Download PHYSICAL SCIENCE

PHYSICAL SCIENCE
Electricity
Part 3: Circuits
13.3 Circuits Objectives
Use schematic diagrams to represent
circuits.
Distinguish between series and parallel
circuits.
Calculate electric power using voltage
and current.
Explain how fuses and circuit breakers
are used to prevent circuit overloads.
Defining Circuits
An electric circuit is an electrical device
connected so that it provides one or more
complete paths for the movement of charges.
In a circuit involving a battery and light bulb,
electrons move through the wires and bulb
from the negative terminal to the positive
terminal.
The battery then adds energy to the charges
as they move within the battery from the
positive terminal back to the negative one.
Defining Circuits
The electrons follow a closed-loop path.
This complete path is called a closed
circuit, if the path becomes incomplete,
it becomes an open circuit.
The voltage source is always a part of
the conducting path of a closed circuit.
A switch is a device used to open or
close a circuit.
Circuits – Open and Closed
Schematic Diagrams
A schematic diagram is a graphic
representation of an electric circuit or
apparatus, with standard symbols for the
electrical devices.
Schematic diagrams tend to use universal
standard symbols.
Each element used in a piece of electrical
equipment is represented by a symbol that
reflects the element’s construction or
function.
Schematic Diagram Symbols
Series and Parallel Circuits
A series circuit provides
a single conducting
path.
Charges cannot build up
or disappear at a point
in a circuit.
If one element of a
series circuit is
removed, the circuit will
not work.
Series and Parallel Circuits
In a parallel circuit, components in the
circuit are connected across common
points, providing two or more separate
conducting paths.
The voltage across each device is the
same, although the current can vary.
The sum of the current in all devices
equals the total current.
Series and Parallel Circuits
If one bulb of a parallel
circuit were removed,
charges would still
move through the other
loop or loops.
The break of the flow in
one path does not
interrupt the flow of
electric charge in other
paths.
Electric Power and Electrical
Energy
Electrical energy is the energy associated
with electrical charges, whether moving or at
rest.
The rate at which electrical work is done is
called electrical power.
Electrical power (P) is the product of total
current (I) in and voltage (V) across a circuit.
Formula: power = current x voltage, or
P=VI.
Electric Power and Electrical
Energy
The SI unit for power is the watt (W).
The power lost (dissipated) by a resistor can
be calculated by the formula…

P = I2R = V2/R
Electric companies track consumption of
energy in units of kilowatt-hour (kW•h).
One kilowatt-hour is the energy delivered in
one hour at the rate of 1kW.
In SI units, 1 kW•h = 3.6 x 106 J.
Fuses and Circuit Breakers
Electrical wires could carry an overload
– more than a safe level of current.
If the insulation on wires is worn, and
two wires touch, this creates an
alternative pathway for current called a
short-circuit.
Grounding appliances reduces the risk
of electric shock from a short circuit.
Fuses and Circuit Breakers
A fuse is an electrical
device containing a
metal strip that melts
when current in the
circuit becomes too
great.
The melting of the thin
metal strip opens the
circuit to prevent
overload.
Fuses and Circuit Breakers
A circuit breaker
uses a bimetallic
strip that responds
to current overload
by opening the
circuit.
Circuit breakers act
like a switch and can
be reset by turning
the switch back on.