Transcript P5 electric circuits revision

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P5 Electric Circuits
OCR 21st Century
Georg Simon Ohm
1789-1854
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P5.1 Electric Current – a Flow of What?
Static Electricity
An introduction – click here
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Static Electricity
Static electricity is when charge “builds up” on an object and
then stays “static”. How the charge builds up depends on what
materials are used:
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Static Electricity
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Short Static Experiments
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Try the following quick static electricity experiments:
1) Rubbing a balloon on your jumper and “sticking” it to the
wall
2) Charging a plastic rod by rubbing it with a cloth and then
holding it near the water from a smooth-running tap
3) Charging a plastic rod and trying to pick up small pieces of
paper (or someone else’s hair!) with it
4) Rubbing a balloon on someone else’s head – you might want
to ask their permission first…
Can you explain what you saw in each of these experiments?
Gold Leaf Electroscopes
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Consider a gold-leaf electroscope…
Now charge the top:
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This effect was seen
because charges can easily
move through conductors
Electric Current
Electric current is a flow
of negatively charged
particles (i.e. electrons).
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e-
Note that
electrons go
from negative
to positive
By definition, current is “the
rate of flow of charge”
e-
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Basic ideas…
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Electric current is when electrons start to flow around a
circuit. We use an _________ to measure it and it is
measured in ____.
Potential difference (also called _______) is
how big the push on the electrons is. We use a
________ to measure it and it is measured in
______, a unit named after Volta.
Resistance is anything that resists an electric current. It is
measured in _____.
Words: volts, amps, ohms, voltage, ammeter, voltmeter
Understanding Current
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When a voltage is applied it basically causes the electrons in a
conductor to move towards the positive end of the battery:
Negative
Electrons
Ions
Positive
The main difference between conductors and insulators is that
insulators have less of these free electrons.
P5.2 Current and Energy
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Circuit Symbols
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Variable
resistor
Diode
Switch
Bulb
A
V
Ammeter
Voltmeter
LDR
Resistor
Cell
Fuse
Thermistor
Battery
More basic ideas…
If a battery is
added the current
will ________
because there is a
greater _____ on
the electrons
If a bulb is added
the current will
_______ because
there is greater
________ in the
circuit
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Electric Current revisited
Work is done by the battery to
move the electrons around the
circuit.
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e-
When the electons arrive at
the bulb energy is
transferred to it.
e-
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Note that we
usually
assume the
resistance of
these leads is
very small.
Electrical Power revision
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Power is defined as “the rate of transferring energy” and is
measured in units called “Watts” (W).
The amount of power being transferred in
an electrical device is given by:
Power = voltage x current
in W
in V
in A
P
V
I
1) How much power is transferred by a 230V fire that runs
on a current of 10A?
2) An electric motor has a power rating of 24W. If it runs
on a 12V battery what current does it draw?
3) An average light bulb in a home has a power rating of
60W and works on 230V. What current does it draw?
Understanding Resistance
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Recall our previous model of electric current but this time
we’ll use it to explain resistance:
Negative
Electrons
Ions
Positive
Notice that the ions were vibrating and getting in the way of
the electrons – this is resistance. This effect causes the
metal to heat up.
Using this heating effect
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This heating effect can have its advantages and its
disadvantages. For example, consider an old-fashioned light
bulb:
This heating
effect causes the
filament to emit
light…
…but it also causes
a lot of energy to
be wasted to the
environment
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Resistance
Resistance is anything that will
RESIST a current. It is measured
in Ohms, a unit named after me.
Georg Simon Ohm
1789-1854
The resistance of a component can be
calculated using Ohm’s Law:
Resistance
(in )
=
V
Voltage (in V)
Current (in A)
I
R
An example question:
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Ammeter
reads 2A
A
V
Voltmeter
reads 10V
1) What is the resistance across
this bulb?
2) Assuming all the bulbs are the
same what is the total resistance
in this circuit?
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More examples…
3A
6V
12V
3A
2A
4V
2V
1A
What is the
resistance of
these bulbs?
Resistance
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Resistance is anything that opposes an electric current.
Resistance (Ohms, ) =
Potential Difference (volts, V)
Current (amps, A)
What is the resistance of the following:
1) A bulb with a voltage of 3V and a current of 1A.
3
2) A resistor with a voltage of 12V and a current
of 3A
4
3) A diode with a voltage of 240V and a current of
40A
6
4) A thermistor with a current of 0.5A and a
voltage of 10V
20
LDRs and Thermistors
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Two simple components:
1) Light dependant
resistor – resistance
DECREASES when light
intensity INCREASES
Resistance
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2) Thermistor –
resistance DECREASES
when temperature
INCREASES
Resistance
Amount of light
Temperature
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Using Thermistors and LDRs in circuits
A
V
1) What will happen to the resistance of the
thermistor when it gets hotter?
2) How will this affect the brightness of the bulb
and the reading on the ammeter?
3) How will this affect the readings on the
ammeter and voltmeter?
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Resistors in Series and Parallel
Consider the total resistance of these combinations:
10Ω
10Ω
10Ω
10Ω
The total resistance of this
combination is 20Ω as the
battery has to move charges
through both resistors
The total resistance of this
combination is only 5Ω as the
charges have more paths to
move through
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Current-voltage graph for a Resistor
I
V
Resistor
Notice that a currentvoltage graph for a
resistor of fixed value
shows that current
increases in proportion
to voltage.
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P5.3 How do Parallel and Series
Circuits work?
Current in a series circuit
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If the current
here is 2
amps…
The
current
here will
be…
The current
here will
be…
And the
current
here will
be…
In other words, the current in a series
circuit is THE SAME at any point
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Current in a parallel circuit
A PARALLEL circuit is one where the current has a “choice
of routes”
Here comes the current…
Half of the current
will go down here
(assuming the bulbs
are the same)…
And the rest will
go down here…
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Current in a parallel circuit
If the
current
here is 6
amps
And the
current here
will be…
The current
here will be…
The current
here will be…
The current
here will be…
Some example questions…
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3A
6A
4A
2A
1A each
Voltage and Work done
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What does “voltage” mean?
The voltage (or potential difference) between two
points is a measure of the work done per “bit” of
charge moving between these points.
Voltage in a series circuit
If the voltage
across the
battery is 6V…
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V
…and these
bulbs are all
identical…
…what will the
voltage across
each bulb be?
V
V
2V
Voltage in a series circuit
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V
Notice that the voltages
add up to the voltage
across the battery – this
is because the work done
on each unit of charge by
the battery must equal
the work done by it to
the bulbs.
V
V
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What if the Resistances are different?
Q. What would each of these
voltmeters read?
60V
Total resistance = 30Ω
Therefore current = 2A
Therefore V1 = 2x10 =
20V and and V2 = 2x20 =
40V
10Ω
20Ω
V1
V2
The voltage across the bigger resistance is higher as more
work is done by the battery to get the current through it.
Voltage in a series circuit
If the voltage
across the
battery is 6V…
…what will the
voltage across
two bulbs be?
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V
V
4V
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Voltage in a parallel circuit
If the voltage across
the batteries is 4V…
What is the
voltage here?
4V
V
And here?
V
4V
Summary
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In a SERIES circuit:
Current is THE SAME at any point
Voltage SPLITS UP over each component
In a PARALLEL circuit:
Current SPLITS UP down each “strand”
Voltage is THE SAME across each”strand”
An example question:
3A
6V
3A
A1
6V
A2
V1
2A
1A
A3
3V
V2
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V3
3V
Another example question:
3A
10V
A1
1.2A
3A
A2
V1
6.7V
A3
5V
V2
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1.8A
V3
5V
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P5.4 How is Mains Electricity Produced?
Electromagnetic Induction
N
The direction of the induced current is reversed if…
1) The wire is moved in the opposite direction
2) The field is reversed
The size of the induced current can be increased by:
1) Increasing the speed of movement
2) Increasing the magnet strength
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Electromagnetic
induction
The direction of the induced current is
reversed if…
1) The magnet is moved in the opposite
direction
2) The other pole is inserted first
The size of the induced current can be
increased by:
1) Increasing the speed of movement
2) Increasing the magnet strength
3) Increasing the number of turns on
the coil
Transformers
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Current through primary
Time
Magnetic field
Time
Voltage induced in secondary
Time
Transformers
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Transformers are used to _____ __ or step down
_______. They only work on AC because an ________
current in the primary coil causes a constantly alternating
_______ ______. This will “_____” an alternating
current in the secondary coil.
Words – alternating, magnetic field, induce, step up, voltage
We can work out how much a transformer will step up or
step down a voltage:
Voltage across primary (Vp)
No. of turns on primary (Np)
Voltage across secondary (Vs)
No. of turns on secondary (Ns)
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Some transformer questions
Primary
voltage
Vp
Secondary
voltage
Vs
No. of turns
on primary
Np
No. of turns
on secondary
Ns
Step up or
step down?
12V
24V
100
?
?
400V
200V
20
?
?
25,000V
50,000V
1,000
?
?
23V
230V
150
?
?
Some example questions
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Primary
voltage
Vp
Secondary
voltage
Vs
No. of turns
on primary
Np
No. of turns
on secondary
Ns
Step up or
step down?
6V
24V
100
?
?
400,000V
200V
?
1,000
?
25,000V
?
20,000
20
?
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230V
150
1,500
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1) A transformer increases voltage from 10V to 30V. What is the ratio
of the number of turns on the primary coil to the number of turns on
the secondary coil?
2) A step-down transformer has twice as many turns on the primary coil
than on the secondary coil. What will be the output (secondary)
voltage if the input voltage is 50V?
AC Generators
Magnetic
Field
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Slip rings and
brushes
Generators (dynamos)
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The coil already has an iron
core, but the induced
current could still be
increased by:
1) Increasing the speed of
movement
2) Increasing the magnetic
field strength
3) Increasing the number of
turns on the coil
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Large-scale production of Electricity
A generator at Drax power station in England
AC Generator
N
S
N
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S
Current
Induced current can be increased in
4 ways:
1) Increasing the speed of
movement
2) Increasing the magnetic field
strength
3) Increasing the number of turns
on the coil
4) Increasing the area of the coil
Time
DC and AC
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V
DC stands for “Direct
Current” – the current only
flows in one direction and a
common example is a battery:
AC stands for “Alternating
Current” – the current
changes direction 50 times
every second (frequency =
50Hz). In the UK mains
electricity is 230V AC, not
DC, as AC is easier to
generate and transmit over
long distances.
Time
1/50th s
230V
T
V
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P5.5 How do Electric Motors work?
The Motor Effect
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N
S
1) What will happen to this wire?
2) How can you make it move faster?
3) How can you make it move in a different direction?
Electric Motors
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Using Motors
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The following devices are just some examples of devices that
use electric motors: