Quantities
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Click here to read the lecture notes on this topic. Please take the practice quiz below. This is completely anonymous and no records of results are maintained.
If you do not see the quiz above your browser may not support Java Applets. You may wish to download an update for your browser. When a difference of 1 C exists between two bodies, one unit of electrical potential energy exists. This is also referred to as an electromotive force (emf), electrical pressure, or voltage. The unit of measure for emf, electrical pressure, or voltage is the volt. E = W / Q E = The potential difference (V) W = The amount of work or energy expended (J) Q = The charge © (Note that 1 J is the work done by a force of 1 N acting though a distance of 1 m.) When a potential difference exists between two charged bodies that are connected by a conductor, electrons flow in the conductor. This flow is from the negatively charged body to the positively charged body. The flow continues until the two charges are equalized. Voltage Reference It is convenient in most electrical circuits to use one standard reference for all of the various potentials. This is often a common point or ground reference. Voltage Prefixes and Measuring Instruments SI prefixes are used for convenience. The kilovolt kV is used for large values of voltage, whereas the millivolt, mV and microvolt uV are used for small values of voltage. A voltmeter is a device used to measure voltage. THE PRODUCTION OF VOLTAGE A voltage source is a device capable of supplying and maintaining a voltage while some type of electrical device is connected to its terminals. The internal action of the source is such that it continuously removes electrons from one terminal, keeping it positive, and simultaneously supplies these electrons to the second terminal, which maintains a negative charge. There are several methods of producing voltage. Voltage Produced by Friction The Van de Graff generator is a device used to produce millions of volts for research. Voltage Produced by Pressure Certain crystal such as quartz generate a voltage whenever stresses are applied to their surfaces. This phenomenon, called the piezoelectric effect. Is used in microphones and phonograph needles. Voltage Produced by Heat When metal such as copper is heated at one end electrons tend to move away from the hot end toward the colder end. Some metals, such as iron the electrons move toward the warmer end. The thermocouple is a device that uses this principle. Voltage Produced by Light A photoelectric cell is a device that is activated by electromagnetic energy in the form of light waves (photons). Three basic kinds of photoelectric cells exist. The photoconductive cell and photoemissive cell are passive devices, depending on an external current or voltage. The photovoltaic cell is active, converting light energy directly into electricity. These are used in calculators, wristwatches, and satellites. Voltage Produced by Chemical Action An electric cell, or battery, is a device that converts chemical energy into electricity. Strictly speaking a battery consists of two or more cells in series or parallel, but the term is also used for single cells. Any cell consists of a liquid, paste, or solid electrolyte, a positive electrode, and a negative electrode. Batteries in which the chemicals cannot be reconstituted into their original form once the energy has been converted are called primary cells, or voltaic cells. Batteries in which the chemicals can be reconstituted by passing an electric current through them in the direction opposite that of normal cell operation are called secondary cells, rechargeable cells, storage cells, or accumulators. Common examples include lead-acid car batteries and nickel-cadmium batteries for small electronics. Voltage Produced by Magnetism Three fundamental conditions must be met before a voltage can be produced by magnetism. 1. There must be a conductor in which the voltage will be produced. 2. There must be a magnetic field in the conductor's vicinity 3. There must be a relative motion between the field and conductor.
VOLTAGE DROP A voltage drop represents the energy used by the free electrons while engaged in current flow. This energy is usually dissipated as heat in most electronic components. ELECTRIC CURRENT Electric current (I) is defined as the directed flow of free electrons. Electron movement is from negative to positive. Random Drift At room temperature conductors have an abundance of free electrons. These free electrons take a path that is not predictable. Directed Drift If a conductor has a potential difference impressed across it, a direction of movement is imparted to the randomly drifting electrons. The directed movement of the electrons occurs at a relatively low velocity. The effect of this directed movement however, is felt almost instantaneously. Magnitude of Current Flow The magnitude of current flow is affected by the difference of potential. Measurement of Current The magnitude of current is measured in amperes (A). A current of 1 A is said to flow when 1 C of charge passes a point in 1 s. 1 C is equal to the charge of 6.25 x 10**18 electrons. I = Q/T I = current (A) Q = charge © T = time (s) Amperage Prefixes The ampere is frequently too large a unit for measuring current. Therefore, the milliampere (mA), one-thousandth of an ampere is commonly used. The device used to measure current is called an ammeter. ELECTRICAL RESISTANCE Some materials offer little opposition to current flow, whereas others greatly oppose current flow. The opposition of current flow is known as resistance ®. The unit of mesure is the ohm () Factors That Affect Resistance The magnitude of resistance is determined in part by the number of free electrons available within the material. Resistance Prefixes Resistance values for electrical and electronic components can range from several millions of ohms to values of less than 1 ohm. Ohm, kilohm, and megohm are commonly used. Conductance The term that is opposite of resistance is conductance (G). The old unit of conductance was the mho, (ohm spelled backward). The SI unit, the siemens (S) is now more often used. R = 1/G G = 1/R A PRACTICAL ELECTRIC CIRCUIT An electric circuit has four important characteristics: 1. There must be a source of potential difference. 2. There must be a complete path (supplied by a conductor) for current flow from one side of the applied voltage source, through the external circuit and returning to the other side of the voltage source. 3. A resistance that changes the electrical energy into a more useful form. 4. The circuit is usually controlled by some device such as a switch. Schematic Representation A schematic diagram is a picture of a circuit that uses symbols to represent the various circuit components. Click here to move back to the top of this page.
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