Chapter Outline
Electric Charge 352
Static Electricity and Electric Potential 352
Electric Current 352
Resistance and Ohm’s Law 353
Heating Effect of Electric Currents 353
Root Mean Square (RMS) Values 353
Capacitance 353
Energy Stored in a Capacitor 353
Variable Electric Current 353
Electrodes, Cells, and Batteries 354
Thermistors 355
Electric and Magnetic Fields 356
Electromagnetic Field and Electromagnetic Radiation 356
Electromagnetic Induction 356
Electronics and Control Systems 356
Analog and Digital Electronics 356
Electromagnetic Compatibility 359
The Electromagnetic Spectrum 360
Electric Charge
Electricity arises from the physical properties of the elementary particles that comprise atoms and molecules. Some of these particles possess a property known as electric charge , of which there are two forms: a positive form and a negative form. An attractive force occurs between particles of opposite charge and a repulsive force between particles of similar charge.
Atoms comprise a positively charged nucleus surrounded by negatively charged electrons. The aggregate charge on the complete atom is normally zero because the total charge of the electrons is the same magnitude as that of the nucleus, but of opposite sign. Atoms may become charged, however, if electrons are added or removed. Such electrically charged atoms are known as ions . Some compounds, such as sodium chloride, dissociate into charged ions when they dissolve in water. The ions may then move through the solution as carriers of charge.
Electrons may also transport charge by moving from one atom to another through a material, but materials differ in the degree to which electrons can readily move through them. In insulators , electrons do not move, but remain fixed in position; in conductors , they move more readily; in semiconductors , their freedom of movement is more dependent on temperature than in conductors.
The SI unit of charge is the coulomb (C), which is equal to 6.2 × 10 18 times the charge possessed by a single electron.
Static Electricity and Electric Potential
Electrons can be added to an insulator or removed from it, for example, by friction with a different material, and when this takes place, the net charge due to the excess or deficit of electrons tends to remain on the insulator. This accumulation of charge is known as static electricity because the position of the charge is normally fixed.
Although the usual SI unit of energy, the joule (J), can be used to quantify the change that takes place when the charge is moved, a different quantity, the electric potential, is normally used. The electric potential, V , is related to the potential energy ( E ) and the charge (Q) , by the relationship: <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='V=dEdQ’>V=dEdQV=dEdQ
V = dE dQ
The SI unit of electric potential is the volt (V). An alternative term, voltage, may be used to describe the size of an electric potential.
In some circumstances the electric potential produced by static electricity may amount to thousands of volts. The rate of change of potential with distance is the potential gradient. Although air is normally an insulator, a potential gradient of sufficient magnitude can ionize air molecules, thereby causing the air to conduct, resulting in the occurrence of a spark. The energy contained in the spark may be sufficiently great to ignite a flammable mixture.
Electric Current
Electric current is the flow of electrically charged particles such as electrons or ions. It occurs when an electric potential exists across a conductor or a conducting fluid. The SI unit of electric current is the ampere (A), which is equal to a flow of one coulomb of charge per second through any cross-section of the conductor. Current density is current flowing across a unit area; its units are therefore A m −2 ( Figure 28-1 ).
Current, which flows in one direction through a conductor at a fixed rate, is known as direct current (DC). Current, which changes direction periodically, is known as alternating current (AC). An electric current may have both a direct and an alternating component. The waveform of alternating current may be of any shape, but the sine wave is the most fundamental form because all other waveforms can be constructed from a combination of sine waves of appropriate frequencies, amplitudes, and phases. The range of sine wave frequencies of which any waveform consists is known as the frequency range of the waveform.
An electric circuit consists of a source of potential and one or more electrically conducting paths connected to the source that allow current to flow.
Resistance and Ohm’s Law
Materials are characterized by their electrical resistance (R), which measures the potential difference (V) required to produce a certain direct current flow (I). The relationship is given by Ohm’s law: <SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='I=VR’>I=VRI=VR
I = V R
The SI unit of resistance is the ohm (O).
Heating Effect of Electric Currents
The power converted into heat, or some other form of energy, when electric current flows through a resistance is given by
P = V × I
The SI unit of power is the watt (W). The heating effect of electric current is the principle used by the fuse, the electric component used to disconnect a supply of electricity if an excessive current flows. Under normal conditions, the current flows through the fuse without producing any noticeable effect, but when the current flow is excessive, the power converted into heat is sufficient to melt the conductor and interrupt the flow of current.
Root Mean Square (RMS) Values
The instantaneous value of an alternating current varies throughout its cycle. It is more convenient to use a single number to specify the value of an alternating current, and it is common to employ the value of direct current that would produce the same heating effect as the alternating current.
Since the heating effect depends on the power, P, this is represented by
P = VI (see previous discussion)
and as V = IR, (Ohm ’ s law) ,
therefore , P = I 2 R .
To obtain the equivalent direct current, the square of the alternating current is averaged over one complete cycle ( Figure 28-2 ), giving the mean square value. The equivalent direct current is then the square root of this and is known as the root mean square (RMS) value. A similar procedure is used to derive the RMS voltage of an alternating potential. The RMS voltage of the US main supply, for example, is 220 V, although the peak voltage occurring during the cycle is 170 V.
Capacitance
If an electric charge is added to one of two adjacent conducting surfaces, an equal and opposite charge arises on the other conducting surface. This ability of an object to hold electric charge is the property known as capacitance ( C ). It is measured by the relationship between the charge on the conductors ( Q ), and the potential between them ( V ), as follows:
capacitance, C = Q V
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