2. CHAPTER 37

3. Static Electricity
Positive and negative charges
There are two kinds of
electric charge. Positive
charge (+) and negative
charge (-). Like charges (+
and + or – and – ) repel,
whereas unlike charges (+
and -) attract. The force
between them decreases
when the distance
increases.

4. Charges, atoms and electrons
Small central nucleus which contains
positively charged particles called protons,
are surrounded by an equal number of
negatively charged called electrons. A
electrically neutral atom has the same
number of protons and electrons. Every
nucleus, except hydrogen, contains
uncharged particles called neutrons.

5. Electrons, insulators and conductors
An electroscope is a device to determine or
measure the presence of electrostatic charges. In
an insulator all electrons are bound firmly to
their atoms and in a conductor some electrons
can move freely from atom to atom.
Good insulators are plactics such as polyetene,
cellulose acetate, Perspex and nylon. All metals
and carbon are good conductors. Wood, paper,
cotton, the human body and the Earth are poor
conductors and insulators.

6. Electrostatic Induction
Electrostatic induction is a
quick way of using a
charged object to give
something a charge, of the
opposite sign, without
losing any of the original
charge. The attraction of
an uncharged object by
object near it is due to
electrostatic induction.

7. Dangers of static electricity
Lightning
Refuelling
Opering Theatres
Computers
Uses of static electricity
Flue-ash precipitation
Photocopiers
Inkjet Printers

8. Van de Graaff generator
The basic use of
a Van de Graaf
generator is to
separate electric
charges and build
them up,
frequently for
demonstration
purposes
A Van de Graaff generator is
an electrostatic generator which uses a
moving belt to accumulate very high
voltages on a hollow metal globe on the top
of the stand.

9. Electric Fields
It is the region of space where an electric charge
experiences a force due to other charges.

10. CHAPTER 38

11. Electric Current
 Electric current is a flow of electric
charge through a medium
 An electric current has three effects
that reveal its existence :
 Heatingand lightning
 Magnetic
 Chemical

12. • The ampere and the coulomb
▫ Current is measured in ampere (A). One
milliampere (mA) is one-thousandth of an
ampere. Ammeter is the measuring
instrument of current.
▫ The unit of charge, the coulomb (C), is
defined in terms of the ampere. 1C = 1A s.

13. • Circuit diagrams
 Current must have
a complete path or
circuit of
conductors if it is
to flow. These
symbols are the
commonly-used
ones in a circuit.

14.  Series and parallel circuits
 Series
 In a series circuit, the current is the same at all points in a circuit but it
has different numbers of potential different.
 Parallel
 In a parallel circuit, the potential difference is the same at all points in
a circuit but it has different numbers of current.

15. CHAPTER 39

16.  The battery is said to have a potential
difference (p. d.) at its terminals.
Potential difference is measured in
volts (V) and the term voltage is
sometimes used instead of p.d.
 Its defined by V = I x R

17.  Energy transfers and p.d.
 Energy in a circuit is supplied from a source such as a
battery and is transferred to other forms of energy by
devices in the circuit.
 The volt
 The p.d. between two points in a circuit is 1 volt of 1
jould of electrical enegry is transferred to other forms of
energy when 1 coulomb passes from one point to
another.

18. ▫ Cells, batteris and e.m.f.
A battery consists of two or more electric cells. Greater
voltages are obtained when cellls are joined in series.
When no current is drwn from a battery it is said to be an
open circuit and its terminal p.d. is a maximum. The
maximum voltage is called the electrmotive force (e.m.f)
▫ Voltages round a circuit
These are how to calculate the total of V in a circuit;
 Series : V= V1+V2+V3
 Parallel : V1=V2

19. CHAPTER 40

20.  The opposition of a conductor to current is
called its resistance. A good conductor has a
low resistance, a poor conductor has a high
resistance.
 It increases as its length increases, as its
cross-section area decreaes, and it depens
on the material. Silver and copper are the
best conductors.

21.  The Ohm
 The resistance is defined by R = V.I and it is
measured in Ohms (Ω).
 The ohm is the resistance of a conductor in
which the current is 1 when a voltage of 1
volt is applied to it.

22.  Resistors
 Conductors intended to have resistance are called
resistors. They arre usually made from wires of special
alloys or carbon.
 Rheostat is a variable resistor, used for adjusting the
current, can also work as a potential divider to adjust
the p.d. in a circuit.

23. These are how to calculate the total of
resistance in a series;
-Series
R = R1+R2+R3
-Parallel
1/R = 1/R1+1/R2+1/R3

24. The resistor colour code

25. • Resistivity
The resistivity of a material is numerically
equal to the resistance of a 1m length of
it of corss-section area 1m².

26. CHAPTER 41

27. Capacitors
 A capacitor stores electric charge and its usefu in
many electronic circuits. The insulator called the
dielectris is sandwiched between two parallel metal
plates.

28.  A capacitor can blok d.c. and passes a.c.
 Capacitance
The more charge a capacitor can store, the
greater is its capacitance (C). It is measured
in farads but smaller units like microfarad are
more convinient.

29.  Types of capacitor
 Practical Capacitor
 Plastics are the commonly-used material for this
kind of capacitor, with films of metal being
deposited on the plastic to act as the plates.
 Electrolytic capacitor
 It has a very thin layer of aluminium oxide as the
dielectric between two strips of aluminium foul,
giving large capacities.

30. CHAPTER 42

31. Electric power
 Energy transfers were measured by the work
done and power was defined by the equation;
power = work done/time taken = energy
taken/time taken
 Lamp and motor can be used to measure the
electric power in a circuit.
 Instead of using ammeter and a volmeter to
measure the electrical energy transferred by an
appliance, a joulemeter can be used to obtain it
directly in joules.

32. • Electric lightning
▫ Filament lamps
• Compact fluorescent lamps
▫ Fluorescent strips

33.  Electric heating
› Heating elements
• Fuses
› Three-heat switch

34. CHAPTER 43

35.  These are some aspects that are important
to be concerned for electricity in the home;
circuits in parallel, switches and fuses,
staircase circuit, ring main circuit, fused
plug, earthing and safety, circuit breakers
and double insulation.

37. • Paying for electricity
– A killowat-hour is the electrical energy used by a 1
kW appliance in 1 hour.
• Dangers of electricity
– Electric shock
– Fire risks

38. THANK YOU
FOR
STOPPING BY!
Use electricity wisely! : )