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ELECTRICAL CIRCUITS
SNC 1D
An Introduction
• The energy that results from moving
electrons is called electrical energy.
• When electrons move from one place to
another they make an electric current.
• The amount of energy that each
electron delivers is called the voltage.
• The path that the electrons follow is
referred to as an electric circuit.
Simple Circuits
Here is a simple electric circuit. It has a source, a
load and a switch to control the flow of electrons.
source
control
connectors
load
To make the circuit, these components are connected
together with metal connecting wires.
Functions of the Parts
• Source – where the electrons come from.
(Commonly called the cell or battery)
• Load – a device that converts electrical
energy into another form. (Lightbulb,
motor, buzzer, toaster, heater…)
• Control – a device to start and stop the
flow of electrons. (Turn load on and off)
• Connectors – conducting wires to provide
a path for the electrons to follow.
Simple Circuits
When the switch is closed, the lamp lights up. This is
because there is a continuous path of metal for the
electric current to flow around. This is called a
closed circuit.
If there were any breaks in the circuit, the current
could not flow. This is called an open circuit.
The Cell
The cell stores chemical energy and transfers it to
electrical energy when a circuit is connected.
When two or more cells are
connected together we call this
a Battery.
The cells chemical energy is
used up pushing a current round
a circuit.
Types of Cells
• Primary Cells – electric cells that can not
be recharged because the chemical
reaction is not reversible.
• Secondary Cells – electric cells that can
be recharged by forcing the chemical
reaction in the reverse direction.
• Fuel Cells – electric cells that
require a continuous supply of
chemicals to be pumped in.
What is an electric current?
An electric current is a flow of microscopic particles
called electrons flowing through wires and
components.
+
-
In which direction does the current flow?
from the Negative terminal to the Positive terminal of a
cell.
Circuit Diagram
Scientists usually draw electric circuits using symbols;
cell
lamp
switch
wires
Circuit Diagrams
In circuit diagrams components are represented by
the following symbols;
cell
ammeter
battery
voltmeter
switch
motor
lamp
buzzer
resistor
variable
resistor
Current
• Made up of moving electrons
• Measure of how fast the electrons
are moving; how many move past a given
point in a certain amount of time
• Symbol for current is I
• SI unit used to measure current is the
Ampere (A) or Amps
Measuring Current
Electric current is measured in amps (A) using
an ammeter connected in series in the circuit.
A
Measuring Current
This is how we draw an ammeter in a circuit.
A
Connected right into
the circuit
Measuring Voltage
The ‘electrical push’ which the cell gives to the current
is called the voltage. It is measured in volts (V) on a
voltmeter
V
Measuring Voltage
This is how we draw a voltmeter in a circuit.
V
Connected around the
load and/or source
Measuring Voltage
Different cells produce different voltages. The
bigger the voltage supplied by the cell, the bigger the
current.
Unlike an ammeter a voltmeter is connected across
the components (in parallel).
Scientist usually use the term Potential Difference
(E) when they talk about voltage.
Resistance
• A measure of the opposition to the
movement of electrons as they flow
through a circuit.
• SI unit used to measure resistance is
ohms (Ω)
• Measured using a ohmmeter
• All materials have some internal
resistance. The greater the resistance
the lower the current and the warmer the
material becomes.
Factors Affecting Resistance
•
•
•
•
Type of Material
Cross-Sectional Area
Length
Temperature
Relating Current, Voltage, and
Resistance
Gizmo Time!
Ohm’s Law!
Ohm’s Law
As resistance
increases,
current
decreases
As voltage
increases,
current
increases
Ohm’s Law
V=IR
where
V = voltage (V)
I = current (A)
R = resistance (R)
Ohm’s Law
Ohm’s Law
Ohm’s Law
Types of Circuits
There are two types of electrical circuits;
SERIES CIRCUITS
PARALLEL CIRCUITS
SERIES CIRCUITS
The components are connected end-to-end, one
after the other.
They make a simple loop for the current to flow
round.
If one bulb ‘blows’ it breaks the whole circuit and
all the bulbs go out.
PARALLEL CIRCUITS
The components are connected side by side.
The current has a choice of routes.
If one bulb ‘blows’ there is still be a complete circuit to
the other bulb so it stays alight.
Measuring Current
How does current differ in a series and parallel
circuit?
A
A
SERIES CIRCUIT
PARALLEL CIRCUIT
Measuring Current
SERIES CIRCUIT
• current is the same
at all points in the
circuit.
2A
2A
2A
PARALLEL CIRCUIT
• current is shared
between the
components
2A
2A
1A
1A
Measuring Voltage
How does voltage differ in a series and parallel
circuit?
V
SERIES CIRCUIT
V
PARALLEL CIRCUIT
Series Circuit
• voltage is shared between the components
3V
1.5V
1.5V
Parallel Circuit
• voltage is the same in all parts of the circuit.
3V
3V
3V
Measuring Current & Voltage
Copy the following circuits on the next two
slides.
Complete the missing current and voltage
readings.
Remember the rules for current and
voltage in series and parallel circuits.
Using Ohm’s Law, calculate the resistance
of each load in the circuits.
measuring current & voltage
a)
6V
4A
A
V
V
A
measuring current & voltage
b)
4A
6V
A
V
A
V
A
answers
a)
b)
4A
6V
6V
4A
6V
4A
4A
3V
2A
3V
4A
6V
2A