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					         Theme 5 – Electricity in the Home


5.1 The Atom:

                                       Everything around us is made up of atoms.
                                       Atoms are too small to be seen by our eyes and
                                       we imagine them to have the shape of a ball.


                                       An atom is made up of particles. There are
                                       three types of particles, two of which carry an
                                       electrical charge.




(a) THE NUCLEUS :

  The nucleus is made up of:
       (i) protons - particles having a positive (+) charge.
       (ii) neutrons - particles with no charge.



(b) ELECTRONS :

   The   electrons have a negative (-) charge and are orbiting
(moving) around the nucleus.



5.2 Charge:
Normally an atom has an equal quantity of positive and negative charges, and
therefore it is neutral.

However, atoms can gain or lose electrons:

      If an atom   gains   electrons, then it has more negative (-) than positive (+)

       and it becomes       negatively charged.
    If an atom   loses electrons, then it has more positive (+)    than negative (-)
       and it becomes positively charged.


                                                                                     1
This means that an object can be either positively (+) charged or negatively (-)
charged.

For example:

    When a polythene strip is rubbed with a
       duster, some electrons move from the duster
       onto the polythene strip.

This means that the polythene strip has now extra
electrons and becomes negatively charged.



      When a perspex strip is rubbed with a
       duster, some of the electrons move
       from the perspex strip onto the duster.



This leaves the perspex strip with fewer electrons than protons, and so
becomes positively charged.


Note that:
1. It is always the electrons that move, protons remain fixed.
2. Rubbing does not create charge, it simply separates them.



5.3 Attraction and Repulsion:

When two objects having the same charge are placed next to each other, they
try to move away from each other (   Repulsion ).
On the other hand, when two objects having an opposite charge are placed next
to each other, they move towards each other (    Attraction  ).

This is known as the Law of Force which states that:

                         Like electric charges repel.
                      Unlike electric charges attract.




                                                                               2
5.4 Conductors and Insulators:

Conductors   of electricity have loosely bound electrons which are free to move
through them. This means that electrons can move freely from atom to atom.
Thus we do not need to rub a conductor if we want to charge it.

Insulators of electricity have strongly bound electrons which cannot move around
the insulator. Since in insulators the charge produced does not move from
where the rubbing occurs, we say that the charge is static.

A   semi-conductor   is neither a good conductor nor an insulator. These materials,
such as silicon and germanium are used in electronic devices. These can be made
negatively charged (called n-type semiconductor) or positively charged (called p-
type semiconductor).Sometimes these n-type or p-type materials are joined
together to produce a p-n junction diode.


Exercise: Name some examples of conductors and insulators of electricity.

                   CONDUCTORS                             INSULATORS
             All                   Iron                       Rubber
                                  Gold                        Plastic
          Metals
                                  Copper                      Cloth
                        Water                                 Paper
                     Human Body                               Wood




5.5 Earthing :


The human body is a good conductor of charge. If you touch a charged
conductor with your hand, the conductor loses its charge through you
connecting it to earth. This is called earthing.




                                                                                  3
5.6 The Lightning Conductor:

Lightning is produced in thunderstorms when liquid
and ice particles above the freezing level collide,
and build up large electrical fields in the clouds.
Once these electric fields become large enough, a
giant "spark" occurs between them, like static
electricity, reducing the charge separation. The
lightning spark can occur between clouds, between
the cloud and air, or between the cloud and ground. The temperature inside a
lightning bolt can reach 27760oC, hotter than the surface
of the sun.



When a negatively charged cloud passes over a lightning
conductor, it induces a positive charge on the point at the
top of the lightning conductor. The point then repels
positive ions to the cloud to neutralize it, so that it is less
likely to produce a lightning flash.




                                                                           4
5.7 Circuits:


                                            The diagram shows a bulb connected to
                                            a battery. When the switch is closed,
                                            current flows through the circuit, and
                                            the bulb lights up.




WHAT IS A CURRENT?

A current is simply a flow of electrons. When the switch is closed (closed
circuit), electrons flow in the metal wire as shown in the diagram below:


                              Note that a conventional current flows from the
                              positive to the negative end of the battery. A
                              current flowing in one direction is called a direct
                              current (d.c.). On the other hand an alternating
                              current (a.c.) changes direction constantly.



HOW DOES A CURRENT FLOW THROUGH THE CIRCUIT?

For a current to flow through a circuit, we need something to ‘push’ the
electrons around [a source of energy]. In the lab, we usually use a battery as
our source of energy.

                              A battery (cell) is designed such that:
                                   it      maintains   EXTRA    electrons   at    its
                                      negative end (case)
                                     the    positive   end   (rod)   is   short   of
                                      electrons.
                              So there is a difference in charge between the
                              ends of a battery, and this difference makes
                              electrons flow. In fact electrons (-) are pushed
                              out of the negative pole of the cell and are
                              drifting around the circuit from atom to atom in
                              the wire, to the positive pole of the cell.


                                                                                    5
HOW DOES A CURRENT LIGHT A BULB?




      Chemical reaction in the battery produces free electrons which are pushed out
       by the emf:   [chemical energy  electrical energy].
      As electrons move in the bulb, the electrons give up their energy to the bulb
       according to the pd: [electrical energy  light and heat energy].
      Electrons return to the battery to get reloaded.



5.8 Series and parallel circuits:

There are two ways in which you can connect two lamps in a circuit.

                                       battery




                                           bulb



IN SERIES:                                       IN PARALLEL :

The lamps in the diagram above are               The lamps in the diagram above are
connected in series. Note that the               connected in parallel. This allows one
lamps can only be lit up together.               lamp to be on while the other is off.



                                                                                      6
5.9 Current :

       As mentioned previously, a current is the flow of electrons around a
circuit. We measure electric current in amperes or amps. A current of one
ampere (1A) is a flow of charge of one coulomb (1C) per second. We can find the
current flowing in a circuit using:

                        Q            where I = current (Amperes)
                 I =
                                          Q = charge (Coulombs)
                        t
                                           t = time (seconds)

In practice we can find the current flowing in a circuit using an ammeter.



                                      Note that:

                                      1. ammeters are connected in series.
                                      2. same result would have been obtained if
                                         the ammeter was connected on the
                                         other side of the lamp.




5.10 Voltage :

       As we have seen, a battery is needed to drive the electrons around a
circuit. The difference in charge between the ends of the battery is called
potential difference (p.d.). This p.d. provides the force with which the electrons
are pushed out, and is called the electromotive force (e.m.f.).

      The force with which the electrons are pushed out varies according to
the strength of the battery, or its voltage. Voltage is the energy carried by the
flowing charge. We can find the voltage using:

                  E              where V = voltage (Volts)
         V=
                  Q                    E = energy (Joules)
                                      Q = charge (Coulombs)

        Also :         E=IxVxt

In practice we can find the voltage in a circuit using a voltmeter.

                                                                                 7
                                        Note that when we need to find the p.d.

                                       across    a   component,   we   connect   the

                                       voltmeter in parallel to that particular

                                       component.



5.11 Resistance :

      Certain conductors allow flow of electricity better than others.
Therefore electricity flows better in some conductors than in others. This
means that some conductors oppose the flow of electrons. The opposition of a
conductor to current flow is called resistance.


Resistors:

   Resistors are used in circuits to oppose the flow of current. They allow us to
control the flow of electricity in any part of the circuit. They can be either of:

 1. a fixed value (called fixed resistors)

 2. a variable value (called variable resistors or rheostats)

The resistance of a wire increases with length and
decreases with area. The resistance also depends on
what type of material is being used. We measure the
resistance of a resistor in Ohms ().

Note that current passes from paths of least
resistance. This means that a current ‘chooses’ to
pass from where there is least resistance.



   This is why:

   ~ Ammeters should have a low resistance. They should count the flowing
     charge without stopping any of it.

   ~ Voltmeters should have a high resistance. This is because they should
     not take any current from the circuit.
                                                                                   8
5.12 Series vs Parallel Circuits :


                         SERIES                             PARALLEL
Current:      The same current flows through Current in the main circuit is the
              any part of the circuit.       sum of the currents in the
              [Ammeters give same readings]  separate branches.




                I1                       I2            I
                                                                   I1

                                                                   I2

                             I1 = I 2                           I = I1+I2
Voltage:      Total p.d. is the sum of the p.d.’s P.d. across each component is
              across each component.              equal.


                         V                                              V


                                                                    V1


                        V1        V2
                                                                    V2

                        V = V1 + V 2                          V = V 1 = V2
Resistance:   The total resistance is the sum of Resistors in parallel have a
              individual resistors.              smaller total resistance than
                                                 either of the separate resistors.



                                                                         R1
                          R1        R2                        R2

                                                           Total resistance RT
                 Total resistance = R1 + R2                1/RT = 1/R1 + 1/R2




                                                                                 9
5.13 Ohm’s Law :


        +             A single cell is connected in series with a resistor and an
                      ammeter and the current is noted.
                      Consequently, two and then three cells are connected
+
    A                 across the circuit and in each case the current is noted
                      down.

It is found that:
~ when the p.d. is doubled, the current doubles.
~ when the p.d. is three times as much, the current is
   three times as much.


A graph of V against I is then plotted. The straight
line graph shows that the two quantities are
proportional to one another.


Ohm’s Law:      The current flowing through the metal wire is proportional to
                the p.d. across it {provided the temperature remains constant}.

The gradient of the graph is everywhere the same and gives the value of the
resistance.
In general:
                           V
                 R=                    or V = I x R
                           I
Non-Ohmic conductors:

Not all conductors obey Ohm’s Law. Some of them, like a filament lamp, get
hotter when more current flows in and the value of its resistance increases.
Thus the voltage is no longer proportional to the current. Conductors which do
not obey Ohm’s Law are called non-Ohmic conductors.


                               In the case of non-Ohmic conductors, the voltage
                               current graph is not a straight line.




                                                                               10
5.14   Light Dependant Resistor (LDR):

      The resistance of certain semi-conducting materials
depends on the intensity of incident light energy. When these
materials have light shining on them, their conductivity changes
(the resistance changes).


Circuit Symbol for an LDR:




Name two applications for an LDR:     1) Night lamp.

                                      2) Street lights.


5.15 Thermistors:

       The conductivity of certain materials changes when they are heated. This
means that their resistance changes with temperature. These materials are
called thermistors (temperature dependant resistors).


Circuit Symbol for a thermistor:




Name two applications for a thermistor: 1) digital thermometers.

                                            2) automotive.



Q: Look at the voltage current graph for a thermistor shown below. Is a thermistor an
  ohmic conductor. Why?


                                  Since the resistance of a thermistor changes
                                  with temperature, the temperature is not
                                  constant and therefore a thermistor does not
                                  obey Ohm’s Law (graph is not straight line).




                                                                                   11
5.16 Diodes:

       A semi-conductor is neither a good conductor nor
an insulator. These materials, such as silicon and
germanium are used in electronic devices. These can be
made negatively charged (called n-type semiconductor) or
positively charged (called p-type semiconductor).

      Sometimes these n-type or p-type materials are joined
together to produce a p-n junction diode.




These diodes allow current to flow only one way (in one direction).




Figure a shows the diode connected in forward biased. In this case the diode
allows the current to pass and the bulb will light up.
What can you note about the direction of the diode and that of the current?
Same direction

Figure b shows the diode connected in reverse biased. In this case the current
is nearly zero and the lamp will not light up.
What can you note about the direction of the diode and that of the current?
Opposite direction



Exercise: Look carefully at the V-I graph for the diode and explain what is
            happening.



                                  The left part of the graph shows the diode in

                                  reverse biased (current is nearly zero), while

                                  the right part shows the diode in forward

                                  bias.

                                                                              12
5.17   Light Emitting Diodes (LED) :

       Some special diodes are called LEDs because they give out light when a
current passes through them. The most common color is red, but it is possible to
find yellow, blue, white or green LEDs [mostly used in computers, appliances and
bulbs].

Circuit Symbol for an LED:




LEDs offer a lot of advantages over filament bulbs, mainly that they are small in
size, consume little energy, since they do not heat up and last much longer.




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