Electric Theory English文档格式.docx

1、e.g. Silicon.Electric ChargesNeutral state of an atomAn atom with an equal number of electrons and protons is said to be electrically neutral.e.g. Aluminum atom has 13 electrons and 13 protonsPositive and negative chargesThe lack of electrons is called positive charge because there are more protons

2、than electrons. The excess of electrons has a negatives charge. Attraction and repulsion of electric chargesUnlike charges attract Like charges repel CurrentElectricity is the flow of free electrons in a conductor from one atom to the next atom in the same general direction. This flow of electrons i

3、s referred to as current. The symbol of current is A.VoltageVoltage is the force that is applied to a conductor that causes electric current to flow. The force required to make electricity flow through a conductor is call a difference in potential. Current flow through a conductor as same as water f

4、low through a pipe. The symbol of voltage is V or E. ResistanceAll material impedes the flow of electrical current to some extent. The amount of resistance depends on composition, length, cross-section and temperature of material. The symbol of current is .Simple Electric CircuitA fundamental relati

5、onship exits between current, voltage, and resistance in the electric circuit. There must be a complete path for current to flow. Otherwise, the circuit open and then not work.e.g. The following diagram is represent an simple electrical circuit. It is consist of battery, resistance, ammeter and volt

6、meter.Ohms LawThe relationship between current, voltage and resistance is that current varies directly with voltage and inversely with resistance. i.e. Current（ I ） = Voltage（ E ） / Resistance （） Voltage（ E ） = Current（ I ） X Resistance （） Resistance （） = Voltage（ E ） / Current（ I ）The correct answe

7、r of the correct values is: Current is expressed in Amps, A Voltage is expressed in Volts, E or V Resistance is expressed in Ohms, e.g. The simple circuit below, assume the voltage supplied by battery 10 Voltage, and resistance is 5To find how much current:Current（ I ） = Voltage（ E ） / Resistance （）

8、Current（ I ） = 10 volts / 5 = 2 ADC Series CircuitA series circuit is formed when any number of resistor are connected end-to-end so that there is only one path for current to flow.Resistance in series circuitThe values of resistance add in a series. If it is five resistance in a series circuit, the

9、n the formula is Rt = R1+R2+R3+R4+R5Rt = R1+R2+R3+R4+R5Rt = 11k+2k+2k+0.1k+1kRt = 16.1kCurrent in series circuitCurrent is the same anywhere it is measured in a series circuit. For example,I = E / RI = 12/10I = 1.2 AVoltage in series circuitVoltage can be measured across each of the resistors in a c

10、ircuit. The voltage across a resistor is referred to as a voltage drop. In the following diagram, four resistors of 1.5 each have been placed in series with a 12V battery.First, solve for total resistance,Rt = R1+R2+R3+R4Rt = 1.5+1.5+1.5+1.5Rt = 6Second, solve for current,I = 12 / 6I = 1 AThird, sol

11、ve for voltage across any resistor （if want to check the third resistor）:E = I X RE = 2 X 1.5E = 3 VVoltage division in a series circuitIt is often desirable to use a voltage potential that is lower than supply voltage. To do this, a voltage divider, similar to the following diagram, can be used.Rt

12、= R1+R2Rt = 5 + 20Rt = 25I = Ein / RtI = 50 / 25I = 2 AFinally, solve for voltage,Eout = I X R2Eout = 2 X 20Eout = 40 VDC Parallel CircuitResistance in parallel circuitThe parallel circuit is formed when two or more resistances are place in a circuit side-by-side so that current can flow through mor

13、e than one path. If there are three resistors in a parallel circuit, the formulas determine total resistance below:1 / Rt = 1 / R1+1 / R2+1 / R31 / Rt = 1 / 5+1 / 10 +1 / 201 / Rt = 4 / 20+2 / 20 +1 / 201 / Rt = 7 / 20 Rt = 20 / 4 Rt = 2.86 Voltage in parallel circuitWhen resistors are placed in par

14、allel across a voltage source, the voltage is the same across each resistor. The following diagram:Current in parallel circuitCurrent flowing through a parallel circuit divides and flows through each branch of the circuit. i.e. Total current is equal to the sum of the current in each branch.It = I1+

15、I2+I3For example, to find the total current in the following circuit diagram.I1 = E / R1I1 = 12 / 40I1 = 0.3 AI2 = E / R2I2 = 12 / 20I2 = 0.6 AIt = I1+I2It = 0.3 + 0.6It = 0.9 ASeries-Parallel CircuitsSeries-Parallel Circuits are compound circuit. At least three resistors are required to from a seri

16、es-parallel circuit. The following diagrams show two ways a series-parallel combination could be found.First, it is simplifying a series-parallel to a series circuitSecond, it is simplifying a series-parallel to a parallel circuitPowerWhenever forces of any kind cause motion, work is accomplished. I

17、n the diagram below work is done when a mechanical force is used to lift a weight. If I force were exerted without causing motion, then no work is done.Electric powerIn an electrical circuit, voltage applied to a conductor will cause electron to flow. Voltage is the force and electron flow is the mo

18、tion. The rate at which work is done is called power and is represented by the symbol PPower formulaThis is expressed as voltage times current.P = E IP = I2 RP = E2 / RFor example, the following diagram power can be using any of the power formulas,P = 12 X 2P = 22 X 6P = 122 / 6P = 24 WP =24 WMagnet

19、ismThe principles of magnetism are an integral part of electricity. All magnets have two characteristics. They attract and old iron. If free to move, like the compass needle, the magnet will assume roughly a north-south position. The three common forms of magnets are horseshoe, bar and compass needl

20、e.Magnetic line of fluxEvery magnet has two poles, one north pole and one south pole. This is the point at which maximum attraction occurs. Invisible magnetic lines of flux leave the north pole and enter the south pole. By drawing lines the way, the following diagram is obtained.The field lines exis

21、t outside and inside the magnet. The magnetic lines of flux are always from closed loop.Interaction between two magnetsWhen two magnets are brought together, the magnetic flux filed around the magnet cause some form of interaction. Two unlike poles attract each other. Two like poles repel each other

22、.ElectromagnetismLeft-hand rule for conductorsAn electromagnetic field is a magnetic field generated by current flow in a conductor. Every electric current generates a magnetic field. A definite relationship exits between the direction of current flow and the direction of magnetic field. The left-ha

23、nd rule for conductors demonstrates this relationship. The left hand with the thumb pointing in the direction of electron flow, the fingers will point in the direction of the magnetic lines of flux.Current-carrying coilA coil of wire carrying a current acts like a magnet. Individual loop of wire act

24、s as small magnets. The individual field adds together to form one magnet. The strength of the field can be increase by adding more turns to the coil and increase the current.Left-hand rule for coilsA left-hand rule exits for coils to determine the direction of the magnetic field. The fingers of the

25、 left hand are wrapped around the coil in the direction of electron flow. The thumb points to the north pole of the coil.ElectromagnetsAn electromagnet is composed of a coil of wire would around a core. The core is usually a soft iron that conducts magnetic line of force with relative ease. When cur

26、rent is passed through the coil, the core becomes magnetized. A large variety of electrical devices such as motors, circuit breakers, relay, and starters use electromagnetic principles.Introduction to AC The supply of current for electrical devices may come from a direct current （ DC ）, or an altern

27、ating current （ AC ）. In DC electricity, electrons flow continuously in one direction. In AC electricity, electrons flow first in one direction then in another. The AC generator reverses terminal polarity many times a second.AC sine waveAlternating voltage and current vary continuously. The graphic

28、representation for AC is a sine wave. A sine wave can represent current and voltage. When the waveform is above the time axis, current is flowing in one direction. This is referred to as the positive direction. When the waveform is below the time axis, current is flowing in another direction and negative direction. Single-phase and three-phase AC powerAC is divided into single-phase and three-phase types. Single-phase is used for small electrical. Three-phase power is used where large blocks. Three-phase power is a continuous series of three overlapping AC cycle. Each wave