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

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.

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

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.

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

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

The LIVE and NEUTRAL terminals UK mains supply is rated at about 230 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?

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.

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

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.

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

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

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

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

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

current Volt Hertz 50 230V AC voltmeter Amp DC