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Alternating current

Most students learning about electricity begin with what is known as direct current (DC), which is electricity flowing in a constant direction. DC is the kind of electricity made by a battery, with definite positive and negative terminals.

However, we have seen that the electricity produced by some generators alternates and is therefore known as alternating current (AC). The main advantage to AC is that the voltage can be changed using transformers. That means that the voltage can be stepped up at power stations to a very high voltage so that electrical energy can be transmitted along power lines at low current and therefore experience low energy loss due to heating. The voltage can then be stepped down for use in buildings and street lights.

Interesting fact

In South Africa alternating current is generated at a frequency of 50 Hz.

The circuit symbol for alternating current is:

Graphs of voltage against time and current against time for an AC circuit are shown in [link]

Graph of current or voltage in an AC circuit.

In an ideal DC circuit the current and voltage are constant. In an AC circuit the current and voltage vary with time. The value of the current or voltage at any specific time is called the instantaneous current or voltage and is calculated as follows:

i = I m a x sin ( 2 π f t + φ ) v = V m a x sin ( 2 π f t )

i is the instantaneous current. I m a x is the maximum current. v is the instantaneous voltage. V m a x is the maximum voltage. f is the frequency of the AC 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:

I r m s = I m a x 2 V r m s = V m a x 2

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

Exercise - alternating current

  1. Explain the advantages of alternating current.
  2. Write expressions for the current and voltage in an AC circuit.
  3. Define the rms (root mean square) values for current and voltage for AC.
  4. What is the period of the AC generated in South Africa?
  5. If V m a x at a power station generator is 340 V AC, what is the mains supply (rms voltage) in our household?
  6. Draw a graph of voltage vs time and current vs time for an AC circuit.

Capacitance and inductance

Capacitors and inductors are found in many circuits. Capacitors store an electric field, and are used as temporary power sources as well as to minimize power fluctuations in major circuits. Inductors work in conjunction with capacitors for electrical signal processing. Here we explain the physics and applications of both.


You have learnt about capacitance and capacitors in Grade 11.

In this section you will learn about capacitance in an AC circuit. A capacitor in an AC circuit has reactance . Reactance in an AC circuit plays a similar role to resistance in a DC circuit. The reactance of a capacitor X C is defined as:

X C = 1 2 π f C

where C is the capacitance and f is the AC frequency.

If we examine the equation for the reactance of a capacitor, we see that the frequency is in the denominator. Therefore, when the frequency is low, the capacitive reactance is very high. This is why a capacitor blocks the flow of DC and low frequency AC because its reactance increases with decreasing frequency.

Questions & Answers

what does the word emitted mean?
Mwinga Reply
to be ejected or released
Ok thanks
what are the hooke laws
Tyriek Reply
what do really asked in exam
Leiyo Reply
hi please help me how to balance redox reactions?
Brian Reply
Which equation do u wanna balance
i tried to write it but my phone just can't write it Cr2 O72-(aq) + H2S (aq) Cr3 (aq) + S(s)
show is the equation
hello. what is the difference between a primary, secondary and tertiary alcohol
Thokozani Reply
primary the C=bonded to 1 Carbon atom.... secondary=bonded to 2 carbon atoms tertiary=bonded to 3 carbon atoms
I hope you're answered
thanks Christina
My pleasure
Thank you Christina. This is very helpful 😀👍👍💯
Anytime Thokozani
How to calculate pH?
thank you guys i didn't know about the primary, secondary and tertiary alcohols
thank you Christina
Hey guys which topic does AC nd DC generator falls
thank you
can somebody help me with functional isomers
Baningi Reply
function isomer has the same molecular formula but different functional group
what's momentum
Luvelo Reply
the product of an objects mass times velocity. it is mainly prevalent in collisions.
in order to produce an interference pattern, the waves used must be what?
Methane contains C and H. This compound is
Juan Reply
ketones's functional group
Moloi Reply
Why does the carboxyl group have acidc properties?
Evi Reply
carboxylic acids
Besides improving appearance what is another reason why medal is electro plated
Elizabeth Reply
it is made of metal
what is different between Dependent variable and independent variables
difference between A.c and D.c
AC has slip rings DC has split rings commutator
to avoid rust and to improve strength and conductivity
the variable whose value doesn't depend on other is known as independent variable ex time
are UV rays dangerous?
Khathutshelo Reply
what's this?
if there is extensive exposure, it can lead to skin cancer
yes depend upon their intensity
i hv a problem with this chapter because i also have problem with Newton's laws of motion
Kearabetswe Reply
I have a problem with calculation in Newton's law of motion
me also VuyokAzi
what's the question
Which area in particular?
just get the hang if free body diagrams,know the directions.
I know how to draw free diagram I have a problem with calculation in second law
what is the wave model of atom ?
Trazy Reply
guys what is momentum
@Luvelo... Momentum is the product of an object's mass and its velocity.
the model based on wave nature of electron
Can you state the Doppler effect in words
Nombuyiselo Reply
State Coulomb's law in words
the magnitude of the electrostatic force exerted point on each other is directly proportional to the product on magnitude of charges and inversely proportional to the square distance between them
the change in frequency of the sound detected by a listener because sound source and listener have different velocity

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Source:  OpenStax, Siyavula textbooks: grade 12 physical science. OpenStax CNX. Aug 03, 2011 Download for free at http://cnx.org/content/col11244/1.2
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