Physics · Current Electricity · NEET
An ammeter is placed in series, so all the current passes through it. If it had any resistance, it would add to the total resistance of the circuit and reduce the current — then the meter would read a smaller value than the real current. Zero resistance means it adds nothing, so the current stays the same and the reading is correct. In NCERT Example 4.12, a real ammeter of 0.02 ohm gives 0.99 A, but the ideal ammeter (0 ohm) gives exactly 1.00 A.
A voltmeter is placed in parallel with the part whose voltage you want. If it had a finite resistance, some current would flow through the voltmeter itself, changing the currents in the circuit and lowering the voltage you are trying to measure. Infinite resistance means no current is diverted into the voltmeter, so the circuit is untouched and the reading is the true voltage.
Current is the same for elements in series, so to measure the current through a resistor the ammeter must sit in the same series line and carry that same current. Voltage is the same across elements in parallel, so to measure the voltage across a resistor the voltmeter must be connected across it (in parallel). Matching the meter to the quantity it reads is why the connection differs.
An ideal ammeter has almost zero resistance, so connecting it in parallel with a component (or across the cell) creates a near short circuit. A very large current flows through the ammeter and it can burn out or damage the cell. This is why an ammeter is never connected across a source — only in series.
No. An ideal voltmeter draws zero current because its resistance is infinite. It still shows a reading because a voltmeter responds to the potential difference across its terminals, not to current flowing through it. So in a series branch, if an ideal voltmeter is placed in the line, no current flows in that line at all — the branch is effectively open.
In two circuits (same 10 V cell, same 10 ohm resistor; ideal meters, only the order of voltmeter V and ammeter A swapped), the readings of the voltmeters and ammeters will be:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
An ideal ammeter has zero resistance and an ideal voltmeter has infinite resistance. Real ammeters have a very small resistance and real voltmeters have a very large resistance, so they are close to ideal but not perfect.
To make an ammeter, a small resistance (shunt) is connected in parallel with the galvanometer, giving a low overall resistance. To make a voltmeter, a large resistance is connected in series, giving a high overall resistance. This matches the ideal-meter requirements.
No. Because its resistance is zero, it adds nothing to the circuit, so the current stays exactly the same as it would be without the meter. That is why it reads the true current.
A real voltmeter has finite (large) resistance, so a tiny current flows through it. This slightly changes the circuit and lowers the measured voltage. The higher the voltmeter resistance, the closer the reading is to the true value.
If an ideal voltmeter (infinite resistance) is placed in series in a line, it blocks all current in that line — the branch acts like an open switch. This is a common trap: current through such a branch is zero.
