1. Alternating Current

2. 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.

3. Starter Draw and label a waveform with the following labels: Amplitude
Wavelength
Peak
Trough
Also, define frequency

4. Starter Draw and label a waveform with the following labels: Amplitude
Wavelength
Draw and label a waveform
with the following labels:
Amplitude
Wavelength
Peak
Trough
Also, define frequency = number of waves passing a fixed point every second (Hz)
Peak
Amplitude
Trough

5. 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.

6. 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.

7. 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?

8. 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

9. 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.

10. 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.

11. 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

12. 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.

13. 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

14. 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.

15. 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????

16. 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.
Have a go at the others

17. 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.

18. Plenary
Draw an oscilloscope trace in your book and give set y-gains and time bases and set a question for your partner.

19. 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.