Electricity

ELECTRICITY REVISION
Basic elements in an electric circuit:
Components that produce electric current (e.c.)
Components that conduct an e.c.
Components that transform an e.c.
Components that control the flow of e.c.
Basic electric magnitudes:
Voltage
Intensity
Resistance
Ohm's Law
Dpto. Tecnoloxía, IES de Pastoriza, Arteixo

Components that produce e.c.:
Batteries and generators
Components that conduct an e.c.:
Wires of conductor materials.
Components that transform an e.c.:
Electric motor, light bulb, electric fire, resistors
Components that control the flow of e.c.:
Switches, circuit switches, push button N.O. and
N.C.


Basic magnitudes :
MAGNITUDE DEFINITION UNITS

Opposition that materials OHMS (Ω)
RESISTANCE offer to electric current KΩ = 1000 Ω = 10 3 Ω
flow. MΩ = 1000000Ω = 106 Ω

Number of charges that
AMPS (A)
INTENSITY go through the section of
mA = 0,001A = 10-3A
a conductor per unit of
μA= 0,000001A = 10-6A
time.
VOLTS (V)
Energy difference
KV = 1000V = 103 V
VOLTAGE between both of the
MV= 1000000V
extremes of a conductor.


RESISTORS
There are 2 types:

fixed resistors:

variable resistors:

fixed resistor Potentiometers


RESISTORS
Fixed resistors:

These resistors have a fixed value represented by a
colour code that we can observe over their surface.

The most common are those of coal, they are small
cylinders of graphite recovered with a film of paint
and they have two terminals to connect them.


VARIABLE RESISTORS
POTENTIOMETERS
Its value can be changed with a cursor or turning an
axle.
The resistance varies from 0 ohms to a maximum
value indicated at potentiometer
Applications
Thermostat of a heating
Volume control of a radio


Ohm's law

Ohm's law relates the intensity that flows through a
conductor whith its resistance and the applied voltage.

V
I= ―
R


SERIES CIRCUITS
They are circuits where components are connected
one behind the other.

There is a different tension in each component, but
through all circulates the same intensity.

Resolution:

R e = R1 + R2 + R3 + … = ΣR


PARALLEL CIRCUITS
They are circuits where components are connected
with two common points.

Through each component circulates a different
intensity but all have the same voltage.

Resolution:
1 1 1 1 1
= + + =Σ
Re R1 R2 R3 R


MIXED CIRCUITS
They are circuits that have components connected
either in series or in parallel.

Resolution:

There is not an only way to solve these circuits.
We have to calculate equivalent resistances by
sections.

POWER AND ENERGY
Power is the electric magnitude that measures the
consumed energy per unit of time.

We represent it by P and it is measured in watts
(W).

If in a component circulates an intensity I and it is
fed with a voltage V, then, the power that consumes
it is calculated like:

P=V·I

POWER AND ENERGY
Power is a characteristic of all electrical devices.
All electrical appliances present the relation of their
electrical characteristics. We refer to that like
nameplate.

Frecuency
Voltage

Power

POWER AND ENERGY
Energy consumed by an electrical device is
determined by its power and the time in which it is
running:
E= P·t
Consumed electrical energy it is measured in W·s or in
KW·h
If an electrical appliance is has a voltage of 230 V and
consumes 640 W, then we can calculate its resistance
like: R=V2/P
Then R = 2302/640 = 82,65 Ω


POWER AND ENERGY
The consume in the electric bill is measured in
kWh (power in kW and time in hours)
If we multiply by the price of a kWh, we obtain
the cost of use of that electrical appliance.
Example: The cost of use of a hairdryer of 2200
W running during 30 minutes is
P = 2200 W = 2,2 kW
t = 30 min = 0,5 h
E = P.t = 2,2 . 0,5 = 1,1 kWh (consumed energy)
If the price of a kWh is 0.12 €, then the cost is: 1,1 x 0,12 = 0,13 €


Electricity

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