Monitoring and Measuring a.c.

Question 1

what is direct current?

Answer 1

a one-way flow of electrons from negative to positive terminals of a power supply. Produced by chemical cells and batteries.

Question 2

what is alternating current?

Answer 2

electrons change direction back and forth in the conductor. Produced in UK power station for mains electricity.

Question 3

what is the frequency of mains voltage?

Answer 3

50Hz

a wave is completed 50 times a second

Question 4

what is an oscilloscope used for?

Answer 4

to display a.c. waveforms

Question 5

what is the y-axis on an oscilloscope?

Answer 5

the voltage across a component

Question 6

what is the x-axis on an oscilloscope?

Answer 6

shows how voltage changes over time.

Question 7

what 2 values can be determined using an oscilloscope?

Answer 7

1. frequency

2. peak voltage

Question 8

definition of frequency

Answer 8

number of cycles of the waveform in one second

Question 9

how is frequency calculated using an oscilloscope?

Answer 9

using the time taken for one cycle (period)
period is measured using the ‘timebase’ of the oscilloscope
f=1/T

Question 10

how is peak voltage determined using an oscilloscope?

Answer 10

the maximum vertical deviation of the trace from the central line is measured
this number of divisions is multiplied by the Y gain

Question 11

what is the r.m.s. voltage of an a.c. signal equal to?

Answer 11

the value of direct current which would have the same effect or energy change as the a.c.

Question 12

which is always higher: peak or r.m.s. voltage?

Answer 12

peak voltage

Question 13

what does r.m.s stand for?

Answer 13

Root of the
Mean of the
Square of the voltage

Question 14

equation to work out rms voltage

Answer 14

Vrms=Vpeak/ root 2

Question 15

equation to work out rms current

Answer 15

Irms=Ipeak/ root 2