AC/DC
• With your partner complete the card sort
game on symbols and terms.
• Distinguish between alternating and direct current-D
• State the frequency and potential difference of UK
mains electricity-C
• Describe how the potential of the live wires varies with
each cycle-B
• Use oscilloscope traces to compare direct and
alternating potential differences and measure the peak
voltage of an ac source-A
• Measure the period and frequency of an ac source
using an oscilloscope or diagrams of oscilloscope
traces- A/A*
What are the similarities and differences
between…
Write down some ideas in your books
Distinguish between alternating and
direct current-D
DC stands for “Direct Current” –
the current only flows in one
direction:
If the current flows in only one
direction it is called direct current
(DC). Batteries and cells supply DC
electricity, with a typical battery
supplying around 1.5 V. The
diagram shows an oscilloscope
screen displaying the signal from a
DC supply.
Time
Parts of a wave
C
A
E
B
D
F
1. Which letter shows the peak or crest of a wave?
2. Which letter shows the trough of a wave?
3. Which two letters represent the amplitude of a wave?
4. Which two letters represent the wavelength of a wave?
Answers
1. C or E
3. B and C
2. D or F
4. C and E or D and F
Distinguish between alternating and direct
current-D
State the frequency and potential difference of
UK mains electricity-C
If the current constantly changes
direction, it is called alternating
current (AC). Mains electricity is an
AC supply, with the UK mains
supply being about 230 V. It has a
frequency of 50 Hz (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 AC supply.
Describe how the potential of the live
wires varies with each cycle-B
• On your laminated template add the wave for
mains voltage and describe how the potential
of the live wires varies with each cycle-B
information to explaining a.c. current.
Use oscilloscope traces to compare direct and alternating
potential differences and measure the peak voltage of an ac
source-A
Measure the period and frequency of an ac source using an
oscilloscope or diagrams of oscilloscope traces- A/A*
The period of an AC supply is the time taken for one complete
oscillation. You can find this by looking at the time between
one peak and the next, between one trough and the next, or
between any two identical places on adjacent oscillations.
In the oscilloscope trace on this page, one horizontal division
represents 5 ms (five milliseconds).
There are four divisions between two adjacent peaks, so the
period is:
4 × 5 = 20 ms
Use oscilloscope traces to compare direct and alternating
potential differences and measure the peak voltage of an ac
source-A
Measure the period and frequency of an ac source using an
oscilloscope or diagrams of oscilloscope traces- A/A*
Frequency
The frequency of an AC supply is the number of oscillations per
second. You can find it from the period:
Frequency = 1 ÷ period (Equation is not given on formula sheet)
Remember to convert to seconds if needed. In this example, 20 ms = 20
÷ 1000 = 0.020 s.
Frequency = 1 ÷ 0.020 = 50 Hz
UK mains frequency is 50 Hz but you may get other frequencies (and
periods) to calculate in the examination.
• Discuss in pairs whether d.c. or a.c. is used for the
following devices, giving reasons:
•
•
•
•
•
•
iPod
bedside lamp
clock radio
wall clock
television remote control
microwave
• Discuss in pairs whether d.c. or a.c. is used for the
following devices, giving reasons:
•
•
•
•
•
•
iPod (DC)
bedside lamp (AC)
clock radio (AC)
wall clock (DC)
television remote control (DC)
microwave
(AC)
AC and DC
• Copy the diagrams
• Describe what happens in AC and DC circuits
in a few sentences, referring to electron flow
• Distinguish between alternating and direct current-D
• State the frequency and potential difference of UK
mains electricity-C
• Describe how the potential of the live wires varies with
each cycle-B
• Use oscilloscope traces to compare direct and
alternating potential differences and measure the peak
voltage of an ac source-A
• Measure the period and frequency of an ac source
using an oscilloscope or diagrams of oscilloscope
traces- A/A*