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# 20: Electric Circuits

Learning Objectives

• Understand how a battery works.
• Understand Kirchhoff rules and how to apply them.
• Understand how to model a circuit with resistors and/or capacitors.
• Understand how an ammeter and voltmeter function, and how to model them.

In this chapter, we develop the tools to model electric circuits. This will allow us to determine the current and voltages across different components, such as resistors and capacitors, within a circuit. We will also discuss how a battery can provide a current at a fixed potential difference, and how one can construct devices to measure current and voltages.

prelude

If two outlets in your house are connected to the same circuit, are the outlets connected in series or in parallel?

1. series
2. parallel

• 20.1: Batteries and Simple Circuits
A battery is an electric component that provides a constant electric potential difference (a fixed voltage) across its terminals. Luigi Galvani was the first to realize that certain combination of metals placed into contact with each other can lead to an electric potential difference (or rather, they can make the legs of a dead frog twitch, which we now understand to be from the potential difference due to the metals). Effectively, Galvani created the first “electrochemical cell”.
• 20.2: Kirchhoff’s rules
Kirchhoff’s rules correspond to concepts that we have already covered, but allow us to easily model more complex circuits, for instance, those where there is more than one path for the current to take. Kirchhoff’s rules refer to “junctions” and “loops”. Junctions and loops depend only on the shape of the circuit, and not on the components in the circuit.
• 20.3: Applying Kirchhoff’s rule to model circuits
In this section, we show how to model a circuit using Kirchhoff’s rules. In general, one can consider a circuit to be fully modeled if one can determine the current in each segment of the circuit. We will show how one can apply the same procedure to model any circuit that contains batteries and resistors.
• 20.4: Measuring current and voltage
In this section, we describe how one can build devices to measure current and voltage. A device that measures current is called an “ammeter” and a device that measured voltage is called a “voltmeter”. Nowadays, these are usually found within the same physical device (a “multimeter”), which can also measure resistance (by measuring voltage and current, resistance can easily determined). We will limit our description to the design of simple analogue ammeters and voltmeters.
• 20.5: Modeling circuits with capacitors
• 20.6: Summary
• 20.7: Thinking about the Material
• 20.8: Sample problems and solutions