Download P2.3 Current Electricity

P2.3.1 – Static
Electricity
Electrical Charges
• Objectives, to understand that:
– When certain electrical insulators are
rubbed together they become
electrically charged.
– Objects can become charged when
electrons are transferred from one to
another.
– Similarly charged objects repel one
another.
Explain how a charged rod can attract a
neutral piece of paper.
How large a piece of paper can you pick
up?
Explain how a charged rod can attract a
neutral piece of paper.
How large a piece of paper can you pick
up?
• When certain insulating materials are
rubbed against each other they become
electrically charged. Negatively charged
electrons are rubbed off one material and
onto the other.
• The material that gains electrons becomes
negatively charged. The material that
loses electrons is left with an equal
positive charge.
• When two electrically charged objects are
brought together they exert a force on
each other.
• What is electric current?
• What is current measured in?
• What is the difference between
a series and parallel circuit?
• How many circuit symbols can
you draw?
P2.3.2 Electric
Circuits
Electric Circuits
• By the end of today:
– You should be able to draw circuit
symbols for common components.
– Describe what these things actually do.
– Some will be able to use these symbols
to draw circuit diagrams.
Can you light a bulb with just
one wire?
Resistance
• By the end of today:
– You will know where to put an ammeter
and a voltmeter in a circuit.
– You will be able to describe how to
measure the resistance of a component.
– You can state Ohm’s law.
– Some will be able to rearrange the
resistance equation.
Ohm’s Law
The current through a resistor at a
constant temperature is directly
proportional to the potential difference
across the resistor.
This means if you double the current you double the
voltage over a component.
It also means that the resistance of the component does
not change when you put more current through it.
Starter
• How many different circuits
can you draw with 3 cells and
4 bulbs?
• 10?
Series and
Parallel Circuits
Series Circuit
The components in the circuit are lined up in
series, one after each other.
Parallel Circuit
The components in the circuit are lined up in
parallel, in parallel lines to one another.
Series Circuit
The current (electrons) can only go one way
so the current is the same everywhere in the
circuit.
But the energy it has given to it by the
battery is shared equally amongst all the
bulbs.
Parallel Circuit
The components in the
circuit are lined up in parallel,
in parallel lines to one
another.
Resistance of
Components
5.3 Resistance of Components
• By the end of today:
– You should be able to recognise the
graph of current against voltage for a
diode, filament lamp, thermistor and
LDR
A
V
•An ammeter measures the current in
the circuit
•A voltmeter measures the potential
difference across a component.
A
V
How to measure resistance
A
bit of wire
V
http://phet.colorado.edu/en/simulation/circuit-construction-kit-dc
Potential Difference (Volts)
Current (A)
Current (A)
Potential Difference (Volts)
A Resistor
Current (A)
R = V
I
• If the gradient is
constant…
• …then the ratio of
V to I is
constant….
• …so the
resistance is
constant
Potential Difference (Volts)
A Filament Lamp
The filament lamp is a
common type of light bulb. It
contains a thin coil of wire
called the filament. This heats
up when an electric current
passes through it, and
produces light as a result.
A Filament Lamp
The filament lamp does not follow Ohm’s Law. Its
resistance increases as the temperature of its filament
increases. So the current flowing through a filament lamp is
not directly proportional to the voltage across it. This is the
graph of current against voltage for a filament lamp.
The diode
If the diode is this way round, no
current can flow so the lamp stays
unlit.
The diode has a very high resistance in one
direction. This means that current can only
flow in the other direction.
This is the graph of current against potential
difference for a diode.
Conventional Current
By scientific convention the current goes
from the positive end of the battery to the
negative end.
A
Conventional Current
•But current in an electronic circuit is the flow of
electrons.
•Electrons are negatively charged.
•So the electrons actually go the other way around
the circuit to what ‘conventional current’ says.
A
What is negative current
and voltage?
+ve reading
•Consider conventional current (+ve to –ve)
A
What is negative current
and voltage?
-ve reading
•If we turn the battery around we send the current
the other way around the circuit, this gives us a
negative reading on the ammeter.
A
Thermistors
Thermistors are used as temperature sensors - for
example, in fire alarms. Their resistance decreases
as the temperature increases:
•At low temperatures, the resistance of a
thermistor is high and little current can flow
through them.
•At high temperatures, the resistance of a
thermistor is low and more current can flow
through them.
Light Dependent Resistor
LDRs are used to detect light levels, for example, in
automatic security lights. Their resistance
decreases as the light intensity increases:
•In the dark and at low light levels, the resistance
of an LDR is high and little current can flow through
it.
•In bright light, the resistance of an LDR is low and
more current can flow through it.
Extension Tasks
• Collect your equipment from my desk.
• Can you set up the multimeter to find the
resistance of a component?
• Draw a superb, artistic, clear diagram of what
you have just done.
• Measure the resistance of a thermistor and
an LDR.
– How can you change their resistance?
– Does that agree with what you have written in
your notes?
Resistance of a Circuit
R1
R2
The total resistance (known as RT) of a series
circuit is equal to the sum of the resistance of
each individual component.
RT = R1 +R2