# PHYS 2212 Module 6

6: Direct Current Circuits

This circuit shown is used to amplify small signals and power the earbud speakers attached to a cellular phone. This circuit’s components include resistors, capacitors, and diodes, all of which have been covered in previous modules, as well as transistors, which are semiconducting devices covered in Condensed Matter Physics. Circuits using similar components are found in all types of equipment and appliances you encounter in everyday life, such as alarm clocks, televisions, computers, and refrigerators.

In the preceding few modules, we discussed electric components, including capacitors, resistors, and diodes. In this module, we use these electric components in circuits. A circuit is a collection of electrical components connected to accomplish a specific task. The figure above shows an amplifier circuit, which takes a small-amplitude signal and amplifies it to power the speakers in earbuds. Although the circuit looks complex, it actually consists of a set of series, parallel, and series-parallel circuits. The second section of this module covers the analysis of series and parallel circuits that consist of resistors. Later in this module, we introduce the basic equations and techniques to analyze any circuit, including those that are not reducible through simplifying parallel and series elements. But first, we need to understand how to power a circuit.

Most electrical phenomena (everything from light bulbs to toaster ovens to computer screens to lightning bolts…) involve the flow of current. Current flow happens when you have a source of potential difference, and a circuit — a closed path for current to flow. (Since charge is conserved, charges ultimately have to go around in a circuit for the flow to be sustainable.)

In this module, we will discuss electric circuits that contain resistors (like light bulbs or toaster ovens, for example), capacitors, and something to drive the current — a source of the potential difference.

One source of a potential difference is called an “emf” device. You could think of “emf” as a funny spelling of “oomph”. It’s what provides the oomph, the required potential difference to drive current. Examples are generators, solar panels, fuel cells, or batteries. Batteries will be our “typical” emf devices.

#### 6.1 Electromotive Force

• Describe the electromotive force (emf) and the internal resistance of a battery
• Explain the basic operation of a battery

#### 6.2 Resistors in Series and Parallel

• Define the term equivalent resistance
• Calculate the equivalent resistance of resistors connected in series
• Calculate the equivalent resistance of resistors connected in parallel

#### 6.3 Kirchhoff’s Rules

• State Kirchhoff’s junction rule
• State Kirchhoff’s loop rule
• Analyze complex circuits using Kirchhoff’s rules

#### 6.4 Electrical Measuring Instruments

• Describe how to connect a voltmeter in a circuit to measure voltage
• Describe how to connect an ammeter in a circuit to measure current
• Describe the use of an ohmmeter

#### 6.5 RC Circuits

• Describe the charging process of a capacitor
• Describe the discharging process of a capacitor
• List some applications of RC circuits

#### 6.6 Household Wiring and Electrical Safety

• List the basic concepts involved in house wiring
• Define the terms thermal hazard and shock hazard
• Describe the effects of electrical shock on human physiology and their relationship to the amount of current through the body
• Explain the function of fuses and circuit breakers