WEEK TWO
SUBJECT: BASIC SCIENCE
CLASS: GRADE 9
SECOND: TERM
TOPIC: ELECTRICAL ENERGY
FLOW OF ELECTRONS:
When current flows in a wire, what actually moves are the electrons. Electrons
have negative charges. When a current is produced by the movement of free
electrons toward a positive terminal; the direction of electron flow is
opposite to that of that of the current. The electric current flows from the
positive terminal to the negative terminal. Electric current is a flow of
electric charge through a medium. This charge is typically carried by moving
electrons in a conductor such as a wire. It can also be carried by ions in an
electrolyte.
The electrons of different types of atoms
have different degrees of freedom to move around. With some types of materials,
such as metals, the outmost electrons in the atoms are so loosely bound that
they chaotically move in the space between the atoms of that material by
nothing more than the influence of room-temperature heat energy. Because these
virtually unbound electrons are free to leave their respective atoms and float
around in the space between adjacent atoms, they are often called free electrons. In other word types of
materials such as glass, the atoms and electrons have very little freedom to
move around. While external forces such as physical rubbing can force some of
these electrons to leave their respective atoms and transfer to the atoms of
another material, they do not move between atoms within that material very
easily. This relative mobility of electrons within a material is known as electric conductivity. Conductivity is
determined by the types of atoms in a material (the number of protons in each
atom’s nucleus, determining its chemical identity) and how the atoms are linked
together with one another. Materials with high electron mobility (many free
electrons) are called conductors, while
materials with low electrons mobility (few or no free electrons) are called insulators. A few common examples of
conductors are silver, copper, gold, aluminum, iron, steel, brass, bronze,
mercury, graphite, dirty water, concrete, etc. Some examples of insulators are
glass, rubber, oil, fiberglass, porcelain, dry cotton, dry paper, dry wood,
plastic, air, diamond, pure water, etc.
All conductive materials do not have the same
level of conductivity, and not all insulators are equally resistant to electron
motion. For instance, silver is the best conductor in the “conductors” list,
offering easier passage for electrons than any other material cited. Dirty
water and concrete are also listed as conductors, but these materials are
substantially less conductive than any metal. It should also be understood that
some materials experience changes in their electrical properties under
different conditions. Glass, for instance, is a very good insulator at room
temperature, but becomes a conductor when heated to a very high temperature.
Most metals become poorer conductors when heated, and better conductors when
cooled. Many conductive materials become perfectly conductive (this is called
superconductivity) at extremely low temperatures.
The uniform motion of electrons is called electricity, or electric current. To
facilitate the flow of electrons, the wires are made of highly conductive
metals such as copper or aluminum. There can be electric current only where
there exists a continuous path of conductive material providing a conduit for
electrons to travel through.
Electron Source Electron
Destination
However, the flow will be interrupted if the
conductive path formed by the wire is broken:
Electron
no flow! no flow! Electron
Source (break) Destination
If we were to take another piece of wire
leading to the destination and simply make physical contact with the wire
leading to the source, we would once again have a continuous path for electrons
to flow. The two dots in the diagram indicate physical (metal-to-metal) contact
between the wire pieces:
Electron no flow! Electron
Source ( break ) Destination
CIRCUITS AND TYPES:
Electricity is defined by three major attributes: Voltage, current and
resistance. Voltage is the “pressure” that makes an electric charge to
move. Current is the charges “flow”- the rate which the charges moves through
the conductor, measured at any particular point. The conductor offers a certain
amount of resistance to this flow, which varies depending on the conductor’s
composition and size.
Voltage, current and resistance are all
interrelated- you can’t change one without changing another. According to Ohm’s Law, the current is equal to
voltage divided by resistance (commonly written as I=V/R). It means that if you increase the pressure working on an
electric charge or decrease the resistance, more charge will flow. If you
decrease pressure or increase resistance, less charge will flow.
In many practical cases, it is necessary to
increase or decrease the current in an electric circuit. A component used to
regulate current without changing the voltage source is called variable resistance. In an electric
circuit, a device called rheostat is
often used to change the resistance in the circuit. The current through a
conductor depends upon its resistance and the potential difference across its
ends. In various electrical gadgets, we often use resistors in various
combinations. Ohm’s law is applied for the combination of resistors.
Resistors
in Series- When in an electric circuit, the resistors are joined end to
end, they are said to be connected in series.
You will observe that the value of current in
the ammeter is the same, independent of its position in the electric circuit.
It means that in a series combination of resistors, the current is the same in
every part of the circuit or the same current through each resistor.
Resistors
in Parallel: When in an electric circuit, the resistors are joined together
between two points, they are said to be connected in parallel.
We have seen that in a series circuit, the
current is constant throughout the electric circuit. Thus, it is obviously
impracticable to connect an electric bulb and an electric heater in series,
because they need currents of widely different values to operate properly.
Another major disadvantage of a series circuit is that when one component
fails, the circuit is broken and none of the components work. On the other
hand, a parallel circuit divides the current through the electrical gadgets.
This is helpful particularly when each gadget has different resistance and
requires different current to operate properly.
CIRCUIT COMPONENTS
AND THEIR SERIES AND PARALLEL
ARRANGEMENT IN A CIRCUIT:
(a) The cell or Battery: The
cell is the source of electric current. It has a positive and a negative
terminal. When two or more cells are connected as the positive terminal of one
cell to the negative terminal of the second, the combination is called a
battery.
(b) Switch:
It is an important part of a circuit. When the switch is in the ON position,
current flows in the circuit and the circuit is called as closed. When the switch is in the OFF position, current does not
flow in the circuit and the circuit is called an open.
(c)Conducting Wire: Through
these wires, electric current is passed. Examples are copper wires, tungsten
wires, etc.
(d) Appliances: Appliances
such as bulbs use the electric current to glow. A bulb has a thin wire which is
a called a filament. It glows when current passes through it.
Circuit diagram is used to represent the
circuit elements through symbols. Some common symbols are:
wire
open switch
bulb
power
supply
Electrical symbols
Battery
Light
Bulb
A simple Circuit Diagram showing a Battery and a
bulb
Conventional current flow
- switch
battery
+
Light bulb Direction of current flow
ASSIGNMENT
OBJECTIVE:
1.
Which of the following is NOT a component of an electrical circuit? A. Ammeter
B. Fuse C. Resistors D. Speedometer E. Voltmeter
2.
Which of the following statements is true? A. Electron has a positive charge B.
Electron has no charge C. Neutron has a negative charge D. Proton and neutron
have negative charge E. Proton has a positive charge
3.
The combined resistance of 3 ohms and 6 ohms resistors connected in series is
A. 0.5 ohms B. 2.0 ohms C. 3.0 ohms D. 9.0 ohms E. 18.0 ohms
4.
Which of the following is an electric conductor? A. cloth B. copper C. paper D.
plastic E. Glass
5.
Which of the following is an electrical insulator? A. Aluminum B. Copper C.
Glass D. Iron E. Wet wood
THEORY:
1.
Write in detail on the concept of electron flow.
2.
Differentiate between series and parallel circuit.
3.
What are the components of an electric circuit?
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