1. Household Electricity
02 July 2019
Household Electricity
Objectives
Be able to describe what AC and DC are and be able to read oscilloscope traces.
HSW: AF2 – Understanding the applications and implications of science
Used before in: p2.3 – static electricity
Will use again in: lesson 2 – electrical safety
PLTS: Team workers – Work collaboratively with others.
Keywords
AC, DC, Alternating, Direct, Current, Charge, Time, Cathode Ray Oscilloscope, Period, Frequency, Hertz, Potential Difference, Voltage, Volts, sinusoidal

2. Outcomes
All students should be able to describe the difference between AC and DC.
Most students should be able to compare DC voltages on an oscilloscope and read the peak voltages of AC.
Some students should be able to determine the period and frequency of alternating current from reading a cathode ray oscilloscope trace of AC.

3. Direct current
Cells and batteries supply electric current which always flows in the same direction.
This is called direct current (d.c.).
Direct current always flows the same way

4. Alternating current
An alternating current (a.c.) is one which is constantly changing direction. ~
a.c. power supply
Alternating current constantly changes direction.
The lamp works with a.c. and d.c.

5. Oscilloscope
Your teacher will demonstrate what DC and AC look like on an oscilloscope.
Can you sketch the trace for each?
Can you explain why they look like that?
Y-gain
Time- base
Click the link to go to the circuit simulation

6. How electricity arrives into our homes
Mains Electricity
The electricity supplied to our homes is called Mains Electricity.
It is an alternating current supply.
In the UK the current changes direction every 1/100th of a second.
This means it completes a complete cycle of changes every 1/50th of a second.
It therefore has a frequency of 50 cycles per second
or 50 hertz (50 Hz).
How electricity arrives into our homes

7. The LIVE and NEUTRAL terminals
UK mains supply is rated at about volts.
This means that it has the same effect as a 230V d.c. battery on devices like a lamp.
One side of the a.c. supply changes constantly between +325V and – 325V.
This terminal is called the LIVE.
Touching this terminal can be fatal!
The other terminal remains at about 0V.
This terminal is called the NEUTRAL.
NEUTRAL
LIVE
In the diagram, which trace shows which terminal of the plug?

8. Voltage variation of the LIVE terminal
The voltage of the LIVE terminal varies SINUSOIDALLY between +325V and – 325V taking 1/50th or 0.02 second to complete one complete cycle.

9. Choose appropriate words to fill in the gaps below:
_______ current (d.c.) is a current that always flows in ____ direction around a circuit. It is supplied by cells and ________.
Alternating current (a.c.) constantly ________ in direction. The _______ supply to our homes is a.c. In this case the a.c. is supplied at an effective voltage of ______ and a frequency of ______.
An _________ can be used to display and __________ a d.c. or a.c. waveform.
Direct
one
batteries
reverses
mains
230V
50Hz
oscilloscope
measure
WORD SELECTION:
batteries
mains
one
50Hz
measure
Direct
230V
oscilloscope
reverses

10. Measuring d.c. potential difference
All three diagrams below show the trace with the time base on and the Y-gain set at 2V cm-1.
Diagram a shows the trace for pd = 0V.
Diagram b shows the trace for pd = +4V
Diagram c shows the trace for pd = -3V.

11. Measuring a.c. potential difference
Let the time base setting be 10ms cm-1 and the Y-gain setting 2V cm-1.
In this case the waveform performs one complete oscillation over a horizontal distance of 2 cm.
Therefore the period of the waveform is 2 x 10ms
period = 20 ms
as frequency = 1 / period
frequency = 1 / 0.020s
= 50 Hz.
The peak-to-peak displacement of the waveform is about 5cm.
Therefore the peak-to-peak pd is 5 x 2V
Peak-to-peak pd = 10V

12. Question 1
Measure the approximate period, frequency and peak-to-peak pd of the trace opposite if:
Time base = 5ms cm-1
Y-gain = 5V cm-1
peak-to-peak pd ≈ 20V

13. Question 2
Measure the approximate period, frequency and peak pd of the trace opposite if:
Time base = 2ms cm-1
Y-gain = 0.5V cm-1
peak pd ≈ 1.3V

14. Question 3
The trace shows how a waveform of frequency 286 Hz and peak-to-peak pd 6.4V is displayed.
Suggest the settings of the time base and Y-gain amplifier.
The peak-to-peak displacement of the trace is about 3.7 cm.
Therefore the Y-gain setting is 6.4V / 3.7cm
≈ 2V cm-1

15. Tell me two things... (for each)
that you have done well this session
that you know now that you didn’t know at the start
that you could do better next time
that you would like to know more about 2

16. current
Volt
Hertz 50
230V AC
voltmeter
Amp DC