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Alternating Currents

Teacher: Mr. B. BurnettDate: January 28, 2013

Class: 6A- Phys1

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Recall Transformer

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A.C Generator

Revisited(Example)

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What is an Alternating

Current?O The term Alternating current (AC) means

that the direction of current flowing in a circuit

is constantly being reversed back and forthcyclically.

O ACis created by generators in power plants,

and other sources. ThisACis delivered to our

homes and businesses by the power lines we

see everywhere.

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AC vs DC

Alternating

Current(AC) Direct Current (DC)

O Current flow is bi-directional (flows back and

forth in two directions) Eg.

Transformer, AC

Generator

O Current flow is uni-diectional (flows in one

direction only). Eg.

Batteries, Car battery, fuel

cells, solar cells etc.

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Important DefinitionsO Peak value (Symbol: V0, I0)

O - This is the maximum value of current or voltage.

O V0= Vrms x 2, I0 = Irmsx 2O

Peak-to-Peak value(Vpp, Ipp)This is the distance fromthe max in the positive

half of the cycle to the min in thenegative half of the cycle. It is doublethe peak value.

O Frequency- This is the number of cycles per second. Thedirection of current is switching back and forth 60times each second (60Hz) in commercial use.

O RMS (Root-mean-squared) value (Effective value)- Therms value of an ac voltage is the DC value that gives thesameamount of heat or light (or produces the same heatingeffect).

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The RMS voltage

RMS (Dc equivalent)

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AC Waveforms

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A.C

1

2

1st

halfcycle

2nd

half

cycle

On every half-cycle, the

polarity of the

alternating voltage

reverses. Hence, the

direction of the current

changes every half

cycle. In the circuit on

the left, terminal 1 is

positive w.r.t terminal 2

for the 1sthalf cycle then

the polarity changes

where terminal 2 is now

positive w.r.t terminal 2

for the next half cycle

and the process is

repeated.

Resistor, R

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Example of AC (Half-wave

rectification)

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Practice Question 1O A sine wave of voltage varies from zero to

a maximum of 100V. How much is the

voltage at the instant of 300

of the cycle?450, 900, 2700 (H.W)?

O V=V0sin

O V= 100 sin 300= 100 x 0.5

O V= 50V

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Advantages of ACO Can be used to transmit powerover long distances to

run AC systems, e.g radio set (via power lines).

O It is less expensive than DC because you can decrease

or increase the AC power very easily.

O you can use high voltageswith small currents to

reduce losses when you transmit power.

O A home wired for AC power is ready for utility or

generator power if / when it is available.

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Example: Hair DryerO a) Calculate the resistance and the peak

in a 1000-W hair dryer connected to a

110-V line in Jamaica?O b) What happens if it is connected to a

220-V line in Britain?

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