Lesson 19 - current electricity
The most significant difference between the static and current electricity is that
in static electricity the charges are at rest and they are accumulating on
the surface of the insulator. Whereas in current electricity the electrons are
moving inside the conductor. The other differences between the static and
current electricity are explained below in the comparison chart.
Electricity basically has two forms, static electricity and current electricity.
Basis for
Comparison
Static Electricity
Current Electricity
Definition
The electricity which is build up on the
surface of the substance is known as the
static electricity.
The current electricity is
because of the flow of
electrons.
Causes
It induces because of the movement of
the negative charges from one object to
another
The current electricity is
because of the movement of
the electrons.
Material
The static electricity develops both in the
conductor and insulator.
The current electricity
develops only in conductor.
Measuring
Device
Gold leaf electroscope
Analog and digital meter.
Examples
Lightning strokes, it develop by rubbing
the balloons on hair, etc.
The current electricity is used
for running the fan, light, T.V
etc.
Static electricity :
When two insulating materials are rubbed together, electrons are transferred from
one object to the other. One object which loses electrons becomes positive and the
other object which gains negative. A non-contact force exists between charged
objects.
Charging by friction
When insulating materials rub against each other, they may become
electrically charged. Electrons, which are negatively charged, may be ‘rubbed off’
one material and on to the other. The material that gains electrons becomes
negatively charged. The material that loses electrons is left with a positive charge.
Repulsion and attraction
Two charged objects will:

repel each other if they have like charges (they are both positive or both negative)

attract each other if they have opposite charges (one is positive and the other is
negative)
Charging by Induction

When a charged object is held close to a conductor, electrons in the conductor are able to move
towards (or away from) the charged object:

In the diagram above, electrons in the aluminium foil are attracted to the
positively charged rod
This causes the top of the foil to become negatively charged, whilst the bottom
edge of it will be left with a positive charge
The attraction between the positive rod and the negative charges on the top
surface of the foil will cause the foil to be attracted to the rod


Static electricity is detected by gold leaf electroscope.
Current electricity
An electric current is a flow of charge, and in a wire this will be a flow of electrons.
Electric current
An electric current is a flow of charge, and in a wire this will be a flow of electrons. We
need two things for an electric current to flow:
1. something to transfer energy to the electrons, such as a battery or power pack
2. a complete path for the electrons to flow through (an electric circuit)
Electricity flowing through conductors
Materials that allow a current of electrons to pass easily through it are known as
conductors. All metals conduct electricity easily. All metals such as copper, aluminum
and iron are electric conductors. The electrons in the outermost shell of metallic
atoms can be easily detached from the atom. A large number of such detached electrons from the
outermost shell of metal atoms are in random motion. The reason is a current through conductors
, is the free electrons.
The negative terminal of a cell has the ability to repel electrons. Its positive terminal
has the ability to attract electrons.
The SI unit used to measure the electric current is known as the Ampere (A) and the
instrument used to measure electric current is known as the ammeter.
Connect the ammeter series to the circuit in such a way that the entire current passing
through the conductor passes through the ammeter as well.
Potential difference and the electromotive force
The current through a component depends on both the resistance of
the component and the potential difference across the component.
Potential difference is a measure of how much energy is transferred between
two points in a circuit.
To measure the potential difference across a component, a voltmeter must be
placed in parallel with that component in order to measure the difference in energy
from one side of the component to the other. Potential difference is also known
as voltage and is measured in volts (V).
When an electric current is drawn from a cell, the current also passes through the
cell itself. The cell too has an electric resistance. Then a potential difference arises
across the resistance of the cell. When this potential difference is subtracted from
the electromotive force of the cell, the potential difference that provides an electric
current to the external circuit can be obtained.
What is Electromotive Force?


The Electromotive Force (EMF) is the name given to the Potential
Difference (Voltage) of the power source in a circuit
The Electromotive Force (EMF) is measured in Volts (V)
EMF = IR + ir
Electromotive force = potential difference of the external circuit + potential
difference arise across the resistance of the cell
If a graph is plotted using your data, with the voltage difference in the y axis and the
current in x axis it will take the form shown by the following graph,
Resistance
An electric current flows when electrons move through a conductor, such as a metal
wire. The moving electrons can collide with the ions in the metal. This makes it more
difficult for the current to flow, and causes resistance.
There are two main circuit symbols used for resistors.

Resistance is the opposition to current
o For a given potential difference:
The higher the resistance, the lower the current

Potential difference, current and resistance are related by the following equation:
By changing the current through the circuit using the rheostat, obtain readings for
the potential difference across the nichrome coil and the current to calculate the
resistance of the nichrome wire.
Different types of resistors

Resistors are used in virtually all electronic circuits and many electrical ones. Resistors,
as their name indicates resist the flow of electricity, and this function is key to the
operation most circuits.


Resistors are one of the most widely used components in electronic circuits - there are
many different types of resistor available having different properties and used in
different ways in different circuits.
The first major categories into which the different types of resistor can be fitted is into whether
they are fixed or variable. These different resistor types are used for different applications:
Fixed value resistors:

Fixed resistors are by far the most widely used type of resistor. They are used in electronics
circuits to set the right conditions in a circuit with a fixed resistance. . Their values are
determined during the design phase of the circuit, and they should never need to be
changed to "adjust" the circuit.


By increasing the resistance it decreases the current through a circuit.
The colour code of the resistors: Often, the value of a resistor is indicated in coded
form by colour bands marked on its body. The coding system of marking the resistor value using
colored bands is known as the colour code method.
The relationship with current and voltage is follows.
For a fixed resistor, the potential difference is directly proportional to the current.
Doubling the amount of energy into the resistor results in a current twice as fast
through the resistor. This relationship is called Ohm's Law and is true because
the resistance of the resistor is fixed and does not change. A resistor is an ohmic
conductor.
Variable resistors:
These resistors consist of a fixed resistor element and a slider
which taps onto the main resistor element. This gives three connections to the component:
two connected to the fixed element, and the third is the slider. In this way the component
acts as a variable potential divider if all three connections are used. It is possible to connect
to the slider and one end to provide a resistor with variable resistance.
A variable resistor has its resistance changed by altering the length of material
within the resistor. The further the electrons have to travel through the material, the
more opportunities there are for collisions between the electrons and the atoms/ions
within the material.
Light-dependent resistor (LDR)
The resistance of a LDR depends on light intensity. At low light levels, the LDR has a
high resistance. As the light intensity increases, the resistance decreases. LDRs are
made of materials (semiconductors) which have electrons knocked out of orbitals when
photons of light hit them. In the light, there are more electrons that are free to move
around the material, making it easier for current to flow, reducing resistance.
Applications of LDR
Light-dependent resistors are simple and low-cost devices. ...
These resistors are used as light sensors and the applications of LDR mainly
include alarm clocks, street lights, light intensity meters, burglar alarm circuits.
Series and parallel circuits
There are two types of circuit we can make, called series and parallel.
The components in a circuit are joined by wires.
If there are no branches then it's a series circuit.
If there are branches it's a parallel circuit.
Series circuits
If you follow the circuit diagram from one side of the cell to the other, you should pass
through all the different components, one after the other, without any branches.
If you put more lamps into a series circuit, the lamps will be dimmer than before
because the voltage is shared.
In a series circuit, if a lamp breaks or a component is disconnected, the circuit is broken
and all the components stop working.
Series circuits are useful if you want a warning that one of the components in the
circuit has failed. They also use less wiring than parallel circuits.
Current in series circuits
The current in a series circuit is the same at all places in the circuit.
Voltage across components in a series circuit

The supply voltage is shared between components in a series circuit.

The sum of the voltages across components in series is equal to the voltage of the supply.

The voltages across each of the components in series is in the same proportion as
their resistances. This means that if two identical components are connected in series, the
supply voltage divides equally across them.
Resistors in series
When resistors are connected in series, the current through each resistor is the same.
In other words, the current is the same at all points in a series circuit.
When resistors are connected in series, the total voltage (or potential difference) across
all the resistors is equal to the sum of the voltages across each resistor.
In other words, the voltages around the circuit add up to the voltage of the supply.
The total resistance of a number of resistors in series is equal to the sum of all the
individual resistances.
In this circuit the following applies.
I1 = I 2 = I 3
VT = V 1 + V 2 + V 3
and, RT = R1 + R2 + R3
Parallel circuits
In parallel circuits different components are connected on different branches of the wire.
If you follow the circuit diagram from one side of the cell to the other, you can only pass
through all the different components if you follow all the branches.

In a parallel circuit, if a lamp breaks or a component is disconnected from one
parallel wire, the components on different branches keep working. And,
unlike a series circuit, the lamps stay bright if you add more lamps in parallel.
 Parallel circuits are useful if you want components to continue to work, even if
one component has failed. This is why our homes are wired up with parallel
circuits.
Current in parallel circuits
The current in a parallel circuit splits into different branches then combines again
before it goes back into the supply. When the current splits, the current in each
branch after the split adds up to the same as the current just before the split.
Answer
Voltage across components in a parallel circuit
The voltage across components in parallel is the same for each component.
Question
Look at the par
circuit below
allel
Answer
Resistors in parallel
When resistors are connected in parallel, the supply current is equal to the sum of the
currents through each resistor. The currents in the branches of a parallel circuit add up
to the supply current.
When resistors are connected in parallel, they have the same potential difference
across them. Any components in parallel have the same potential difference across
them.
In order to calculate the total resistance of two resistors connected in parallel, this
equation is used.
Difference Between Series and Parallel Circuits
Series
Parallel
The same amount of current flows through all the
components
The current flowing through each component
combines to form the current flow through the
source.
In an electrical circuit, components are arranged in
a line
In an electrical circuit, components are
arranged parallel to each other
When resistors are put in a series circuit, the
voltage across each resistor is different even
though the current flow is the same through all of
them.
When resistors are put in a parallel circuit, the
voltage across each of the resistors is the
same. And even the polarities are the same
If one component breaks down, the whole circuit
will burn out.
Other components will function even if one
component breaks down, each has its own
independent circuit
If Vt is the total voltage then it is equal to
V= V1+V2+V3
If Vt is the total voltage then it is equal to
V1=V2=V3