Everything Science Grade 12

CHAPTER 13. ELECTRODYNAMICS 13.3

time

current OR voltage

Figure 13.9: Graph of current or voltage in an AC circuit.

called the instantaneous current or voltage and is calculated as follows:

i = Imaxsin(2πft + φ)

v = Vmaxsin(2πft)

i is the instantaneous current. Imaxis the maximum current. v is the instantaneous
voltage. Vmaxis the maximum voltage. f is the frequency of theAC and t is the time
at which the instantaneous current or voltage is being calculated.
The average value we use for AC is known as the root mean square (rms) average. This
is defined as:

Irms =

Imax

√

2

Vrms =

Vmax

√

2

Since AC varies sinusoidally, with as much positive as negative, doing a straight average
would get you zero for the average voltage. Therms value by-passes thisproblem.

Exercise 13 - 2

1. Explain the advantages of alternating current.


2. Write expressions forthe current and voltagein an AC circuit.


3. Define the rms (rootmean square) values forcurrent and voltage for AC.


4. What is the period of the AC generated in South Africa?


5. If Vmaxat a power station generator is 340 V AC, whatis the mains supply (rms
   voltage) in our household?


6. Draw a graph of voltage vs time and currentvs time for an AC circuit.

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