Transcript Alternating Current - The Place Programme

Alternating Current
Learning Outcomes
• Distinguish between alternating and direct
current.
• State the frequency of UK mains electricity.
• Describe how the potential of the live wires
varies each cycle.
• State that the potential of the neutral wire is
approximately zero.
• Use oscilloscope traces to compare direct and
alternating potential differences.
Starter
Draw and label a waveform
with the following labels:
•Amplitude
•Wavelength
•Peak
•Trough
•Also, define frequency
Starter
Wavelength
Peak
Trough
Draw and label a waveform
with the following labels:
•Amplitude
Amplitude
•Wavelength
•Peak
•Trough
•Also, define frequency =
number of waves passing a
fixed point every second
(Hz)
A.C. D.C. – not the band!!
• Direct current and
alternating current are
different types of
electric current
• You should know the
differences between
direct current (d.c.) and
alternating current
(a.c.) electrical supplies.
Direct Current
• If the current flows in
only one direction it is
called direct current, or
d.c.
• Batteries and cells supply
d.c. electricity, with a
typical battery supplying
maybe 1.5V.
• The diagram shows an
oscilloscope screen
displaying the signal from
a d.c. supply.
We can’t see current but we can put it
In to an image using a oscilloscope.
Alternating Current
• If the current constantly
changes direction, it is called
alternating current, or a.c..
• Mains electricity is an a.c.
supply, with the UK mains
supply being about 230V.
• It has a frequency of 50Hz (50
hertz), which means it changes
direction, and back again, 50
times a second.
• The diagram shows an
oscilloscope screen displaying
the signal from an a.c. supply.
Why does the graph go positive and
negative?
Match em’ up
• Match the subject with
the type of current a.c.
or d.c.
•
•
•
•
•
•
Torch
Lamp in the living room
Bicycle lamp
Power shower
Mobile phone
Mobile phone charger
Match em’ up
• Match the subject with
the type of current a.c.
or d.c.
• Torch – d.c.
• Lamp in the living room
– a.c.
• Bicycle lamp – d.c.
• Power shower a.c.
• Mobile phone d.c.
• Mobile phone charger
a.c.
Power pack
• Why on a power pack does the a.c. supply
have yellow plastic circles around the points
whereas d.c. have red and black?
– What do the red and black represent?
D.C.
A.C.
Power pack
• Why on a power pack does the a.c. supply
have yellow plastic circles around the points
whereas d.c. have red and black?
– What do the red and black represent?
– Red = positive +
– Black = negative –
– It flows from negative to positive
DEMO
• Will a bulb light in a.c. and d.c.?
• In a direct current the electrons are travelling
all around the circuit. In a a.c. current they
oscillate only a few centimetres in each
direction – they pass energy on through
energy transfer.
Mains Electricity
• Main circuits have live and neutral wire.
• Mains has an a.c. supply and is very dangerous as
it repeatedly changes form + to – and reaches
over 300V both ways.
– If we were to look at this on an oscilloscope what
would happen if we increased the potential difference
(voltage) – it should make the waves taller.
– What happens if we increase the frequency – it
increases the number of waves/sec so it makes them
more squashed on the screen
HIGHER!!
• 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 that, although the mean
voltage of the mains supply is
about 230V, the peak voltage is
higher.
Oscilloscope Traces
• There are two things that you need to know about on the
oscilloscope
– the time base
– the Y-gain
• The time base allows us to look at how long a wavelength
takes. We can alter the setting of the time-base on the
oscilloscope so e.g. 10 milliseconds per centimetre shows
that each cm on the screen is a time interval of 10s
• The Y-gain can also be set e.g if it is set at 0.5V/cm the it
means that each cm of height is due to 0.5 V.
• Confused????
Alternating current Problems!
• Q: 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.
• The Y-gain setting of the
oscilloscope is 0.5 V/cm.
Determine the amplitude of
the alternating potential
difference.
Practical
• Use the oscilloscopes and make notes on what
happens when you alter the time base setting
and the Y gain.
• You need to be able to read information off
oscilloscopes about frequency.
Plenary
• Draw an oscilloscope trace in your book and
give set y-gains and time bases and set a
question for your partner.
Learning Outcomes
• Distinguish between alternating and direct
current.
• State the frequency of UK mains electricity.
• Describe how the potential of the live wires
varies each cycle.
• State that the potential of the neutral wire is
approximately zero.
• Use oscilloscope traces to compare direct and
alternating potential differences.