All about electricity

1. Electricity
Cambridge
IGCSE

2. Static Electricity
• Def: a stationary electric charge, typically
produced by friction, that causes sparks or
crackling or the attraction of dust or hair.

3. • Triboelectric effect
The triboelectric effect is a simple process in
which an object becomes electrically charged by
rubbing against another object.

4. Insulators & Conductors
• Conductors
Materials that permit electrons to flow freely
from particle to particle.
Eg. metals, aqueous solutions of salts, graphite,
and the human body

5. Insulators & Conductors
• Insulators
Materials that impede the free flow of electrons
from atom to atom and molecule to molecule.
Eg. plastics, Styrofoam, paper, rubber, glass
and dry air.

6. Electrostatic induction
• Modification in the distribution of electric
charge on one material under the influence of
nearby objects that have electric charge.
• Advantages to charging something by
induction.
– The originally charged object never loses any
charge so it need not be recharged.
– The induced charge can be quite strong and
subsequent charges will be equally strong

7. Comparison of charging by Conduction
& charging by Induction.
Charged object touches the electroscope.
Charged object does not touch the
electroscope.
Electroscope ends up similarly charged to
the object used to charge it.
Electroscope ends up oppositely charged to
the object used to charge it.
The first charge is strong but gets weaker
each time the electroscope is
recharged. (This is due to the original
object giving up some charge every time it
is touched.)
The first charge is strong and stays strong
each time the electroscope is
recharged. (This is due to the original
object not losing any charge in the
process.)

8. Electric current
• Def: An electric current is a flow of electric
charge. In electric circuits this charge is often
carried by moving electrons in a wire. It can
also be carried by ions in an electrolyte, or by
both ions and electrons such as in a plasma.

9. Ampere
• Ampere is the unit of current, which is defined
by using the magnetic effect.
• Measured by a Ammeter
• SI unit : A
• Symbol: I
• It is named after André-Marie Ampère (1775–
1836), French mathematician and physicist,
considered the father of electrodynamics.

10. Coulomb
• Coulomb is equal to the
quantity of charge transferred
in one second across a
conductor in which there is a
constant current of one ampere.
• SI unit : C
• Symbol : Q
• Named for French chemist Charles-Augustin de
Coulomb (1736-1806), who devised a method of
measuring electrical quantity
• Q = I x t

12. Series Circuits
• In a series circuit, the current through each of
the components is the same, and the voltage
across the circuit is the sum of the voltages
across each component.

13. Parallel Circuits
• In a parallel circuit, each device is placed in its
own separate branch.

14. Types of Current
• Direct current (DC) which is a constant stream
of charges in one direction.
• Alternating current (AC) that is a stream of
charges that reverses direction.

15. Direct Current
• The current in the circuits is moving in a
constant direction.
• Refined by Thomas Edison in the 1800s.

16. Alternating current
• Alternating current is an electric current in
which the flow of electric charge periodically
reverses direction

17. Cells, batteries & e.m.f
• Electrical cell is a device that is used to generate
electricity, or one that is used to make chemical
reactions possible by applying electricity.
• Electric battery is a device consisting of one or
more electrochemical cells with external
connections provided to power electrical devices.
• A battery has positive terminal, or cathode, and
a negative terminal, or anode

18. Cells, batteries & e.m.f
• Electromotive force, (denoted and measured
in volt), is the voltage developed by any
source of electrical energy such as a battery or
dynamo. It is generally defined as the
electrical potential for a source in a circuit.

19. Frequency
• The number of periods or regularly occurring
events of any given kind in unit of time,
usually in one second.
• SI unit : Hz
• Symbol : F

20. Potential difference
• A potential difference, also called voltage,
across an electrical component is needed to
make a current flow through it.
• Cells or batteries often provide the potential
difference needed.

21. Calculate the potential difference
• The potential difference between two points
in an electric circuit is the work done when a
coulomb of charge passes between the points.
• V = W ÷ Q
– V is the potential difference in volts, V
– W is the work done (energy transferred) in joules,
J
– Q is the charge in coulombs, C

22. Resistance
• An electron traveling through the wires and loads
of the external circuit encounters resistance.
• Resistance is the hindrance to the flow of charge.
For an electron, the journey from terminal to
terminal is not a direct route.
• While the electric potential difference established
between the two terminals encourages the
movement of charge, it is resistance that
discourages it.

23. Ohm’s law
• The electric potential difference between two
points on a circuit (ΔV) is equivalent to the
product of the current between those two
points (I) and the total resistance of all
electrical devices present between those two
points (R).

25. Resistors
• A resistor is a passive two-terminal electrical
component that implements electrical
resistance as a circuit element. Resistors may
be used to reduce current flow, and, at the
same time, may act to lower voltage levels
within circuits.

26. Resistor in series
• Resistors can be connected in series; that is,
the current flows through them one after
another.

27. Resistors in Parallel
• Resistors can be connected such that they
branch out from a single point (known as a
node), and join up again somewhere else in
the ciruit.

28. Colour coded Resistors

29. Capacitors
• A capacitor (originally known as a condenser)
is a passive two-terminal electrical component
used to store electrical energy temporarily in
an electric field.

30. Charging & Discharging a capacitor
• A Capacitor is a passive device that stores
energy in its Electric Field and returns energy
to the circuit whenever required. A Capacitor
consists of two Conducting Plates separated
by an Insulating Material or Dielectric.

31. Charging
• As soon as the switch is closed in position 1 the battery
is connected across the capacitor, current flows and
the potential difference across the capacitor begins to
rise but, as more and more charge builds up on the
capacitor plates, the current and the rate of rise of
potential difference both fall. Finally no further current
will flow when the p.d. across the capacitor equals that
of the supply voltage Vo.
The capacitor is then fully charged.

32. Discharging
• As soon as the switch is put in position 2 a 'large' current starts to
flow and the potential difference across the capacitor drops. As
charge flows from one plate to the other through the resistor the
charge is neutralised and so the current falls and the rate of
decrease of potential difference also falls.
Eventually the charge on the plates is zero and the current and
potential difference are also zero - the capacitor is fully discharged.
Note that the value of the resistor does not affect the final potential
difference across the capacitor – only the time that it takes to reach
that value.
The bigger the resistor the longer the time taken.

33. Magnetic fields
Properties of Magnetic Lines
• They seek the path of least resistance between
opposite magnetic poles. In a single bar magnet
as shown to the right, they attempt to form
closed loops from pole to pole.
• They never cross one another.
• They all have the same strength.
• Their density decreases (they spread out) when
they move from an area of higher permeability
to an area of lower permeability.

34. Properties of Magnetic Lines cont.
• Their density decreases with increasing
distance from the poles.
• They are considered to have direction as if
flowing, though no actual movement occurs.
• They flow from the south pole to the north
pole within a material and north pole to south
pole in air.

35. Magnetic fields caused by electric
currents
• The magnetic field around a straight wire
consists of ‘concentric circles’ (circles around
the same centre). These are at right angles to
the direction in which the electric current
flows.
• A solenoid consists of a long piece of wire
made into several coils. Its magnetic field is
the same as the magnetic field produced by a
simple bar magnet.

36. Maxwell’s Right Hand rule
• The nature of magnetic field around a
straight current carrying conductor is like
concentric circles having their center at the
axis of the conductor. The direction of these
circular magnetic lines is dependent upon the
direction of current.

37. Electromagnets
• An electromagnet is a type of magnet in
which the magnetic field is produced by an
electric current. The magnetic field disappears
when the current is turned off.

38. Electric motors
• Electric motors involve rotating coils of wire
which are driven by the magnetic force
exerted by a magnetic field on an electric
current. They transform electrical energy into
mechanical energy.

39. Fleming’s left hand rule

40. Generators
• Generators induce a current by spinning a coil
of wire inside a magnetic field, or by spinning
a magnet inside a coil of wire.

41. Electromagnetic induction
• A bar magnet is kept stationary while the coil is
moved back and forth within the magnetic field;
an electric current would be induced in the coil.
• Then by either moving the wire or changing the
magnetic field we can induce a voltage and
current within the coil and this process is known
as Electromagnetic Induction
• Is the basic principal of operation of
transformers, motors and generators.

42. Faraday’s law
• Any change in the magnetic environment of a
coil of wire will cause a voltage (emf) to be
"induced" in the coil. No matter how the
change is produced, the voltage will be
generated.

43. Lenz’s Law
• A principle stating that an electric current,
induced by a source such as a changing
magnetic field, always creates a counterforce
opposing the force inducing it. This law
explains such phenomena as diamagnetism
and the electrical properties of inductors.

44. Fleming’s right hand rule
(Dynamo rule)

45. Transformers
• Alternating current is passed through the
primary coil (the input)
which creates a changing magnetic field in the
iron core.
The changing magnetic field then induces
alternating current
of the same frequency in the secondary coil
(the output).

46. Transformers
• A step up transformer has more turns of wire
on the secondary coil, which makes a larger
induced voltage in the secondary coil.
• It is called a step up transformer because the
output voltage
is larger than the input voltage.

48. References
• http://study.com/academy/lesson/what-is-
static-electricity-definition-causes-uses.html
• https://learn.sparkfun.com/tutorials/what-is-
electricity
• http://science.howstuffworks.com/electricity.
htm
• hyperphysics.phy-
astr.gsu.edu/hbase/magnetic/magfie.html

49. References
• http://www.physics4kids.com/files/elec_magn
eticfield.html
• http://global.britannica.com/science/magneti
c-field
• https://www.khanacademy.org/science/physic
s/magnetic-forces-and-magnetic-fields