Why it is necessary to have voltmeter to have highbresistance?
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Voltmeter is supposed to have infinite/very high resistance to avoid loading effect.
If the resistance of voltmeter is low, it will start acting as a load itself and draws current from source. Since the voltmeter is connected in parallel to the load, the current drawn by the actual load also changes.
As V= IR, when current changes through load, voltage also changes.
When voltmeter has very high resistance, it draws very low value of current from the source and the load current is pretty much the same as it was without the voltmeter. Hence voltage across the load is approximately same and we get the correct voltage reading.
Let's assume a load resistance of 500 ohms in series with a 100 ohm resistance supplied by a 100 v battery and see the effect of voltmeter resistance in the voltage across load.
Actual voltage across the load is 83.33v but lower resistance voltmeter is incorrectly measuring it as 71.428 v.
If the resistance of voltmeter is low, it will start acting as a load itself and draws current from source. Since the voltmeter is connected in parallel to the load, the current drawn by the actual load also changes.
As V= IR, when current changes through load, voltage also changes.
When voltmeter has very high resistance, it draws very low value of current from the source and the load current is pretty much the same as it was without the voltmeter. Hence voltage across the load is approximately same and we get the correct voltage reading.
Let's assume a load resistance of 500 ohms in series with a 100 ohm resistance supplied by a 100 v battery and see the effect of voltmeter resistance in the voltage across load.
Actual voltage across the load is 83.33v but lower resistance voltmeter is incorrectly measuring it as 71.428 v.
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