Download WHAT IS ELECTRIC CURRENT

WHAT IS ELECTRIC CURRENT?
An electric current is a flow of microscopic particles called ELECTRONS flowing through wires and
electronic components.
It can be compared to the flow of water through pipes and radiators etc.
As water is pushed through pipes by a pump; electric current is pushed through wires by a battery.
Electrons have a negative charge.
Electric current will flow from the positive terminal of a battery, through the lamp, to the negative
terminal.
This is called electron current flow. The current flows round the circuit. This is the conventional
current flow, and it is so because as electrons are negative charged, when they leave the electrode, it
remains positive.
RESISTORS
Resistors are electronic components which oppose the flow of electronic current.
The higher the value of resistance (measured in ohm =) the lower the current will be. This was
discovered by Mr Ohm.
The ohm is a small unit, so sometimes it is necessary to use some multiples. The most important are:
k= 1000  = 1x103
M = 1000000  = 1x106 = 1000 k = 1x103k
In electronic textbooks sometimes we can find the resistors values written as 3k7 that means 3,7k
and 4M6 that means 4,6M.
THE RESISTOR COLOUR CODE
Most resistors have three coloured bands close together at one end and one single band at the other.
Each band means a digit according to its position in the resistor, so the three adjacent bands give the
resistor value. First you have to place the separated band to the right position and then:
The first band on the left, gives the value of the first digit. e.g Brown = 1.
The second band gives the value of the second digit e.g. Red = 2
The third band gives the number of zeros which follows the two digits e.g. orange = 3 zeros = 000. It is
called the Multiplier band.
Therefore a resistor with brown, red, orange bands would have a value of 12000 ohms.
TO KNOW HOW TO USE THE RESISTORS COLOUR CODE, FOLLOW THIS LINK:
http://ealnet.com/m-eal/resistor/resistor.htm
The fourth band indicates the tolerance. e.g. brown indicates plus or minus 1%.
a 100 ohm 1% resistor can be in value between 99 ohms and 101 ohms.
Tolerance band: brown 1%, red 2%, gold 5%, silver 10%, none 20%
If there is a fifth pink band this indicates a high stability resistor.
STANDARD PREFIXES FOR INTERNATINAL SYSTEM OF UNITS
NAME
MULTIPLES
SUBDIVISIONS
SYMBOL
deca-
hecto-
da
kilo-
h
k
MegaM
giga-
tera-
G
T
FACTOR
101
102
103
106
109
1012
NAME
deci-
centi-
mili-
micro-
nano-
pico-
SYMBOL
FACTOR
d
10-1
c
10-2
-1-
m
10-3

10-6
n
10-9
p
10-12
TASKS – PART I
1. COMPLETE THE FOLLOWING TABLE:
COLOURS
VALUES ( in Ohms -  )
Red-red-brown—gold
Brown-brown-brown—silver
Blue-yellow-green –-red
Green-yellow-orange—brown
Red-green-brown—none
Orange-red-blue—red
Brown-black-black—gold
White-grey-orange—none
Violet-blue-orange—silver
33000±10%
27000000±5%
4K7±2%
9800±1%
100±2%
5M8±20%
6900 ±10%
17K±5%
2. Convert the following values:
a)- 1M2 into k
b)- 1K8 into 
c)- 5M6 into K
d)- 4K7 into 
e)- 3M9 into k
3. What is a resistor?
4. Which is the unit used to measure resistance?
5. Name the most common multiples of Ohm and explain their equivalences.
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OHM’S LAW
Different conductors resists current flow by different amounts under different conditions, this
property is called resistance R. Ohm’s law states that the current through a conductor is directly
proportional to applied p.d. (v), and inversely proportional to the intensity, (I) ,of the current applied.
The unit of resistance is Ohm (), the unit of potential difference (p.d.) is V (Volt) and the unit of
intensity, I, is A (Ampere).
TYPES OF CIRCUITS
There are two main types of circuits SERIES and PARALLEL. Some circuits have some components
connected in series and other in parallel. They are called MIXED CIRCUITS.
SERIES CIRCUIT
The p.d. across the cell (V) is equal to the sum of the p.d. across
the two components (V1 and V2).
V= V1+V2
The current (I) is the same through all parts of the circuit.
I = I1 = I2
Total resistance is equal to the sum of the values of all the resistor
in the circuit:
RT = R1+R2
PARALLEL CIRCUIT
The current from the source (I) is equal to the sum of the currents
in the two branches (I2 and I2)
IT = I1+I2
The potential difference (p.d.) across the two components is equal
to the value of Volts from the source (V).
V= V1=V2
The formula used to calculate total resistance is:
1
R
T
ELECTRIC POWER
(potencia)
P

1

1
R R
1
 ... 
2
1
R
N
Electric power is a magnitude that measures how fast is
electric energy used in the circuit. The SI unit for power is
watt (W).
The relationship with other electrical magnitudes are
P=I·V and as V=I·R (according to Ohm´s Law) so
P=I2·R
Units: 1 kW = 1000W = 103 W
ELECTRIC ENERGY: P·t // Units : kW·h
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CIRCUIT ELEMENTS AND SYMBOLS
WIRES & CONNECTIONS
Component
Circuit Symbol
Function of the component
Wire
To pass current very easily from one
part of a circuit to another.
Wires joined
A 'blob' should be drawn where wires
are connected (joined), but it is
sometimes omitted.
Wires not joined
In complex diagrams it is often
necessary to draw wires crossing
even though they are not connected.
Power Supplies
Cell
Supplies electrical energy.
A single cell is often wrongly called a
battery, but strictly a battery is two
or more cells joined together.
Battery
Supplies electrical energy. A battery
is more than one cell.
DC supply
Supplies electrical energy.
DC = Direct Current, always flowing
in one direction. Electrons leave the
+ive pole and goes to the –ive pole.
Output Devices
Lamp
A transducer which converts
electrical energy to light.
Motor
A transducer which converts
electrical energy into kinetic energy
(motion).
Bell / buzzer
A transducer which converts
electrical energy to sound.
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SWITCHES
This switch allows current to flow only
when the button is pressed. This is the
switch used to operate a doorbell.
Push Switch
(push-to-make)
This type of push switch is normally
closed (on), it is open (off) only when the
button is pressed.
Push to break switch
On off switch
An on-off switch allows current to flow
only when it is in the closed (on) position.
A 2-way switch directs the flow of
current to one of two routes according
to its position.
Two way switch
A relay is an electrical switch that
opens and closes under the control
of another electrical circuit
Relay switch
HOW DOES A RELAY WORK?
The switch is operated by an electromagnet to open or close one or many sets of contacts. The
control circuit of the relay is connected to a coil with an iron nucleus that becomes a magnet when
the current flows through it. So, the contacts of the controlled circuit will move and open or close
the circuit. Label the parts of the following relays.
TO LEARN MORE ABOUT RELAYS, FOLLOW THESE LINKS:
http://www.youtube.com/watch?v=zY7XXSG6nbo
http://www.youtube.com/watch?v=E8Os4WFUALA&feature=related
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RESISTORS
Resistor
It restricts the flow of current, for
example to limit the current passing
through an LED.
Variable resistor
(potentiometer)
It has got 3 contacts. It is usually
used to control voltage. Examples
include: adjusting lamp brightness,
adjusting motor speed…
METERS
Voltmeter
A device used to measure potential
difference (voltage). It must be
always connected in parallel to the
points between the p.d. is being
measured.
Ammeter
A device used to measure current
intensity. It must be always
connected in series in the branch
where intensity is being measured.
Ohmmeter
A device used to measure resistance.
TASKS
1. Calculate the intensity that flows through a 100W lamp connected to 220V.
2. Your music equipment electric power is 40W. If you listen to music for 5 hours, how
many kW.h did you use? How much must you pay to the electric company if 1 kW.h
costs 0,08€?
3. The electric power used by an iron is 2400 W. If you are ironing for 2 hours, how many
kW.h did you use? How much must you pay to the electric company if 1 kW·h costs 0,08
€?
4. Represent, using the adequate symbols, a series circuit with 4 different elements and
indicate the position of a voltmeter and an ammeter in it,
5. Represent, using the adequate symbols, a parallel circuit with 4 different elements and
indicate the position of a voltmeter and an ammeter in it,
6. Investigate the electric values for 5 electric or electronic devices you use at home. Use a table
to record your investigation.
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