Transcript P2.4.1 Household electricity

P2.4.1 Household Electricity
P2 Physics
Mr D Powell
Connection
•
•
•
Connect your learning to the
content of the lesson
Share the process by which the
learning will actually take place
Explore the outcomes of the
learning, emphasising why this will
be beneficial for the learner
Demonstration
• Use formative feedback – Assessment for
Learning
• Vary the groupings within the classroom
for the purpose of learning – individual;
pair; group/team; friendship; teacher
selected; single sex; mixed sex
• Offer different ways for the students to
demonstrate their understanding
• Allow the students to “show off” their
learning
Activation
Consolidation
• Construct problem-solving
challenges for the students
• Use a multi-sensory approach – VAK
• Promote a language of learning to
enable the students to talk about
their progress or obstacles to it
• Learning as an active process, so the
students aren’t passive receptors
• Structure active reflection on the lesson
content and the process of learning
• Seek transfer between “subjects”
• Review the learning from this lesson and
preview the learning for the next
• Promote ways in which the students will
remember
• A “news broadcast” approach to learning
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Index
P2.4.1 Household Electricity (Part A)
a) Cells and batteries supply current that always
passes in the same direction. This is called direct
current (d.c.).
b) An alternating current (a.c.) is one that is
constantly changing direction.
c) Mains electricity is an a.c. supply. In the UK it
has a frequency of 50 cycles per second (50
hertz) and is about 230 V.
d) Most electrical appliances are connected to
the mains using cable and a three-pin plug.
e) The structure of electrical cable.
f) The structure and wiring of a three-pin plug.
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Index
P5.1 – Alternating Current p164
a) Cells and batteries supply current that
always passes in the same direction. This is
called direct current (d.c.).
b) An alternating current (a.c.) is one that is
constantly changing direction.
c) Mains electricity is an a.c. supply. In the UK
it has a frequency of 50 cycles per second (50
hertz) and is about 230 V.
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Signal Generator & Oscilloscope - Video
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Signal Generator & Oscilloscope
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D.C. versus A.C.
Scale = 1V/cm
Direct Current
If the _______flows in only one direction it is called ______
_______, or dc. Batteries and cells supply ___
_____electricity. The diagram shows the trace from an
oscilloscope showing the signal from a DC supply. The
voltage is ___V.
Alternating Current
Scale = 300V/cm
If the _______ constantly changes direction it is called
___________ current, or ac. Mains electricity is an ____
______ supply. The UK mains supply is about 230V. It has a
frequency of ___ Hz (50 Hertz), which means that it changes
direction __ times a second. The diagram shows an
oscilloscope screen displaying the signal from an ac supply.
The peak voltage is ___V.
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Index
1 AC Problems
1) An a.c. source is connected to an oscilloscope.
The waveform of the alternating potential
difference from the source is displayed on the
oscilloscope screen as below.
a)
The Y-gain setting of the oscilloscope is 0.5
V/cm. Determine the amplitude of the
alternating potential difference.
The time-base setting of the oscilloscope is 0.02
seconds per cm.
a) 0.5V/cm x 2cm = 1V
b.
Show that the time for one cycle of the
waveform is 0.06 s.
b) 1 cycle is 3 cm so ;3 x
0.02 s/cm = 0.06s
c.
Calculate the frequency of the alternating
potential difference.
c) 1/T = f so 1/0.06s =
16.67s-1 or 17 Hz
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Index
2 AC Problems...
2) An a.c. source is connected to an oscilloscope. The
waveform of the alternating potential difference from
the source is displayed on the oscilloscope screen as
below.
a)
The Y-gain setting of the oscilloscope is 2.0 V/cm.
Determine the amplitude of the alternating
potential difference.
The time-base setting of the oscilloscope is 0.010
seconds per cm.
b) Determine the frequency of the alternating
potential difference.
c)
The a.c. supply is disconnected and a horizontal
flat trace across the middle of the screen is seen.
Sketch the trace that you would expect to see if
the a.c. supply was replaced by a battery of p.d.
3.0 V.
a) 2V/cm x 2.5cm = 5V
b) 1 cycle is 2 cm so ;
2 x 0.01 s/cm = 0.02s
1/T = f so 1/0.02s = 50s-1 or
50 Hz
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Index
P5.2 – Cables and Plugs – p166
d) Most electrical appliances are connected to
the mains using cable and a three-pin plug.
e) The structure of electrical cable.
f) The structure and wiring of a three-pin plug.
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Index
Sockets – Ring Mains
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3 Pin Plug
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Plug Features
Copper is used for the wires because it is a
good electrical conductor and soft or
malleable. This means it bends when you
flex the wire.
Plastic is a good electrical insulator and
therefore prevents anyone touching the
cable from receiving an electric shock.
Brass (copper/ zinc) is used for the pins as
it is a hard and durable alloy metal.
Two-core cables are used for appliances
which have plastic cases
(e.g. hairdryers, radios).
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Index
Plug Features II
The brown wire is connected to the live
pin.
The blue wire is connected to the
neutral pin.
The green-yellow wire (of a three-core
cable) is connected to the earth pin.
The plug contains a fuse between the
live pin and the live wire. The fuse melts
and cuts the live wire off if too much
current passes through it.
A two-core cable does not have an earth
wire.
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Index
P2.4.1 Household Elect (Part A)
P2.4.1 Household Elect (Part A)
P2.4.1 Household Elect (Part A)
a) Cells and batteries supply current
that always passes in the same
direction. This is called direct
current (d.c.).
a) Cells and batteries supply current
that always passes in the same
direction. This is called direct
current (d.c.).
a) Cells and batteries supply current
that always passes in the same
direction. This is called direct
current (d.c.).
b) An alternating current (a.c.) is
one that is constantly changing
direction.
b) An alternating current (a.c.) is
one that is constantly changing
direction.
b) An alternating current (a.c.) is
one that is constantly changing
direction.
c) Mains electricity is an a.c. supply.
In the UK it has a frequency of 50
cycles per second (50 hertz) and is
about 230 V.
c) Mains electricity is an a.c. supply.
In the UK it has a frequency of 50
cycles per second (50 hertz) and is
about 230 V.
c) Mains electricity is an a.c. supply.
In the UK it has a frequency of 50
cycles per second (50 hertz) and is
about 230 V.
d) Most electrical appliances are
connected to the mains using cable
and a three-pin plug.
d) Most electrical appliances are
connected to the mains using cable
and a three-pin plug.
d) Most electrical appliances are
connected to the mains using cable
and a three-pin plug.
e) The structure of electrical cable.
e) The structure of electrical cable.
e) The structure of electrical cable.
f) The structure and wiring of a
three-pin plug.
f) The structure and wiring of a
three-pin plug.
f) The structure and wiring of a
three-pin plug.
P2.4.1 Household Electricity (Part B) – 5.3 Fuses p168
g) If an electrical fault causes too
great a current, the circuit is
disconnected by a fuse or a circuit
breaker in the live wire.
h) When the current in a fuse wire
exceeds the rating of the fuse it
will melt, breaking the circuit.
Some circuits are protected by
Residual Current Circuit Breakers
(RCCBs).
J) Appliances with metal cases are
usually earthed.
k) The earth wire and fuse
together protect the wiring of the
circuit.
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Index
A.C. In Plugs
The ………………………… of the live terminal
varies between a large ……………….. value and
a large ………………… value.
However, the …………………….. terminal is at a
potential difference close to …………., which is
zero.
The diagram shows an oscilloscope screen
displaying the signals from the mains supply.
The ……….. trace is the ……….. terminal and
the blue trace the neutral terminal.
The mean voltage of the mains supply is
about …………, the peak voltage is higher.
(325V) We use ………… in normal
calculations.
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Index
A.C. In Plugs
The potential difference of the live terminal
varies between a large positive value and a
large negative value.
However, the neutral terminal is at a
potential difference close to earth, which is
zero.
The diagram shows an oscilloscope screen
displaying the signals from the mains
supply.
The red trace is the live terminal and the
blue trace the neutral terminal.
Note: although the mean voltage of the
mains supply is about 230V, the peak
voltage is higher. We use 230V in normal
calculations.
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Summary Questions
neutral
live
parallel
Earth
series
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How is it all connected?
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Heater Circuit
Inside 3 Pin Plug
3 Pin Plug
/ Mains
Appliance
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Circuit Breakers
A circuit breaker is an electromagnetic
switch that opens (i.e. ‘trips’) and cuts
the current off if the current is greater
than a certain value.
It can then be reset once the fault that
made it trip has been put right.
Circuit breakers are sometimes fitted in
‘fuse boxes’ in place of fuses.
They work faster than fuses and can be
reset quicker.
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Facts Fuses
 Fuses are thin pieces of wire which resist a large current
 They melt when too much current flows
 Circuit is broken
 Have to be replaced
Facts Circuit Breakers
 Circuit breakers are designed to flip over when too large a current flows
 They break the circuit
 Rely on magnetic forces
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Fuses & Earth Wire - Faults
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How Science Works
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Summary Questions
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Fuses Extra Questions
1. A short-circuit in an electrical cable or appliance happens if a live wire
……………… an earthed metal part or a neutral wire. When a shortcircuit happens, the current in the live wire ……………… and
……………… the fuse wire in the appliance plug. When the fuse wire
………………, it causes a gap in the circuit which ……………… the live
wire, so no current passes through the appliance any more.
2. The recommended fuse for the mains plug of a certain hairdryer is a 3 A
fuse. When the hairdryer is used one day, the fuse melts.
3. What would happen if the fuse was replaced with a new 3 A fuse without
repairing the hairdryer?
4. Why would it be dangerous to replace the fuse with a 5 A fuse or a 13 A
fuse?
breaks
cuts off
melts
rises
touches
Mr Powell 2012
Index
P2.4.1 Household Elect (Part B)
P2.4.1 Household Elect (Part B)
g) If an electrical fault causes too great a current, the
circuit is disconnected by a fuse or a circuit breaker in
the live wire.
g) If an electrical fault causes too great a current, the
circuit is disconnected by a fuse or a circuit breaker in
the live wire.
h) When the current in a fuse wire exceeds the rating
of the fuse it will melt, breaking the circuit. i) Some
circuits are protected by Residual Current Circuit
Breakers (RCCBs).
h) When the current in a fuse wire exceeds the rating
of the fuse it will melt, breaking the circuit. i) Some
circuits are protected by Residual Current Circuit
Breakers (RCCBs).
J) Appliances with metal cases are usually earthed.
J) Appliances with metal cases are usually earthed.
k) The earth wire and fuse together protect the wiring
of the circuit.
k) The earth wire and fuse together protect the wiring
of the circuit.
P2.4.1 Household Elect (Part B)
P2.4.1 Household Elect (Part B)
g) If an electrical fault causes too great a current, the
circuit is disconnected by a fuse or a circuit breaker in
the live wire.
g) If an electrical fault causes too great a current, the
circuit is disconnected by a fuse or a circuit breaker in
the live wire.
h) When the current in a fuse wire exceeds the rating
of the fuse it will melt, breaking the circuit. i) Some
circuits are protected by Residual Current Circuit
Breakers (RCCBs).
h) When the current in a fuse wire exceeds the rating
of the fuse it will melt, breaking the circuit. i) Some
circuits are protected by Residual Current Circuit
Breakers (RCCBs).
J) Appliances with metal cases are usually earthed.
J) Appliances with metal cases are usually earthed.
k) The earth wire and fuse together protect the wiring
of the circuit.
k) The earth wire and fuse together protect the wiring
of the circuit.