chapter three transformer

Question 1

diff and menthion one used on the following

1. step down
2. step UP

Answer 1

1. To decrease the voltage and increases the intensity of A.C ( this is used to operates some electric devices )
2. To increase the voltage and decreases the intensity of A.C ( this used to transfer A.C current through wires without large loss of electric )

Question 2

what is the construction and properties of Step down OR step up

Answer 2

1. primary coil connected to A.C source (copper , thick)
2. secondery coil connected to load resistance (copper ., thick)
3. siliconic soft iron core ( thin insulated shets )

Question 3

what is the idea of operation of step down or step up

Answer 3

depends on the mutual induction between two coils

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when A.C currens passes through the primary coil will be associated with variable magnetic flux which is concentrated by the aid of the silicones soft iron core and link the turns of secondary coil so mutual induction takes place

Question 4

waht is the code when you tell step up or step down

Answer 4

step up N_s > N_p , V_s > V_p

step down N_p > N_s , V_p > V_s

Question 5

P_out = ??

P_in = ??

Answer 5

I_s X V_s

I_p X V_p

Question 6

diff efficiency of transforemer and sympol and the rule and ratio

Answer 6

it is the ratio between the electric energy produced from the secondary coil that consumed in the primary coil

OR

itis the ratio between the electric power produced in secondary coil to that given from primary coil

n = I_s x V_s / I_p x V_p = P_out /P_in

n x V_p / V_s =N_p / N_s = I_s / I_p

Question 7

when you say transformer od effciency 80% that means

Answer 7

80 \ 100 is the ratio between the electric energy produced from the secondary coil that consumed from primary coil OR

80\100 it is the ratio between the out put power to the in put power

Question 8

efficiency of transformer has NO unit GR

Answer 8

because it is the ratio betwenn two similar physical quantities

Question 9

effciency of transformer alwys less than 100% GR

Answer 9

because part of electric energy is lost as

1. thermal energy in coil dueto their ohmic resistance
2. thermal energy in soft iron core due to formation of eddy current
3. mechanical energy caused to move the molecules of the metalic core in the magnetic filed
4. loss in magnetic flux when it travel from the primary coil to the secondary coil

Question 10

what is the suggestions to increase the efficiency of the transformer and why

Answer 10

1. the coils should be made of copper with large cross sectional area [to decrease its ohmic resistance as copper has low resistavity and R & 1\A SO the electric energy lost in form of heat energy in coils due to its ohmic resistance wil decrease that will increase the efficiency]
2. metallic core should be divided into thin insulated sheets [to minimize the loss of energy as thermal energy due to eddy current that will increase the efficiency]
3. the metallic core should be made from silliconic soft iron [because it has high resistivity so it will decrease the intensity of the eddy current and minmize the loss of energy as thermal energy ,, its molecules can move easily in the magnetic filed that will minmize the electric energy consumed as mechanical energy to arrange the molecules to increase efficiency of transformer]
4. the primary coil should be placed inside the secondary coil [To decrease the loss of the magnetic flux that is will increase the efficiency]

Question 11

transformer cant be used D.C source GR

Answer 11

because direct current is associated with sable magnetic filed SO no mutal induction tkes palce as there in no vartion in magnetic flux that link the secondery coil except only at the vmoment of switching ON or OF

Question 12

transaformet doesnt consumed electric energy if the circuit of its secondary coil is opened ( switched off ) GR

Answer 12

because opposing E.M.F is generated by self induction in the primary coil which equels E.M.F of the source oppose it

Question 13

using step up transformer in generating stations while step down transformers are used in distribution stations GR

Answer 13

1. to increase the volage and decrease the current intensity ( I_eff ) so the rate of consuming of electric energy as thermal energy through wires due to their ohmic resistance will decrease will decrease P&I²
2. while using step down transformer in distriburion stations to decrease E.M.F to 220 and increase the effective current to suitable value

Question 14

step up trnasformer increases voltage and decreases current while step on trnasformer decreasesvoltage and increases current GR

Answer 14

from the ratio N_p\N_s = V_p\V_s = I_s\I_p

step up

N_s > N_p

Vs > Vp

I_s > I_p

from the ratio N_p\N_s = V_p\V_s = I_s\I_p

N_p > N_s

V_p > V_s

Ip > Is

Question 15

electric power is transferd from the electric generating power stations to the consumers using high voltage and low current GR

Answer 15

By increasing the voltage and decreasing the effective current intensity the rate of consuming of electric energy as thermal energy through wires due to ohmic resistance will decrease P&I²

Question 16

the transformer does not operate if the secondary coil circuit is opened in spite that its primary coil is connected to A.C current

Answer 16

because opposing E.M.F is generated by self induction in the primary coil which equels and oppose that of the source

Question 17

the transformer operates in case when its secondary coil switched on

Answer 17

Depends on the mutual induction between two coils

• when AC current passes through the primary coil it will be associated with variable magnetic flux which is concentrated by the aid of the sillicones soft iron core and link the turns of seconderay coil so mutual induction takes place

Question 18

the core of the transformer is made of thin insulated sheets of silliconic soft iron

Answer 18

To minmize the bad effect of eddy current by decreasing the thickness of the sheets and using siliconic soft iron as it has high resistivity that will increases the efficiency of the transformer

Question 19

the wires of the two coilss of the tranforemer are made of copper

Answer 19

becauce copper has high conductivity and low resistivity so that the energy lost due to its ohmic resistance will decreases and that will increases the efficiency of the transformer