Question 1

Question 6You are required to make an electromagnet from a soft iron bar by using a cell, an insulated coil of copper and a switch. (a) Draw a circuit diagram to represent the process. (b) Label the poles of the electromagnet.

Accepted Answer

Below circuit diagram shows an electromagnet made from a soft iron bar by using a cell, an insulated coil of copper and a switch with its poles labelled:

Question 2

Question 13State two differences between an electromagnet and a permanent magnet.

Accepted Answer

Comparison of an electromagnet with a permanent magnetElectromagnetPermanent magnetIt is made up of soft iron.It is made up of steel.It produces the magnetic field as long as current flows in it's coil. i.e., it produces the temporary magnetic field.It produces a permanent magnetic field.

Question 3

Question 7Draw a labelled diagram to show the various components of a step up transformer.

Accepted Answer

Below diagrams shows the various components of a step up transformer:

Question 4

Question 8Draw a labelled diagram of a step up transformer and explain how does it work. State two characteristics of the primary coil as compared to it's secondary coil.

Accepted Answer

Labelled diagram of a step up transformer is given below:The transformer used to change a low alternating voltage to a high alternating voltage (of same frequency) is called a step up transformer (i.e. Es > Ep).When the terminals of primary coil are connected to the source of alternating e.m.f. a varying current flows through the primary coil. This varying current produces a varying magnetic field in the core of the transformer.Thus the magnetic flux linked with the secondary coil vary due to which an e.m.f. is induced in it. The induced e.m.f. varies in the same manner as the applied e.m.f. in the primary coil varies and thus the induced e.m.f. has the same frequency as that of the applied e.m.f.In a step up transformer the number of turns in the secondary coil is more than the number of turns in the primary coil ( Ns > Np) i.e., turns ratio n > 1 as shown above.Since current in primary coil is more than in the secondary coil (or Ip > Is), so the wire in the primary coil is kept thicker than in the secondary coil.Two characteristics of the primary coil as compared to it's secondary coil are —The number of turns in the primary coil are less than that of the secondary coil.The wire of the primary coil is thicker than that of the secondary coil.

Question 5

Question 10(a) Complete the following diagram in figure of a transformer and name the parts labelled A and B.(b) Name the part you have drawn to complete the diagram in part (a)(c) What is the material of the part named above ?(d) Is this transformer a step-up or step-down ? Give reason.

Accepted Answer

(a) Completed and labelled diagram is shown below:(b) The part drawn is the core.(c) It is a soft iron core.(d) This is a step down transformer because the number of turns in the secondary coil are less than the primary coil.

Question 6

Question 11The diagram in figure shows the core of a transformer and it's input and output connections.(a) State the material used for the core and describe it's structure.(b) Complete the diagram of the transformer and connections by labelling all parts joined by you.(c) Name the transformer : step up or step down.

Accepted Answer

(a) The material used for the core is soft iron. It is made up from thin laminated sheets of soft iron of T and U shape, placed alternately one above the other and insulated from each other by paint (or varnish) coating over them.(b) Completed diagram is shown below:(c) As 220 V is converted to 44 V hence, it is a step down transformer.

Question 7

Question 12A step up transformer ............... the current whereas a step down transformer ............... the current.increases, increasesdecreases, decreasesdecreases, increasesincreases, decreases

Accepted Answer

decreases, increasesReason — In a step up transformer the number of turns in the secondary coil is more than the number of turns in the primary coil ( Ns > Np) i.e., turns ratio n > 1. It increases the a.c. voltage and decreases the current. i.e., Es > Ep and Is < Ip.In a step down transformer the number of turns in the secondary coil is less than the number of turns in the primary coil ( Ns < Np) i.e., turns ratio n < 1. It decreases the a.c. voltage and increases the current. i.e., Es < Ep and Is > Ip.

Question 8

Question 14Assertion (A): A current carrying solenoid when suspended freely sets itself in the north-south direction just like a bar magnet.Reason (R): The end of the solenoid where the direction of current is anti-clockwise behaves as a north pole and the end where the direction of the current is clockwise behaves as a south pole.Both A and R are true and R is the correct explanation of ABoth A and R are true and R is not the correct explanation of Aassertion is false but reason is trueassertion is true but reason is false.

Accepted Answer

Both A and R are true and R is not the correct explanation of A.ExplanationAssertion (A) is true. A current-carrying solenoid behaves like a magnet and aligns itself in the north-south direction when freely suspended, similar to a bar magnet. This behavior is due to the magnetic field produced by the current flowing through the solenoid.Reason (R) is true. The direction of the magnetic field inside a solenoid depends on the direction of the current. The end of the solenoid where the current circulates anti-clockwise acts as a north pole, while the end where the current circulates clockwise acts as a south pole.But here reason does not justify its assertion.

Question 9

Question 16Assertion (A): A transformer can increase the voltage of an alternating e.m.f.Reason (R): This is achieved by keeping the number of turns in the secondary coil less than the number of turns in the primary coil.Both A and R are true and R is the correct explanation of ABoth A and R are true and R is not the correct explanation of Aassertion is false but reason is trueassertion is true but reason is false.

Accepted Answer

assertion is true but reason is false.ExplanationAssertion (A) is true. A transformer can increase the voltage of an alternating electromotive force (e.m.f.). This is accomplished through electromagnetic induction, where a changing magnetic field in the primary coil induces a voltage in the secondary coil.Reason (R) is false. To increase the voltage in the secondary coil, the number of turns in the secondary coil must be greater than the number of turns in the primary coil, not less. This allows for a step-up transformation of voltage.

Question 10

Question 17Assertion (A): A transformer cannot be used with a direct current.Reason (R): The lamination of core prevents the loss of energy due to induced currents in the core.Both A and R are true and R is the correct explanation of ABoth A and R are true and R is not the correct explanation of Aassertion is false but reason is trueassertion is true but reason is false.

Accepted Answer

Both A and R are true R is not the correct explanation of A.ExplanationAssertion (A) is true. A transformer cannot be used with direct current (DC) because it relies on the principle of electromagnetic induction, which requires a changing magnetic field. In DC, the current flows in one direction without changing, so there is no alternating magnetic field to induce voltage in the secondary coil of the transformer.Reason (R) is true. The lamination of the core in a transformer is used to prevent energy loss due to induced currents in the core, known as eddy currents. By laminating the core, the magnetic flux is confined to the individual lamination, reducing the eddy current losses

Question 11

Question 18Assertion (A): The magnitude of induced e.m.f. becomes maximum when the magnetic flux linked with the coil reduces to zero from its maximum value.Reason (R): This happens when the plane of the coil lies in the direction of the magnetic field.Both A and R are true and R is the correct explanation of ABoth A and R are true and R is not the correct explanation of Aassertion is false but reason is trueassertion is true but reason is false.

Accepted Answer

Both A and R are true and R is the correct explanation of A.ExplanationAssertion (A) is true. According to Faraday's law of electromagnetic induction, the magnitude of the induced electromotive force (e.m.f.) in a coil is maximum when the rate of change of magnetic flux through the coil is maximum. This occurs when the magnetic flux linked with the coil reduces to zero from its maximum value, resulting in the maximum rate of change of magnetic flux.Reason (R) is true. This happens when the plane of the coil lies in the direction of the magnetic field. When the coil is in this position, the magnetic flux through the coil is changing most rapidly, resulting in the maximum induced e.m.f.

Question 12

Question 1The magnetic flux through a coil having 100 turns decreases from 5 milli weber to zero in 5 second. Calculate the e.m.f. induced in the coil.

Accepted Answer

Given,Number of turns (N) = 100 turnsDecrease in the flux = 5 milli weber (0.005 weber) to 0,Time = 5 secondFrom relation,e.m.f. induced e = N × rate of increase of magnetic fluxSubstituting the values in the formula we get,e=100×0.0055⇒e=0.1Ve = 100 \times \dfrac{0.005}{5} \[0.5em] \Rightarrow e = 0.1 V \[0.5em]e=100×50.005​⇒e=0.1VHence, the e.m.f. induced in the coil = 0.1 V = 100 m V

Question 13

Question 2The primary coil of a transformer has 800 turns and the secondary coil has 8 turns. It is connected to a 220 V a.c. supply. What will be the output voltage?

Accepted Answer

Given,Np = 800Ns = 8Ep = 220 VEs = ?Now,EsEp=NsNp\dfrac{E_s}{E_p} = \dfrac{N_s}{N_p}Ep​Es​​=Np​Ns​​Substituting the values in the formula we get,Es220=8800⇒Es=220×8800⇒Es=2.2V\dfrac{E_s}{220} = \dfrac{8}{800} \[0.5em] \Rightarrow E_s = 220 \times \dfrac{8}{800} \[0.5em] \Rightarrow E_s = 2.2 V \[0.5em]220Es​​=8008​⇒Es​=220×8008​⇒Es​=2.2VHence, output voltage = 2.2 V

Question 14

Question 3A transformer is designed to give a supply of 8 V to ring a house bell from the 240 V a.c mains. The primary coil has 4800 turns. How many turns will be in the secondary coil?

Accepted Answer

Given,Es = 8 VEp = 240 VNp = 4800Ns = ?We know that,Now,EsEp=NsNp\dfrac{E_s}{E_p} = \dfrac{N_s}{N_p}Ep​Es​​=Np​Ns​​Substituting the values in the formula we get,8240=Ns4800⇒Ns=4800×8240⇒Ns=160\dfrac{8}{240} = \dfrac{N_s}{4800} \[0.5em] \Rightarrow N_s = 4800 \times \dfrac{8}{240} \[0.5em] \Rightarrow N_s = 160 \[0.5em]2408​=4800Ns​​⇒Ns​=4800×2408​⇒Ns​=160Hence, the number of turns in the secondary coil = 160

Question 15

Question 4The input and output voltages of a transformer are 220 V and 44 V respectively. Find (a) the turns ratio (b) the current in input circuit if the output current is 2 A.

Accepted Answer

(a) Given,Ep = 220 VES = 44 VNow,EsEp=NsNp\dfrac{E_s}{E_p} = \dfrac{N_s}{N_p}Ep​Es​​=Np​Ns​​Substituting the values in the formula we get,44220=NsNp⇒NsNp=44220⇒NsNp=15\dfrac{44}{220} = \dfrac{N_s}{N_p} \[0.5em] \Rightarrow \dfrac{N_s}{N_p} = \dfrac{44}{220} \[0.5em] \Rightarrow \dfrac{N_s}{N_p} = \dfrac{1}{5} \[0.5em]22044​=Np​Ns​​⇒Np​Ns​​=22044​⇒Np​Ns​​=51​Hence, turns ratio = 1 : 5(b) Is = 2 AIp = ?From relation,EsIs=EpIp{E_s}{I_s} = {E_p}{I_p}Es​Is​=Ep​Ip​Substituting the values in the formula we get,44×2=220×Ip⇒Ip=44×2220⇒Ip=0.4A44 \times 2 = 220 \times I_p \[0.5em] \Rightarrow I_p = \dfrac{44 \times 2}{220} \[0.5em] \Rightarrow I_p = 0.4 A \[0.5em]44×2=220×Ip​⇒Ip​=22044×2​⇒Ip​=0.4AHence, input current = 0.4 A

Question 16

Question 5A student in a laboratory wants to light a bulb of 6 W, 11 V rating with a supply of 220 V. Mention the kind of transformer he should use. Also, calculate the turn ratio for the transformer recommended by you.

Accepted Answer

Since the supply voltage (220 V) is much higher than the required voltage (11 V), the student needs a step down transformer to reduce the voltage.Given,Ep = 220 VEs = 11 VLet number of turns at primary coil be Np and at secondary be Ns respectively.Now,Turn Ratio = NsNp=EsEp=11220=120\dfrac{N_s}{N_p}=\dfrac{E_s}{E_p}=\dfrac{11}{220}=\dfrac{1}{20}Np​Ns​​=Ep​Es​​=22011​=201​Hence, recommended turn ratio should be 120\dfrac{1}{20}201​

Question 17

Question 15Name the coil of which the wire is thicker in a (i) step up, (ii) step down transformer. Give reason to your answer.

Accepted Answer

(i) Step up transformer — The wire in the primary coil is thicker than that in the secondary coil. It is done because the current in the primary coil is more than in the secondary coil and a thick wire results in less resistance and therefore less loss of energy as heat.(ii) Step down transformer — The wire in the secondary coil is thicker than that in the primary coil. It is done so because the current in the secondary coil is more than in the primary coil and a thick wire results in less resistance and therefore less loss of energy as heat.

Question 18

Question 19Give two points of difference between a step up and step down transformer.

Accepted Answer

Difference between a step up transformer and step down transformer are:Step up transformerStep down transformerIt increases the a.c. voltage and decreases the current. i.e., Es > Ep and Is < Ip.It decreases the a.c. voltage and increases the current i.e., Es < Ep and Is > Ip.The wire in the primary coil is thicker than that of the secondary coil.The wire of the secondary coil is thicker than the primary coil.

Question 19

Question 20Name the transformer used in the (i) power generating station, (ii) power sub station. State the function of each transformer.

Accepted Answer

The transformer used in the —(i) Power generating station — Step up transformer is used in Power generating station to increase the alternating voltage from 11 kV to 132 kV.(ii) Power sub station — Step down transformer is used in Power sub station to decrease the alternating voltage from 132 kV to 33 kV and then further to 11 kV and to 220 V.

Question 20

Question 9How are the e.m.f. in the primary and secondary coils of a transformer related with the number of turns in these coils?

Accepted Answer

E.m.f. in the primary and secondary coils of a transformer are related with the number of turns in these coils as follows —EsEp\dfrac{E_s}{E_p}Ep​Es​​ = NsNp\dfrac{N_s}{N_p}Np​Ns​​where s stands for secondary, p stands for primary, E is E.m.f and N is number of turns in the coils .

Electro-magnetism

Exercise 10A Long Questions

Exercise 10A Short Questions

Exercise 10C Long Questions

Exercise 10C Multiple Choice Type

Exercise 10C Numericals

Exercise 10C Short Questions

Exercise 10C Very Short Questions