the circuit will be zero, so the ammeter in parallel to the lamp will also read zero

Answer 4

if you use the ampmeter across the leads it goes boom. Voltage is read across the lamp, amps are read inline. And ampmeter can have low resistance and thus it is like a short circuit.
MIKE