Physics Unit 1 Alternating Currents

1) Alternating current and power

2) Using an oscilloscope

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Alternating current measurements

An ac is a current that repeatedly reverses its direction. In one cycle of an ac, the charge carriers move in one direction then the reverse direction then re-reverse direction.

The frequency of an ac is the number of cycles it passes through each second. unit= hertz (hz)

The peak value of an ac is the maximum current or pd which is the same in either direction.

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Observing alternating current

Using an oscilloscope to display the waveform of the alternating pd from a signal generator:

1) Increasing the output pd makes the oscilloscope trace taller. This shows the peak value of the alternating pd has been made larger.

2) Increasing the frequency increases the number of cycles on the screen. This is because the number of cycles per second of the alternating pd has increased.

Connect the signal generator to a torch lamp and make the frequency low enough so you can see the brightness of the lamp vary:

1) At a very low frequency, the lamp light will light up and fade repeatedly.  The lamp is brightest at its peak value.

2) If the frequency is raised gradually, the lamp flickers faster and faster until the variation is too fast to notice. The mains frequency is too high to cause flickering.

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Ac equations

The direct current that would give the same power as the mean power is called the root mean square value of the alternating current, Irms.

The root mean square value of an alternating current is the value of direct current that would give the same heating effect as the alternating current in the same resistor.


The root mean square of an ac or pd=

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An oscilloscope

An oscilloscope tube

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Usinn an oscilloscope

To display a waveform:

1) The Xplates are connected to the oscilloscope's time base circuit which makes the spot move at constant speed left to right across the screen, then back and forth much faster.

2) The pd to be displayed is connected to the Yplates via the Y input so the spot moves up and down as it moves left to right across the screen. As it does it traces out a waveform on the screen.

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Using an oscilloscope

The diagram show the trace when an alternating pd is applied to the Y input. The screen is marked with a centimetre grid

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Using an oscilloscope as a dc voltmeter

An oscilloscope can be used to measure a constant pd as well as the peak valure of an ac. Applying a constant pd to the Y input make the spot move across the screen at a constant height. Its vertical displacement does not change for a constant pd. The spot therefore appears to be a horizontal line on the screen.

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Measuring the speed of ultrasound

The time base circuit of an oscilloscope can be used to trigger an ultrasonic transmitter so it send out a short pulse of ultrasonic waves. An ultrasonic receiver can be used to detect the pulse. If the receiver signal is applied to the Yinput of the oscilloscope, the waveform can be seen on the oscilloscope screen.

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Miss Meera J


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