2.9: Summary

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In this chapter we have examined the basic quantities that make up an electrical circuit. Charge, measured in coulombs, is a characteristic of subatomic particles; protons being positively charged and electrons being negatively charged. Like charges repel and opposite charges attract. The Bohr model of an atom shows that electrons are contained in energy shells and a single electron in the outermost shell is only loosely bound to an atom. With an applied external energy source these electrons (and thus their charge) can be moved from place to place. The rate of charge movement over time is called current and is measured in amps. Energy is defined as the ability to do work and is measured in joules. Voltage is defined as the energy required to move a charge divided by that charge, with units of volts. Power is the rate of energy usage over time and is measured in watts.

Some materials allow the easy flow of current and are called conductors. In contrast, materials that inhibit the flow of electrical current are called insulators. The measure of this inhibition is called resistance and has units of ohms. Its reciprocal is called conductance and is measured in siemens.

Resistors are devices designed to restrict the flow of current. They are available in a wide range of resistive values, power ratings, and physical sizes and configurations. Common resistors use a series of colored stripes, or color code, to signify their resistance value and tolerance.

Efficiency is the ratio of useful power output to applied power input, normally expressed as a percentage. The higher a system's efficiency, the less energy it will use, the less it will cost to run, and generally the less heat it will generate. If the system is battery powered, higher efficiency leads to longer battery life. Battery capacity is measured in amp-hours. All other factors being equal, the higher the amp-hour rating, the longer the battery will last.

The digital multimeter, or DMM, is a versatile tool for measuring voltage, current, resistance, and possibly other circuit or electrical device parameters. The accuracy of a DMM is dependent on its resolution which is in turn set by the number of digits used, or its total count. The count specification is added to a percent tolerance to arrive at a worst case error value for some specific reading.

Review Questions

1. Describe the Bohr atomic model.

2. How does charge relate to current and voltage?

3. What is the relationship between energy and power?

4. What is the relationship between resistance and conductance?

5. Define efficiency. How does it relate to operating cost?

6. Define count in terms of a digital multimeter accuracy specification.

7. Describe the resistor color code and its use.