College Physics

# Introduction to Circuits and DC Instruments

College PhysicsIntroduction to Circuits and DC Instruments

### Chapter Outline

21.1 Resistors in Series and Parallel
• Draw a circuit with resistors in parallel and in series.
• Calculate the voltage drop of a current across a resistor using Ohm’s law.
• Contrast the way total resistance is calculated for resistors in series and in parallel.
• Explain why total resistance of a parallel circuit is less than the smallest resistance of any of the resistors in that circuit.
• Calculate total resistance of a circuit that contains a mixture of resistors connected in series and in parallel.
21.2 Electromotive Force: Terminal Voltage
• Compare and contrast the voltage and the electromagnetic force of an electric power source.
• Describe what happens to the terminal voltage, current, and power delivered to a load as internal resistance of the voltage source increases (due to aging of batteries, for example).
• Explain why it is beneficial to use more than one voltage source connected in parallel.
21.3 Kirchhoff’s Rules
• Analyze a complex circuit using Kirchhoff’s rules, using the conventions for determining the correct signs of various terms.
21.4 DC Voltmeters and Ammeters
• Explain why a voltmeter must be connected in parallel with the circuit.
• Draw a diagram showing an ammeter correctly connected in a circuit.
• Describe how a galvanometer can be used as either a voltmeter or an ammeter.
• Find the resistance that must be placed in series with a galvanometer to allow it to be used as a voltmeter with a given reading.
• Explain why measuring the voltage or current in a circuit can never be exact.
21.5 Null Measurements
• Explain why a null measurement device is more accurate than a standard voltmeter or ammeter.
• Demonstrate how a Wheatstone bridge can be used to accurately calculate the resistance in a circuit.
21.6 DC Circuits Containing Resistors and Capacitors
• Explain the importance of the time constant, τ , and calculate the time constant for a given resistance and capacitance.
• Explain why batteries in a flashlight gradually lose power and the light dims over time.
• Describe what happens to a graph of the voltage across a capacitor over time as it charges.
• Explain how a timing circuit works and list some applications.
• Calculate the necessary speed of a strobe flash needed to “stop” the movement of an object over a particular length.
Figure 21.1 Electric circuits in a computer allow large amounts of data to be quickly and accurately analyzed.. (credit: Airman 1st Class Mike Meares, United States Air Force)

Electric circuits are commonplace. Some are simple, such as those in flashlights. Others, such as those used in supercomputers, are extremely complex.

This collection of modules takes the topic of electric circuits a step beyond simple circuits. When the circuit is purely resistive, everything in this module applies to both DC and AC. Matters become more complex when capacitance is involved. We do consider what happens when capacitors are connected to DC voltage sources, but the interaction of capacitors and other nonresistive devices with AC is left for a later chapter. Finally, a number of important DC instruments, such as meters that measure voltage and current, are covered in this chapter.