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CIE A2-LEVEL

ALTERNATING CURRENT

9702

Characteristics of alternating currents
 AC = Current that is changing all the time
...
In certain applications, different waveforms are used, such
as square waves
...


Alternating current
Amplitude, I0

Alternating voltage
Amplitude, V0

I  I o sin t

V  Vo sin t

 Power generated at any instant is written as:

P  Vo I o sin 2 t

Power is varies all the time because current and voltage are varying all the time
...



R
...
S Value
...
M
...
C, Vrms and Irms will give the same amount of average power in a resistor
 RMS value of constant voltage – the value of constant voltage that produces the same power as
alternating voltage
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[2]
 that value of the direct current producing same (mean) power / heating
 in a resistor
Q2: The mean value of an alternating current is zero
...
[2]
2 2
 heating depends on current /I and
2 2
 current /I is always positive

FAUZIAH IBRAHIM

1

CIE A2-LEVEL

ALTERNATING CURRENT

9702

The Transformer
 The great thing about A
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 When alternating emf (Vp) is applied to a primary coil, then emf is induced in the secondary coil
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This produces an alternating magnetic field in the soft iron
core
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There is thus
an alternating e
...
f
...

 The voltage ratio between primary and secondary coil is dependent on the ratio of the number of turns on the
two coils
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 Structure of transformer
 Np turns and Ns turns on the primary and secondary coils wound around the core
 The core is laminated with soft iron to reduce heating due to:
 Repeated magnetized and demagnetized of the core
 Reduces energy lost as thermal energy due to eddy current
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C must be supplied to primary coil
 Change in flux linkage, induce e
...
f
 If D
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m
...

 The core is designed for maximum flux linkage
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Resistive heating in the primary and secondary coils
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Heating of the core due to eddy currents
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 Laws & Transformer
 Faraday’s Law : the emf induced is proportional to the rate of change of magnetic flux linkage
 Changing current in primary coil will give rises to changing flux in core
 Flux links with the secondary coil
...

 Induced current in secondary produces magnetic field which oppose field produced in primary
 Thus, the input p
...
d are NOT in phase

Example Questions :
Q1: State why the iron core is laminated? [2]
 to reduce power loss in the core
 due to eddy currents/induced currents
Q2: State why the primary and secondary coils are wound on a core made of iron [1]
 e
...
prevent flux losses / improve flux linkage
Q3: Suggest why thermal energy is generated in the core when the transformer is in use [3]
 flux in core is changing
 e
...
f
...
Suggest why :
(i) high voltages are used,
...
[2]
 voltage can be (easily) stepped up/down
 transformers only work with a
...

 generators produce a
...

 easier to rectify than invert

FAUZIAH IBRAHIM

4

CIE A2-LEVEL

ALTERNATING CURRENT

9702

Rectification
Rectification – a process converting AC to DC, achieve using diode
Function : allow current to flow in one direction only

Type

Half Wave

Full Wave (Bridge rectifier)

Number of diode

Single diode

Four diodes

Circuit

Input & output
graph

Example Question:
Q1 : Suggest one advantage of the use of a bridge rectifier, rather than a single diode, for the rectification of
alternating current [1]
 power supplied on every half-cycle or
 greater average/mean power

FAUZIAH IBRAHIM

5

CIE A2-LEVEL

ALTERNATING CURRENT

9702

 Smoothing
 In order to produce steady d
...
d/voltage/current does not fall to zero







Capacitor in parallel with resistor
Capacitor is charges up and maintains the voltage at high level
Voltage drop: capacitor discharges gradually
Voltage rise: capacitor charges again
Rectified AC will be smoother if:
 Capacitance has larger value
 C x R has larger value, hence C will discharge more slowly

Example Questions
Q1 : Explain what is meant by smoothing? [1]
 (output) p
...
/ voltage / current does not fall to zero or
 range of (output) p
...
/ voltage / current is reduced (any sensible answer) or
 reduction in the variation of the output voltage/current
Q2 : State the effect of the value of the capacitance of the smoothing capacitor in relation to smoothing

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