I- Put () or () infront each of the follwoing statements:
1- When the No of flux lines linked with a coil decreases, a forward e.m.f. is induced between its terminals.
1-
2- The induced e.m.f in the dynamo's coil is max. when its plane is perpendicular to flux lines.
2-
3- In transformers, the ratio is applicable whether the transformer is ideal or not.
3-
4- The applied torque on the coil of an electric motor is max. when its coil is perpendicular to flux lines.
4-
5- The time of current decay (break) in a coil is larger than that in straight wire.
5-
6- Lenz's rule is used to detect the direction of the induced current in a coil.
6-
7- A step up-transformer is used between power stations and distribution lines.
7-
8-The induced e.m.f in a coil increases when the coil has an iron core.
8-
9- The induced e.m.f in a straight wire is max. when it moves parallel to flux lines.
9-
10- The instantaneous e.m.f in a dynamo could be written as: .
10-
11- The function of Induction furnace is based on the eddy currents.
11-
12- The induced e.m.f in secondary coil is "backward" when the primary circuit is switched off.
12-
13- Fleming's left-hand rule is used to detect the direction of induced current in sec. coil.
13-
14- The role of commutator is to give unidirectional current inside the coil of a dynamo.
14-
15- The role of commutator is to reverse the electric current in the coil of an electric motor.
15-
16- The transformer that step-up the a.c. voltage, will step-up the current intensity too.
16-
17- Transformers could be used to step-up or down a D.C. voltage.
17-
18- The mutual inductance of two coils depends on the seperation between them.
18-
II- Complete the following:
1- The instantaneous e.m.f in dynamo in terms of the max. e.m.f is given by the relation……..
1- .
2- When the plane of coil in dynamo is parallel to flux lines, the produced e.m.f is......
2- maximum.
3- The produced e.m.f by a dynamo is reduced to half its max. value, when the plane of its coil makes an angle of..... to the flux lines.
3- .
4- Transformers are used to step-up or down..... voltage.
4- a.c.
5- When the two slip rings are replaced by a commutator, in a dynamo, the obtained current is......
5- unidirectional.
6- A step-up transformer is designed such that the Number of sec. coil is..... than that of the............ one.
6- greater - primary
7- The efficiency of a transformer is the ratio between..... to....., and is expressed as percentage.
7- sec. power - prim. power.
8- The delay in current's growth of an induction coil is due to..... when the coil's circuit is switched......
8- backward e.m.f. - on
9- The effective value of an a.c. current is given by:Ieff= ………...
9- = 0.707 Imax.
10- The induced e.m.f by self induction in a coil is given by . V, where E is....., L is....., is..... & V is......
10- e.m.f - self inductance - current rate of change - unit of E "volt".
11- In a D.C. dynamo, the number of divisions of commutator is..... that of coils, and is fixed to..... such that the two pieces in contact with..... have their coil..... to flux lines.
11- double - armature - bruches - parallel.
12- The role of commutator in dynamo is to..... the produced current in..... circuit, while its role in electric motors is to..... the current inside..... each......
1- The uniformity of the speed of rotation of the electric motor.
1- Because of the induced e.m.f . The current in coil is , where E is the e.m.f of battery, R total resistance of circuit. regulates. "I" and the speed of rotation. i- When the speed increases for some reasons, also will increase, leading to a decrease in "I" and the speed of rotation till it reachs the uniform speed. ii- When the speed decreases for some reason, also will decrease, leading to an increase in "I" and the speed of rotation till it reaches the uniform speed.
2- The rate of decay of current intensity in a straight wire is greater than that in an induction coil.
2- Due to the larger self induction of coil, since it contains N turns,.
3- The torque acting on the coil of electric motor is zero when its plane is normal to flux lines.
3- Because the two brushs are in contact to the isolator in commutator, and no current passes in coil. Also the moment of couple vanishes when the its plane is normal to flux lines.
4- A step-up transformer is used between power station and distribution lines.
4- To step-down the current intensity in cables, and hence reduce, to a great extent, the lost power in this cables. (Since power a I2).
5- A transformer couldn't step-up or down D.C. voltages.
5- Because the D.C in prim. coil will build a D.C. magnetic field in iron core. And a steady magnetic flux linked with the sec. coil produce no e.m.f in the sec. coil. .
6- There is no transformer of 100% efficiency.
6- Because energy loss is one or more of the following: a- Heat energy generated in coils due to its resistance. b- Eddy currents in the iron core. c- Mechanical energy in moving the molecules of core. d- Leakage of flux lines from prim.coil.
7- A neon lamp, connected in parallel to an Electro-Magnet, will glow when the circuit is switched off.
7- Because the induced backward e.m.f by self induction is large.
8- Replacing the two slip rings by a commutator, the produced current by a dynamo is rectified.
8- Because the commutator is fixed to armmatur in a way that makes the two pieces incontact with brushs recive a current has the same direction in both halfcycles.
9- The induced e.m.f in dynamo is zero when its coil is normal to flux lines.
9- Because when
10- The usage of the effective value of a.c. current, not its average value, to evaluate the consumption of electric energy.
10- Because the avarge value of a.c. current = zero so the effective value is used. Which is based on the heat effect of the electric current.
11- In the dynamo, the commutator is fixed to armature such that the plane of its isolating material is normal to plane of coil.
11- To get a contact between the two brushs and the isolating material in commutaton when the coil is normal to flux lines, where no e.m.f is generated.
12- The transfromer that steps-up a.c. voltage, steps-down the current intensity.
12- Because the prim. power = sec. power
13- An iron core is used to link the prim. & sec. coils in transformers.
13- To concentrate flux line, between the two coils since the permiability of iron is large.
14- The iron core of the transformers and electric motors is laminated and isolated.
14- To reduce the intensity of eddy current inside the core.
15- The sec. coil in transformer is wounded around the prim. coil.
15- To minimize the leak of magnetic flux lines between the two coils.
IV- Give the scientific term for each of the following:
1- The ratio between the sec. power, in a transformer, to that of the prim.
1- Effeciency of a transformer.
2- Numerically equal to the induced e.m.f, by self-induction, between the terminals of an induction coil when its current changes in a rat of one amp. per sec.
2- Self inductance of a coil.
3- The D.C. currents that produce the same power or same rate of thermal effect produced by the a.c. current.
3- The effective value of an a.c. current.
4- The self inductance of a coil that produces between its terminals, an induced e.m.f of one volt when its current changes by one amp. per sec.
4- The Henry.
5- Whenever an electromotive force is induced, the induced current must be in a direction such as to oppose the change producing it.
5- Lenz's rule.
6- .
6- Faraday's law.
7- The phenomenon of producing an induced e.m.f between the terminals of coil, due to the change of current intensity of another coil linked with it.
7- Mutual induction.
8- Extend the thump, forefinger and middle finger, of the right hand, mutually perpendicular to each other. Let the forefinger points to the direction of the field and the thumb to the direction of motion, then the middle finger will points to the direction of the induced current.
8- Fleming's right-hand rule.
9- The phenomenon in which an induced e.m.f is produced between the terminals of a coil due to the changes in current intensity of the current it supports.
9- Self induction.
10- The induced current in sec. coil when the current intensity of the primary coil is increased.
10- Backward current.
11- The induced current in sec. coil when the current intensity of the primary coil is decreased.
11- Forward current.
V- Show what may happen to the following under the given conditions:
1- The current produced by an a c dynamo WHEN the two slip rings are replaced by a commutator.
1- The current is a unidirectional or D.C.
2- A neon lamp connected in parallel to an Electro-manger WHEN the circuit is switched off.
2- The lamp glows inistantaneously.
3- The applied torque on the coil of an electric motor WHEN its coil is parallel to flux lines.
3- The torque is max.
4- The produced e.m.f. by an a.c. dynamo WHEN its coil is parallel to the flux lines.
4- The e.m.f. is max.
5- The intensity of eddy current in the iron core of a transformer WHEN the core is laminated and isolated.
5- The eddy current is reduced to a minimum.
6- The speed of rotation of an electric motor WHEN its speed is reduced for some reason.
6- The induced backward e.m.f causes the speed to return to the uniform speed.
7- The magnitude of the induced e.m.f. by self induction WHEN an iron core is induced inside coil.
7- The induced e.m.f. increases.
8- The current intensity of an a.c. source WHEN its voltage is stepped-up by a transformer.
8- The current intensity is decreased.
9- The power loss in transferring a.c. power over long distance using a cable system WHEN a step-up transformer is used between the power station and cable system.
9- The power loss is decreased to a great extent.
VI- Choose the most correct answer form brackets:
1- The function of the induction furnace is based on (forward current - backward current - eddy currents)
1- eddy currents.
2- The induced e.m.f in a.c. dynamo is max. when its coil is (parallel - normal - inclined) to flux lines.
2- parallel.
3- The self inductance of a coil is measured in .
3- all the previous.
4- The directions of the induced current in an induction coil is determined by the rule of (Fleming's left-hand - Fleming's right-hand - Lenz)
4- Lenz's rule
5- The direction of the induced current in a straight wire is determined by the rule of (Fleming's left-hand - Fleming's right-hand - Lenz).
5- Fleming's right-hand.
6- The applied torque on the coil of an electric motor is max. when its coil is (parallel - normal - inclined) to the flux lines.
6- parallel.
7- To reduce the intensity of the eddy currents in the iron core of a transformer, it is made to be (laminated - isolated - made from an alloy of Fe & Si - all the previous)
7- all the previous.
8- The mutual inductance between two coils increases when the two coils (acquire an iron core - get closer - all the previous).
8- all the previous.
9- The transformer with 100% efficiency is considered (normal - actual - ideal).
9- ideal.
10- The coil of the electric motor reaches a uniform speed of rotation due to the induced (current - back e.m.f - forward e.m.f).
10- backward e.m.f.
11- The current of a dynamo is rectified by replacing the two slip rings by (two brushes - a double slit metalic piece - commutator).
11- commutator.
12- The current in the coil of an electric motor is reversed by (two brushes - a double slit metalic piece - commutator).
12- commutator.
13- To calculate the effective value of an a.c. current, it's max. value is divided by
13-
VII- Problems:
1- A coil of 700 turns, each of 4 . Find the max. and average induced e.m.f. over one complete cycle, when it rotated inside a magnetic field of 0.1 T with a speed of 6000 cycle per minute.
1- Avarage e.m.f = zero, because it varies from + Emax to - E max over one cycle.
2- In the shown fig. the prim. coil has 1000 turns, each of 6 area and the resistance of rheostat was 30 . The sec. coil has 2000 turns have the same area and a load resistance of 1 K. . Find: i- The current intensity in prim. cirucuit when switched on. ii- The magnetic flux of the prim. coil. iii- If the circuit is opened and the current collapse to zero in 0.05 sec, find the average e.m.f. induced in coil. iv- Find the induced e.m.f and current intensity in sec. coil. v- Repeat steps, i to iv, assuming that an iron core is used to link the two coils, given that . vi- Why and iron core is used to link the two coils of a trnasformer? Use the obtained answers to account your answers.
2- i- ii- iii- iv- v- After using an iron core: i- ii- iii- iv- - The existance of an iron core raised the e.m.f in sec. coil from 0.0684 m.v to 2.736 v.
3- A dynamo has a coil of 8758 turns each of 0.8 area, rotates with a uniform speed in a magnetic field of 1 T. i- What will be the speed of rotation equals, in rad/sec, to get a max. e.m.f. of 220 v? ii- Find the frequency & period of the produced current, consider . iii- Find the passed time to get an instantaneous e.m.f. equals to the effective value. iv- What would be the produced instantaneous e.m.f. when the coil has an angle of to the flux lines.
3- i- ii- iii- Eeff = Emax X 0.707 = 220 X 0.707 = 155.54 V iv- The angle between the coil's axis and flux lines = 90 - 30 = 60 Einst = Emax sin 60 = 190.5 V.
4- An induction coil of 700 turns and 22 cm long, has a cross-sectional area of 2 . Find its self-inductance "L".
I- Put (
) or (
) infront each of the follwoing statements:
is applicable whether the transformer is ideal or not.
.
.
.
= 0.707 Imax.
. V, where E is....., L is....., 
is..... & V is......
. The current in coil is 
, where E is the e.m.f of battery, R total resistance of circuit. 
.
halfcycles.
.
.
. Find the max. and average induced e.m.f. over one complete cycle, when it rotated inside a magnetic field of 0.1 T with a speed of 6000 cycle per minute.
. The sec. coil has 2000 turns have the same area and a load resistance of 1 K. 
. 
. 
to the flux lines.
= 90 - 30 = 60 
