27.10
Ammeter
An ammeter is a current measuring instrument. In the circuit, it is represented by the symbol A. The ammeter is placed in series with the device or co…
An ammeter is an instrument used to measure the current values in a circuit. It is connected in series with the element across which current must be measured.
An ideal ammeter has zero resistance; however, real ammeters have some finite but negligible resistance.
If the resistance is not negligible, the ammeter in the circuit would change the circuit's equivalent resistance and modify the current being measured.
Any galvanometer can be used to measure current by adding a shunt resistance in parallel with it.
The parallel connected shunt resistor divides the current and hence allows the galvanometer to measure DC currents several times greater than its full-scale deflection current.
The shunt resistance needed is determined using the known values of the galvanometer's full-scale deflection current and coil resistance.
This is done by equating the voltage drops across the galvanometer and the shunt resistance.
Here, Ia is the total current passing through the parallel combination, and Ia minus Ifs is the current passing through the shunt resistor.
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Q1: Why is an ammeter connected in series with a circuit component?
An ammeter measures current and must be connected in series because objects in series have the same current passing through them. This placement ensures the ammeter reads the actual current flowing through the component without bypassing it, providing an accurate measurement of the circuit's current flow.
Q2: What happens if an ammeter has significant resistance in a circuit?
If an ammeter's resistance is not negligible, it changes the circuit's equivalent resistance and modifies the current being measured. An ideal ammeter has zero resistance, but real ammeters have finite resistance. To avoid measurement errors, ammeters are designed with negligible resistance, typically a fraction of a milliohm.
Q3: How many ammeters are needed to measure current in series versus parallel resistors?
For two resistors in series, a single ammeter is needed because current is identical through both. For two resistors in parallel, three separate ammeter readings are necessary: one for the battery current and one for each resistor's current. The number of ammeters depends on the circuit configuration and measurement requirements.
Q4: How can a galvanometer be converted into an ammeter?
A galvanometer can measure current by adding a shunt resistance in parallel with it. The parallel shunt resistor divides the current, allowing the galvanometer to measure DC currents several times greater than its full-scale deflection current. The required shunt resistance is determined by equating voltage drops across the galvanometer and shunt resistor.
Q5: What is the relationship between shunt resistance and galvanometer current capacity?
The shunt resistance is calculated using the galvanometer's known full-scale deflection current and coil resistance. By placing a shunt resistor in parallel, the total current divides between the galvanometer and shunt. This allows measurement of larger currents while protecting the galvanometer from excessive current that would damage it.
Q6: Why do ammeters typically contain a fuse?
Since current in a circuit travels through the ammeter, it contains a fuse to protect the meter from damage caused by excessively high currents. The fuse acts as a safety device, breaking the circuit if current exceeds safe levels and preventing internal damage to the ammeter's sensitive components.
Q7: How does ammeter placement affect measurements in circuits with multiple resistors?
Ammeter placement depends on circuit configuration. In series circuits, one ammeter measures the same current through all components. In parallel circuits, multiple ammeters are needed to measure total current and individual branch currents. Proper placement ensures accurate current measurement without altering the circuit's equivalent resistance or affecting household wiring and electrical safety.