The ammeter your physics instructor uses for in-class...

The ammeter your physics instructor uses for in-class demonstrations has internal resistance $R_{\mathrm{i}}=75.0 \Omega$ and measures a maximum current of $1.50 \mathrm{~mA}$. The same ammeter can be used to measure currents of much greater magnitudes by wiring a shunt resistor of relatively small resistance, $R_{\text {shunt }}$ in parallel with the ammeter. (a) Sketch the circuit diagram, and explain why the shunt resistor connected in parallel with the ammeter allows it to measure larger currents. (b) Calculate the resistance the shunt resistor has to have to allow the ammeter to measure a maximum current of 15.0 A

Key Concepts

Parallel Circuit and Current Division

When components are connected in parallel, the overall current divides among the branches based on their resistances. In the context of using a shunt resistor with an ammeter, the principle of current division allows most of the current to be safely bypassed to the shunt resistor, enabling the ammeter to measure a reduced and manageable current.

Circuit Diagrams

Circuit diagrams are simplified representations of electrical circuits, illustrating the layout and interconnection of components. They are useful for visualizing how the ammeter and shunt resistor are arranged in parallel, clarifying the path of current flow and the mechanism that allows the measurement of large currents through controlled division.

Ammeter Operation and Internal Resistance

An ammeter is designed to measure electric current and is constructed with a low internal resistance to ensure it minimally disturbs the circuit being measured. However, this also implies that the device is sensitive to overload if the current exceeds its designed maximum. Understanding this characteristic is key to safely expanding its measurement range.

Shunt Resistor

A shunt resistor is a low-resistance device connected in parallel with the ammeter to divert the majority of the current away from it. This setup allows only a small, controlled portion of the high current to pass through the ammeter while the bulk flows through the shunt resistor, thereby protecting the instrument and extending its measurement range.

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