Alternating currents and Transformers

Question 1

alternating current def

Answer 1

A current which periodically varies between a positive to a negative value with time

(This means the direction of an alternating current varies every half cycle)

Question 2

what kind of curve is the variation of current, or p.d., with time in an alternating current

Answer 2

a sine curve

Question 3

what is peak current and peak voltage

Answer 3

The maximum value of the alternating current or voltage

Question 4

What is shown by the the amplitude of a current-time or voltage-time graph

Answer 4

peak current, or peak voltage

Question 5

Draw a graph of alternating current against time showing the time period, peak current and peak-to-peak current

Answer 5

Question 6

What do Root-mean-square (rms) values of current, or voltage mean

Answer 6

represent the direct current, or voltage, values that will produce the same average power in a resistor as the alternating current, or voltage

Question 7

rms value of an alternating current definition

Answer 7

The square root of the mean of the squares of all the values of the current in one cycle, which is the equivalent direct current that produces the same power

Question 8

What do the components of the Irms equation mean

Answer 8

Question 9

rms value of an alternating voltage definition

Answer 9

The square root of the mean of the squares of all the values of the voltage in one cycle

Or:

The equivalent dc voltage that produces the same power

Question 10

what is the rms current roughly equal to

Answer 10

0.707 × I0, which is about 70% of the peak current I0

Question 11

Vrms vs peak voltage on a graph

Answer 11

Question 12

Equation linking average power supply, Irms and Vrms

Answer 12

Average power = Irms × Vrms

Question 13

Practical applications of alternating current and voltage

Answer 13

power stations produce alternating current

Question 14

What is an a.c voltage on an oscilloscope represented by

Answer 14

a transverse wave

Question 15

What is an d.c voltage on an oscilloscope represented by

Answer 15

a horizontal line at the relevant voltage

Question 16

What are the components needed to produce a wave on an oscillospcope

Answer 16

1. Time-base:
   - When the time-base is switched off, only a vertical line on the voltage-gain axis is seen with its relevant amplitude
   - When the time-base is switched on, a wave will appear across the whole screen and the time period can be measured
2. Voltage-gain (sensitivity):
   - This controls the vertical deflection, or amplitude, of the wave
   - The peak voltage (V0) is the maximum vertical displacement measured from the time axis
   - The peak-to-peak voltage is the vertical displacement between the minimum and maximum values of voltage
   - When the voltage-gain is switched off, only a horizontal line on the time axis will be seen

• For a d.c waveform, only a horizontal line is displayed at the relevant voltage.
  The time-base settings are irrelevant since there is no time period
  The voltage-gain setting is relevant since this is used to read the value of the d.c voltage

Question 17

What is a transformer

Answer 17

A device that changes high alternating voltage at low current to low alternating voltage at high current, and vice versa

Question 18

Rms values of voltage and current equations

Answer 18

Question 19

How is an oscilloscope adjusted so that a trace is shown

Answer 19