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 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

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. Have a go at the others

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.