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8/14/2019 Section 17.3 Transformer
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Manhattan Press (H.K.) Ltd. 1
Section 17.3Section 17.3
TransformerTransformer
Section 17.3Section 17.3
TransformerTransformer
Voltage ratio and turns ratioVoltage ratio and turns ratio
Efficiency of transformerEfficiency of transformer
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change magnitude of
a.c. voltage
17.3 Transformer (SB p. 162)
Transformer
Go to
Discussion 2Discussion 2
soft-iron core woundwith two coils
transformer
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17.3 Transformer (SB p. 162)
Transformer
low voltaged.c. power
supply centre-zerogalvanometer
switch
coilA coil B
transformer
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(1) at the instant
switch is closed
17.3 Transformer (SB p. 162)
Transformer
magnetized
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(2) switch remains closed
17.3 Transformer (SB p. 163)
Transformer
field no changeno induced current
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(3) at the instant
switch is open
17.3 Transformer (SB p. 163)
Transformer
oppose thechange
Iflows inopposite direction
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17.3 Transformer (SB p. 163)
Transformer
change current in a coil
induce current inanother coil
mutual induction
primary coil secondary coil
to an energy
source
soft-iron core
output
CAL
Workshop 2
Transformer
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17.3 Transformer (SB p. 164)
Transformer
light
bulb
low voltage a.c.
power supply
transformer
secondary
coil
light
bulb
primary
coil
a.c. sets upalternating
magnetic field in
core
a.c. induced
glows
continuously
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Expt. 17D
Transformer
to a.c. powersupply
CRO
Voltage ratio and turns ratio
17.3 Transformer (SB p. 164)
secondary coilprimary coil
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s
p
s
p
N
N
V
V=
Vp
Np
VsNs
Voltage ratio and turns ratio
17.3 Transformer (SB p. 165)
Thinking 2Thinking 2
voltage ratio turns ratio
primarycoil
secondarycoil
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input
voltage
(Vp)
output
voltage
(Vs)
soft-iron core
NsNp
circuit symbol
(Ns>Np)
Voltage ratio and turns ratio
17.3 Transformer (SB p. 165)
step-up transformer
(Vs>Vp)
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input
voltageoutput
voltage
soft-iron core
Ns
Np
circuit symbol
(Ns
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Efficiencyof a transformer
Efficiency of transformer
17.3 Transformer (SB p. 166)
Go to
Discussion 3Discussion 3
Thinking 3Thinking 3
%100powerelectricalInput
powerelectricalOutput=
%100pp
ss=
IV
IVe
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inputpower
Efficiency of transformer
17.3 Transformer (SB p. 167)
outputpower
ideal
transformer
(e = 100%)
ppss IVIV =
s
p
p
s
s
p
N
N
I
I
V
V
==
Forideal transformer
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transformer
input
power
output
power
input
poweroutput
power
power
loss
transformer
ideal
(e= 100%)
practical
(e< 100%)
Efficiency of transformer
17.3 Transformer (SB p. 167)
Thinking 4Thinking 4
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Methods to improve efficiency of transformer
17.3 Transformer (SB p. 167)
Thinking 5Thinking 5
input
voltageoutput
voltage
soft-iron core
secondary
coil
primary
coil
(1) resistance
coils heat up
useconduc
tingwiresof
smallerresis
tance
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input
voltageoutput
voltage
soft-iron core
secondary
coil
primary
coil
Methods to improve efficiency of transformer
17.3 Transformer (SB p. 167)
reduceedd
ycurrentby
laminationof
core
(2) eddycurrent
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Methods to improve efficiency of transformer
17.3 Transformer (SB p. 167)
(3) continuousmagnetization&
demagnetization
heats up coils
usesoft-ironcore
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Example 4:Example 4:In a TV set, a 220 V a.c. is stepped up to 22 000 V by a
transformer.(a) What the turns ratio is needed for the transformer?
(b) If the input power is 100 W and the efficiency of the
transformer is 95%, what is the output current?
Solution
(b) Output power = Input power Efficiency
= 100 95%
= 95 W
ByP= VI95 = 22 000 I
I= 0.004 3 A
The current is small because
the voltage is high.
17.3 Transformer (SB p. 168)
(a) By
1001
22000
220
s
ps
p
s
p
s
p
=
=
=
N
N
N
N
N
N
V
V
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Class Practice 3:Class Practice 3:A transformer is used to step down a 220 V mains supply to
run an electrical appliance of 110 V, 550 W.(a) If the primary coil has 1 000 turns, what is the number of
turns of the secondary coil?
By
________ = ________
Ns = ________
Ans
wer
500
110220
17.3 Transformer (SB p. 168)
s
p
s
p
N
N
V
V
=
s
0001
N
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Class Practice 3: (Cont)Class Practice 3: (Cont)(b) What is the current drawn by the appliance?
Current = __________________________
(c) It is found that the current drawn from the mains is 2.8 A.
What is the efficiency of the transformer?
Output power = _______________
Input power = _______________
Efficiency = _______________
Ans
wer
Ans
wer
Ans
wer
Ans
wer
550 W
VI= 220 2.8 = 616 W
A5110
550==
V
P
17.3 Transformer (SB p. 169)
%89%100616
550=
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To section 17.4
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Discussion 2:Discussion 2:
One day, Louis found that the 6 V battery of his
shaver was running out. Luckily, he got an
adaptor. Then the shaver was connected to a
socket through the adaptor. Louse realized that
the electricity from the socket was an a.c. outputof 220 V and wondered why the shaver did not
overload. Do you know why? Answer
The adaptor is a transformer, which changesthe voltage from 220 V a.c. to 6 V a.c. and then 6 V d.c.
Therefore the shaver can work properly.Return to
TextText
17.3 Transformer (SB p. 162)
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Thinking 2Thinking 2
Two resistors Pand Q are connected to an idealtransformer. What are the changes in the
readings of the ammeter and the voltmeter when
the switch Sis open?
Ans
wer
17.3 Transformer (SB p. 165)
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Thinking 2 (Cont)Thinking 2 (Cont)
Return to
TextText
1. When Sis open, the current only passes throughP. The output power is decreased. Since the input
power is directly proportional to the output
power, it is decreased too. ByP=IpV
p, the
current in the primary coil is decreased. Therefore,
the reading of the ammeter decreases.
17.3 Transformer (SB p. 165)
2. The reading of the voltmeter remains unchanged
because by Vp / Vs =Np /Ns, the secondary voltage
depends on the primary voltage and the turns ratio
of the transformer only.
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Discussion 3:Discussion 3:
After Sally has used an adaptor to charge a
mobile phone, the adaptor becomes very hot.
Why? What is the danger if the adaptor becomes
too hot? What should you do to avoid it?Ans
wer
17.3 Transformer (SB p. 166)
The heat produced is due to the energy lost during the
process when the input electrical energy is converted
to the output electrical energy. If the adaptor becomes
too hot, it causes fire. Therefore, we should not usean adaptor to charge an electrical device for too long.
Return to
TextText
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Thinking 3Thinking 3
Recharger of electrical toothbrushHave you used an electrical toothbrush as
shown? Somehow, the plastic recharger can
charge the toothbrush without a metal electricalcontact. How can the recharger charge the
toothbrush?Ans
wer
17.3 Transformer (SB p. 166)
recharger
electrical
toothbrush
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Thinking 3 (Cont)Thinking 3 (Cont)
Return to
TextText
When the toothbrush is placed on the recharger, thecoil inside the toothbrush rests inside another coil in
the recharger. An alternating current passes through
the coil of the recharger to produce a variable
magnetic field. Therefore, a current is induced in the
coil of the toothbrush to charge the toothbrush.
17.3 Transformer (SB p. 166)
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Thinking 4Thinking 4
Can an efficient transformer step up energy?Ans
wer
17.3 Transformer (SB p. 167)
input
voltage(Vp)
output
voltage(Vs)
soft-iron core
secondar
y coilprimarycoil
No. Transformer can only step up
voltage. In all cases, energy is
conserved.
Return to
TextText
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Thinking 5Thinking 5
Explain why thicker wire is used to make coils intransformers. Ans
wer
17.3 Transformer (SB p. 167)
VpV
s
soft-iron core
NsNp
Since the resistance of a wire of greater
cross-sectional area is smaller. It can reduce theheat loss by the coil.
Return to
TextText
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