electric circuits, symbols, voltmeter, ammeter

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Electric Circuits

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Teacher’s notes included in the Notes Page
Flash activity (these activities are not editable) Virtual experiment
Accompanying worksheet
For more detailed instructions, see the Getting Started presentation

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What is an electric circuit?
At the beginning of the lesson, your teacher had to press a
switch to turn the interactive whiteboard on. Pressing the
switch completed an electric circuit.
Look around you now. What objects can
you see that contain electric circuits?
An electric circuit is made up of the following:
a source of electricity, such as a battery
a conducting material, usually a wire,
through which electricity can flow
a component to use some of the
electricity, such as a light bulb
a switch to break the circuit.

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Drawing circuit diagrams
Electricians, architects and engineers may need to draw
diagrams of electric circuits.
To make diagrams
easier to understand,
electric circuits can be
drawn using symbols
to represent the
components.
Wires are shown as
straight lines.

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Circuit symbols

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What is current?
Current is a measure of how much electric charge flows
through a circuit. The more charge that flows, the larger
the current.
Current is measured using
an ammeter, which must
be connected in series in
the circuit.
When two or more circuit components are connected in
series, the same current flows through each component.
The units for current are
amps (A).

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What is voltage?
Voltage is a measure of how much energy the electrons are
carrying around the circuit.
When two or more circuit components are connected in
series, the total voltage of the supply is shared between them.
Voltage is measured using a
voltmeter, which must be
connected in parallel with a
component in the circuit.
The units for voltage are
volts (V).

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Current and voltage in series circuits

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Case study: electricity and the heart
Most of the body is controlled by electrical
impulses from the brain or different
organs. A small area within the heart,
called the sinoatrial node (SAN), controls
its beating.
Sometimes the rhythm of the heart can be
disrupted – beating too quickly or in an
uncontrolled way. This can lead to death if
a normal rhythm isn’t established quickly.
A defibrillator can be used to shock the heart into
re-establishing a normal rhythm. Unsynchronized current flows
from the defibrillator into the heart, causing it to momentarily
stop beating. It can then restart with a more controlled rhythm.

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What is resistance?

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Factors affecting resistance

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What is a resistor?
A resistor is a component designed to reduce the current.
Most electronic circuits need to
have resistors to work properly.
Resistors are often used with
components that require less
current than is being supplied
to the whole circuit.
Resistors can be produced that reduce the current by
different amounts.

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True or false?

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Case study: the heating effect of resistance
High resistance in a wire can produce heat.
Resistance is important in a filament light bulb.
The filament is made from a very thin, long
piece of high resistance wire. The resistance is
so high that when electricity flows through the
wire, it glows ‘white hot’ and emits light.
Electricity is transmitted across the
country with a very high voltage and a
low current. This reduces the resistance
in the wire and the heat produced.

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Ohm’s law
Depending on the values given,
the formula can be used to
calculate voltage, current and
resistance in a circuit.
Ohm’s law is a formula that links voltage, current and
resistance. Resistance is measured in ohms (Ω).
What two other equations
could you write from this
Ohm’s law formula triangle?
voltage = current × resistance
V = I × R

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What does Ohm’s law mean?
These three equations can be derived from the Ohm’s law
formula triangle. But what do they actually mean?
If the voltage increases, then either the
current flowing through a material or the
resistance will also be increased.
V = I × R
R = V/I
The resistance of a material is constant for a
given temperature, which means that current
and voltage must be proportional.
I = V/R
For a low resistance material, more current is
allowed to flow for a given voltage. For a high
resistance material, less current will flow at the
same voltage.

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Ohm’s law calculations

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Using Ohm’s law
Ohm’s law can be used to calculate the resistance of a
component in a circuit.
Which equation would you use to
calculate resistance?
It is difficult to measure the resistance of a component in a
circuit directly, so it is usually done by measuring the voltage
and current through the component instead.
Which instrument would be used to
measure the current?
Which instrument would be used to
measure the voltage?
R = V/I
voltmeter
ammeter

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Investigating resistors

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Case study: dimmer switches
Dimmer switches provide control over
the brightness of a light bulb.
Dimmer switches used to contain
variable resistors. These are similar to
normal resistors, but the level of
resistance can be changed.
Changing the resistance changed the amount of current
passing through the bulb. However, this produces a lot of heat
and so these dimmer switches were dangerous.
New dimmer switches switch the light circuit on and off very
rapidly to reduce the total amount of current flowing.
The circuit is automatically switched off and on at different
voltage values during alternating current cycles.

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You are an electrical engineer and have been
asked to train some new employees.
You should include the following:
a brief description of how a voltmeter and ammeter are
used in a circuit to measure voltage and current
a diagram of a series circuit showing how to measure
the current and voltage across the battery and another
component
an explanation of how to use Ohm’s law to calculate
resistance, current and voltage, with worked examples.
Prepare a handout about Ohm’s law and electric circuits.

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Multiple-choice quiz