Safety in Using Electricity
Grade 7 (3rd
Motion in One
Dimension(in terms of
displacement, speed, or
velocity and acceleration)
transverse vs. longitudinal,
Heat (heat transfer)
Electricity (charges and
Grade 8 (1st
Laws of Motion
Work, Power, and Energy
Sound (propagation of
sound through SLG)
Heat (heat and
temperature; it's effect)
relationship; electric power;
electric energy; home
Activity 1 Current and Voltage
Activity 2 Current and
Activity 3 What's the
Activity 4 Stay safe!
How do voltage and resistance affect
What are the safety precautions
needed in using electricity?
A circuit is any arrangement of a source of
energy (battery), connecting wires, and
load (e.g. bulbs)
A complete or a closed circuit provides a
path for electrical charges to flow.
Electric current is a measure of the number of
electrical charges passing through a
cross- section of a conductor in a given
The direction of conventional current or
simply current is from the positive
terminal of the battery to the negative
The symbol for current is capital letter I.
The unit, ampere (A), is named after Andre-
Marie Ampere, a French physicist who made
important contributions to the theory of
electricity and magnetism.
An ammeter measures electric current. The
positive terminal of an ammeter is connected to
the positive terminal of the energy source (e.g.
battery) while the negative terminal is
connected to the negative terminal of the
In a circuit, work must be done
on the charges to make them
The battery supplies the energy
in electric circuits.
The chemical energy in the
battery is transformed to
This electrical energy moves the
A battery consists of several
dry cells or wet cells.
Dry and wet cells contain a
conducting medium called
(The batteries we use in
flashlights and watches are dry
Symbol for voltage is capital
The unit, volts (V)
Named after the Italian
physicist AlessandroVolta who
invented the voltaic pile, the
forerunner of what we now call
the dry cell.
A voltmeter measures voltage.
The voltmeter should be
connected across the load being
The positive terminal of a
voltmeter is connected to the
positive terminal of the bulb while
the negative terminal is
connected to the negative
terminal of the bulb.
The more dry cells used,the brighter the
The current is higher for two dry cells as
compared to one dry cell.
The bulb glows brighter when another dry
cell was added to the circuit and reading
on voltmeter increases.
The voltage is bigger for two dry cells as
compared to one dry cell.
For a constant load (one bulb), when the
voltage increases the current also
Dry cell provides the energy that
moves the charges in a circuit
Dry cell must be connected by
conducting wires to a load to
form a complete circuit.
Adding dry cells in series
increases the voltage in a circuit.
Adding dry cells increases the current
in a circuit as shown by the ammeter
Brightness of the bulb also indicates
the amount of current passing through
The bigger the current through the
bulb, the brighter it glows.
Both the meter readings and the
brightness of the bulb show that
voltage and current are related.
As the voltage increases, the current
When electric charges flow through
the wires and loads of the circuits they
encounter resistance or a hindrance
to their movement.
The symbol for resistance is capital
The unit, ohms (Ω) is named after the
German physicist Georg Simon Ohm.
Resistance of the material opposes
the flow of charges.
Resistance can also be measured and
they are expressed in units called
A lower resistance would mean that
there is less opposition in the flow of
charges and therefore bigger current.
Different materials have different
amounts of resistance.
Conductors definitely have very
little resistance and therefore
allow more charges to pass
Insulators are materials that have
very high resistance and therefore
flow of charges would be difficult.
For a fixed resistance (one bulb),
as the voltage increases, the
current also increases.
Keeping the voltage the same (2
dry cells), when the resistance
increases, the current decreases.
The length and thickness of the
conducting wire are factors that
affect resistance encountered by
The longer the wire the greater will
be its resistance and the greater the
cross sectional area (a measure of
the thickness of the wire), the lower
will be its resistance.
The resistance of an object also
changes when the object
Dry human skin for instance has a
resistance of 100,000 ohms but
when it gets wet its resistance is
reduced to 1,000 ohms.
It is important to dry the hands when
plugging an electrical appliance to
reduce any chance of getting a lot of
current if an accident occurs.
below 1 milliampere Generally not perceptible
1 milliampere Faint tingle.
5 milliampere Slight shock felt not painful
Average individual can let go. Strong
involuntary reactions can lead to other
6-25 milliampere (F) Painful shock, loss of muscular
9-30 milliampere (M) The freezing current or let go range
individual cannot let go but can be thrown away from the
circuit if extensor muscles are stimulated
50-150 milliampere Extreme pain, respiratory
arrest (breathing stops) severe
muscular contractions. Death is
1,000-4,300 milliampere Rhytmic pumping action of the
heart ceases. Muscular contraction and
nerve damage occur, death likely.
10,000 milliampere Cardiac arrest and severe
burns occur. Death is probable.
15,000 milliampere Lowest overcurrent at which a
typical fuse or circuit breaker opens
loads form a single pathway for
charges to flow
A gap or a break anywhere in
the path stops the flow of
charges.When one bulb is
removed from the socket, a gap
is created.The other bulb turns
off as there is no longer current
in the circuit.
The total resistance in a series circuit is equal to
the sum of the individual resistances of the load
Current is the same in every part of the circuit.
The current is equal to the voltage divided by the
total resistance. As more load (bulb) is added in a
series circuit, the smaller the current as
reflected by the brightness of the bulb.
The voltage across each load depends on the
The sum of the voltage across each load is equal
to the total voltage.
loads form branches; each provides
a separate path for charges to flow.
A gap or a break in any branch will
not affect the other branches.Thus,
when one bulb is removed from the
socket, a gap is created only for that
branch.The other bulbs still glow as
their path is still complete.
In a parallel connection the voltage
is the same across each load.
The total current is equal to the
sum of the currents in the
The amount of current is
inversely proportional to the
resistance of the load.
Table 4 Series connection Parallel connection
Total current Same as current in individual load Equal to the sum
of current in individual
Total voltage Equal to the sum of the Same anywhere
voltages across each load two points in the circuit
Total resistance Increases with increasing load Decreases with
Fires can happen when the wires start
heating up causing combustible parts
of the house to be set on fire.
The wires heat up when the current
passing is more than what the wires
can carry. In this case there is an
overloading of the circuit.
An example of how the circuit gets
overloaded is by plugging a lot of
appliances in a common outlet like an
Another instance of overloading of the
circuit is the presence of short circuits.
Short circuits happen when wires with
defective rubber insulation touch each
other so the current does not pass to the
supposed path it should take.
It is a circuit where the current encounters
very little resistance and therefore the
amount of current will increase rapidly.
Such increase in the amount of current
leads to the overloading of the circuit and
can lead to fires.
In the wires the electrons that flow
in a closed circuit collide with the
atoms of the conducting wire.
As the collisions take place the
kinetic energy of the metal atoms
The increased kinetic energy of
the atoms is dissipated as heat.
The higher the kinetic energy of
the particles, the higher will be its
The higher the current passing
through the wire, the more
collisions between the electrons
and the atoms of the wire take
In the end the wire will become
hot. So just imagine how much
heat will be generated from an