Transcript Electric circuits - World of Teaching

Electric circuits
What is the job of an
electrical circuit?
• Electrical energy can be transferred
through it
• This electrical energy comes from a
battery or other power supply
• The energy can be transferred to
components in the circuit
• The unit of energy is the joule (J)
What is a current?
• Current is a flow
of electrons
round the circuit
• Current will only
flow through a
component in a
circuit if there is
a voltage across
that component
What is direct current?
• Direct current is travels in one
direction only
• Cells and batteries supply direct
current
• Direct current is also known as
d.c. current or d.c. supply
Direct current
• With d.c.
current the
voltage is
constant
voltage
• The cathode
ray
oscilloscope
(CRO) trace will
show d.c.
supply as a
horizontal line
What is alternating
current?
• Alternating current is also
known as a.c. current of a.c
supply
• This current is always changing
direction, so the voltage goes up
and down all the time too
Alternating current
• a.c. supply will
produce a wave
on the cathode
ray oscilloscope
• The voltage with
a.c. supply is
constantly going
up and down
Types of circuits:
•
•
•
•
SERIES
These circuits go in
one continuous ‘loop’
The current is the
same anywhere in
the circuit
An ammeter can be
placed anywhere in
the circuit
If we get one gap in
the circuit the
current stops flowing
The other type of circuit:
PARALLEL
• These circuits have two or
more routes for the current
to flow round
• If we get a gap in the circuit
not all components will go
off
• Parallel circuits are used for
house and car lights
• The current leaving the
power supply will split up to
go through different parts of
the circuit
• The current returning to the
battery is the same as what
had left it
Circuit symbols:
Measuring current
• An ammeter is used to measure
current
• Current is measured in amperes
or amps (A) for short
• In physics electrical current is
shown as (I)
Heating effect
• When electrical charge flows
through a resistor it will
generate heat
• This heat is useful in the
following:
• Hairdryers
• Immersion heaters
• Light bulbs
Voltage
• A voltmeter is used to measure
voltage
• Voltage is also known as
potential difference
• The unit of voltage is volts (V)
Resistance
• Resistance tells us how easy or
difficult it is for electrical charge to
flow through a conductor
• Resistance (R) is measured in ohms
()
• We can use the following equation to
find out the resistance of a wire or a
component:
• Voltage = current x resistance
• V=IR
What does a variable resistor
do?
• You change change
the resistance in a
circuit by twisting a
knob or moving a
slider on an oldfashioned variable
resistor
• We can use a variable
resistor to change the
current flowing
through a circuit
What is a light
dependant resistor?
• In bright light the
resistance will drop
in an LDR
• When it is dark the
resistance is highest
• This makes an LDR
useful in automatic
circuits such as
automatic night
lights and burglar
detectors
What do thermistors do?
• A thermistor is a
temperature dependant
resistor
• The resistance drops
when it is hot
• When it is cold the
resistance goes up
• Thermistors are useful
temperature detectors,
e.g. in car engines and
electronic thermostats
What else can affect the
resistance in a circuit?
• Thick wires have a
low resistance as it
is easier for current
to flow through
• Thin wires have a
high resistance as
it is harder for
current to flow
through
• Long wires in a
circuit have a
higher
resistance than
short wires as
the current has
further to travel
What else affects
resistance?
• Some metals are better
conductors than others
• Therefore some metals give a
higher resistance than others
• Copper is a good conductor of
electricity
• Copper has a low resistance
Temperature and
resistance
• If a wire gets warmer, the
resistance increases
• The atoms in the wire start to
vibrate more when they have
more energy
• This makes it harder for the flow
of electrons to flow through the
wire.
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