Alternating Currents

Question 1

Define the frequency (f) of an alternating current and give its units

Answer 1

The number of cycles it passes through each second (in Hz)

Question 2

Define the peak value for an alternating current

Answer 2

The maximum current which is the same in either direction

Question 3

Define the peak-to-peak value for an alternating current

Answer 3

The difference between the peak value one way and the peak value in the opposite direction
(i.e. twice the peak value)

Question 4

Name the shape of how an alternating current varies with time

Answer 4

Sinusoidal

Question 5

Describe 2 ways in which you can observe an alternating current

Answer 5

1) Using an oscilloscope to display the waveform (i.e. variation with time) of the alternating pd from a signal generator
2) Connect the signal generator to a torch lamp and make the frequency low enough so you can see the brightness of the lamp vary

Question 6

Explain the effect on increasing the output pd from a signal generator on the trace on an oscilloscope

Answer 6

It makes the trace taller, showing that the peak value of the alternating pd has been made larger

Question 7

Explain the effect on increasing the frequency of the signal from a signal generator on the trace on an oscilloscope

Answer 7

It increases the number of cycles on the screen, because the number of cycles per second of the alternating pd has increased

Question 8

Give the equation to show the heating effect of an electrical current

Answer 8

P = IV = I²R

where R is the resistance of the heater element

Question 9

Describe how the power of from an AC source varies with time and how it relates the the variation of current with time

Answer 9

The power increases as the current deviates from 0 and decreases as the current returns back to 0

Question 10

For a sinusoidal current, give the mean power over a full cycle

Answer 10

Half the peak power

Question 11

Define the root mean square of an alternating current

Answer 11

The value of direct current that would give the same heating effect as the alternating current in the same resistor
(The direct current that would give the same power as the mean power for the alternating current)

Question 12

Give the equation to find the root mean square value of an alternating current or pd

Answer 12

rms = peak value / √2

Question 13

Give the equation for mean power supplied to a resistor for rms values of V and I

Answer 13

P = I(rms)²R = V(rms)²/R = I(rms)V(rms)

Question 14

Describe the set up of an oscilloscope

Answer 14

An electron gun (filament) at one end of the glass tube emits electrons in a beam towards a fluorescent screen at the other end of the tube. Light is emitted from the spot on the screen where the electron beam hits the screen.
Deflecting plates in both the X and Y axis deflect the beam of electrons passing through them. The deflection of electrons is determined by the pd across the plates and determines the position of the electron beam on the fluorescent screen

Question 15

Describe how an oscilloscope displays a waveform

Answer 15

• The X-plates are connected to the oscilloscope’s time base circuit which changes the pd across the plates at a constant speed (moving the spot left to right across the screen) then reverses the pd very quickly. It is calibrated to mscm⁻¹.
• The AC pd to be displayed is connected to the Y-plates so the spot moves up and down as the pd across the plates changes. It is calibrated to Vcm⁻¹ and is called the y-sensitivity or y-gain

Question 16

Describe how you would use an oscilloscope as a dc voltmeter

Answer 16

Applying a constant pd to the Y-input makes the spot move across the screen at a constant height because its vertical displacement does not change for a constant pd.
The spot traces a line above or below the ‘0 pd’ line according to whether the pd applied is positive or negative.
By measuring the displacement of the line, we can measure the direct pd

Question 17

Describe how you would use an oscilloscope to measure the speed of ultrasound

Answer 17

By using the time base circuit to trigger an ultrasonic transmitter to send out a short pulse of ultrasonic waves. If the receiver is applied to the Y-input of the oscilloscope, the waveform of the received pulse can been seen on the oscilloscope screen.
By measuring the horizontal distance of the screen from the leading edge of the pulse to the start of the spot’s sweep, the travel time from the transmitter to the receiver can determined

Question 18

Give 2 examples for a use of an oscilloscope rather than to show the waveform of an alternating current

Answer 18

1) Using the oscilloscope as a dc voltmeter

2) Measuring the speed of ultrasound