TECHNOLOGIES ESO 4

UNIT 1: ELECTRICITY AND ELECTRONICS

3rd YEAR SUMMARY

1. WHAT IS ELECTRIC CURRENT?

Electric current is the flow of electrons around a circuit.Electrons have to “scape” from the nucleus of the atom to flow, the electrical conductivity depends on the atomic structure of material.Pay attention to these atoms:

What are the differences in the atomic structure of this elements ?

2. ELECTRICAL PROPERTIES

____________ allow electric currents to pass through them. All metals belong to this group: copper, iron, silver…

___________don’t allow electric currents to pass through them. Plastic, wood and glass are examples of this group.

_________________ have properties which are intermediate between previous groups. The most important examples are silicon and germanium.

CONDUCTORS

INSULATORS

SEMI-CONDUCTORS

3. COMPONENTS OF AN ELECTRICAL CIRCUIT

Generators provide the energy for electrons to circulate. The power for many electrics an electronics circuits comes from batteries. A battery stores electric energy and, when connected to a circuit, it provides the force to the electrons to run along the wires.

3.1 GENERATORS

Dynamos and alternators are used for large scale power generation and industry.

They transform electrical energy into another form of energy. For example, light bulbs, lamps or LEDs transform electrical energy into light, motors transform electrical energy into movement, buzzers transform electrical energy into sound.

3.2 RECEIVERS

3. COMPONENTS OF AN ELECTRICAL CIRCUIT

These are elements used to control the flow of electrons through the circuit. Switches are mechanical devices that can make (connect), break (disconnect) a circuit or divert the current from one conductor to another (circuit switch or crossover switch). A push button allows the current to flow only when pressure is mantained.

3.3 CONTROL ELEMENTS

3. COMPONENTS OF AN ELECTRICAL CIRCUIT

These are elements used to protect the circuit against excessive current, voltage or current leakage. The most important elements in this group are fuses (that cuts the circuit by melting when the current is too high), breaker switches (automatic fuses) and differential switches (than protect against current leakage).

3.4 PROTECTION ELEMENTS

3. COMPONENTS OF AN ELECTRICAL CIRCUIT

BATTERIES: LIGHT BULB/LAMP:

MOTOR:

BUZZER/BELL:

LED:

SWITCH:

PUSH BUTTON:

CIRCUIT SWITCH:

RESISTORS:

4. REPRESENTATION AND SYMBOLS

INTERESTING WEB:http://www.clarvis.co.uk/version2/symbols.html

5. ELECTRIC QUANTITIES

Voltage or tension is the energy that a battery or generator can provide for each electron. Voltage is measured in volts (V).

5.1 VOLTAGE

5.2 ELECTRIC CURRENTElectric current is the charge, or number of electrons, that flows through a conductor per second. I = Q/t Electric current is measured in amperes o amps (A).

5.3 RESISTANCEResistance is a measure of how easily (or with what difficulty) electrons will flow through the device. Copper wire has a very low resistance, so a small voltage will allow a large current to flow. Likewise, the plastic insulation has a very high resistance, and prevents current from flowing from one wire to those adjacent.

5. ELECTRIC QUANTITIES

I = V/R

5.4 OHM´S LAWVoltage, current and resistance are quantities related. Current flowing through a circuit is increased in relation to the voltage and may also change depending on the material used. Ohm’s law expresses these relations as follows:

5.5 ELECTRIC POWER AND ENERGY

P = V·I E = V·I·t = P·t

Electric power is measured in watts (W) and also kilowatt (kW)Energy in SI units is measured in joules (J), calories (cal) and kilowatt-hour (kWh)

1 J = 0,24 cal

1 kWh = 3,6 · 106 J

6. SI UNITS

Mega 1000,000 1x106 M Ohmskilo 1000 1x103 k OhmsUnitsmilli 0.001 1x10-3 m Ampsmicro 0.000,001 1x10-6 μ Faradsnano 0.000,000,001 1x10-9 n FaradsPico 0.000,000,000,001 1x10-12 p Farads

p n μ m 1 k M-12 -9 -6 -3 0 +3 +6

am x an = am+n

am / an = am-n

7. SOLVING CIRCUITS

Resistors can be wired in parallel or in series, or any combination there of, so that values greater or smaller than normal or with higher power or voltage can be obtained. This also allows us to create new values, not catered for in the standard values.

Some Resistor CombinationsSeries: When wired in series, the values simply add

together. A 100 ohm and a 2k2 resistor in series will have a value of 2k3.

R = R1 + R2 (+ R3, etc.)

Parallel: In parallel, the value is lower than either of the resistors. A formula is needed to calculate the final value

1/R = 1/R1 + 1/R2 (+ 1/R3 etc)

7.1 SERIES AND PARALLEL RESISTANCES

7. SOLVING CIRCUITS

Some elements are connected in series and some in parallel: 7.2 MIXED CIRCUITS

RT = RP + RS; 1/Rp= 1/10 + 1/7; RP = 4,1k; RT = 5k + 4,1k = 9,1k

IT = VT/RT; IT = 9V/9,1k= 0,98 mA

V5k=IT·R5k = 0,98 mA·5k= 4,9 V ; V7k=V10k = 9V-4,9V = 4,1 V

I7k = V7k/R7k = 4,1V / 7k = 0,58 mA ; I10k= 4,1V/10k = 0,41 mA

(IT = I7k + I10k = 0,58mA+0,41mA=0,99mA)

Rs = 250+1000=1250Ω; 1/RT= 1/Rp= 1/100 + 1/1250; RT = 92,6Ω

IT = VT/RT; IT = 9V/92,6Ω = 0,097A; V100= 9V; I100=V100/R100 = =9V/100Ω=0,09A; IT=I100+I1250; I1250=0,097A-0,09A=0,007A; V250=0,007A·250=1,75V; V1k = 9-1,75V = 7,25V

Rs = 10k+10k=20k; 1/RT= 1/Rp= 1/20 + 1/10; RT = 6,7k

IT = VT/RT; IT = 9V/6,7k = 1,34mA; V10k= 9V; I10k=V10k/R10k = =9V/10k=0,9mA; IT=I10k+I20k; I20k=1,34-0,9=0,44mA;

V10k-10k=0,44mA·10k=4,4V