Notes Ch # 17

Chapter # 17

ELECTROMAGNATISM Q#1: Choose the correct answer of the statement from the given choices I.

Magnetic field is formed around a current carrying wire in the form of (a) line (b) circle (c) concentric circle (d) square

II.

Electric current and magnetic field are mutually (a) Perpendicular (b) parallel (c) intersect each other (d) non of these

III.

If the current passing through a wire in upward direction then the magnetic field around it is (a) Clockwise (b) anti clockwise (c) perpendicular (d) parallel

IV.

A current carrying solenoid behaves as a _________ magnet. (a) Horse shoe (b) bar (c) point (d) all of these

V.

We can determine the polarity of a solenoid by___________ (a) Right hand rule (b) left hand rule (c) faraday’s law of induction (d) coulomb’s law

VI.

__________ is a device used to convert electrical energy into mechanical energy. (a) Motor (b) generator (c) ups (d) vehicle

VII.

A machine which converts direct current energy into mechanical energy is called________ (a) D.C Motor (b) DC generator (c) AC motor (d) AC generator

VIII.

A machine which converts alternating current energy into mechanical energy is called________ (a) D.C Motor (b) DC generator (c) AC motor (d) AC generator

IX.

A machine which converts mechanical energy into alternating electrical energy is called_____ (a) D.C Motor (b) DC generator (c) AC motor (d) AC generator

X.

When magnetic flux of current carrying coil changes a ____________ is produced in it. (a) Current (b) emf (c) voltage (d) potential

XI.

Value of induced emf is directly proportional to rate of change of flux. This is known as ___ law (a) Right hand rule (b) Flemings left hand rule (c) faraday’s law of induction (d) coulomb’s law

XII.

If a current is induced in a circuit due to a change in the current of another circuit, this phenomenon is known as (a) Self induction (b) mutual induction (c) magnetic induction (d) electric induction

XIII.

If the current through a coil or a circuit changes and this change induces an emf in the circuit itself, the phenomenon is known as (a) Self induction (b) mutual induction (c) magnetic induction (d) electric induction

XIV.

The effect on the current in a circuit due to the change in the current is called________ (a) Self induction (b) mutual induction (c) magnetic induction (d) electric induction

XV.

Transformer works on the principle of _________ (a) Self induction (b) mutual induction (c) magnetic induction (d) electric induction

XVI.

A device which is used to increase or decrease the value of alternating voltage is called ____ (a) Motor (b) generator (c) transformer (d) ups

XVII.

There are _________ types of transformer. (a) 1 (b) 2 (c) 3 (d) 4

XVIII.

The coil in which change in current produces, induced current in another coil is called________ . (a) Primary coil (b) secondary coil (c) any of these (d) inducting coil

XIX.

The coil in which induced current is produced due to change in current of another coil is called________ . (a) Primary coil (b) secondary coil (c) any of these (d) inducting coil

XX.

Step up transformer are used to increase the value of___________ (a)Current (b) voltage (c) resistance (d) potential

XXI.

Step down transformer are used to decrease the value of___________ (a)Current (b) voltage (c) resistance (d) potential

XXII.

Step up transformer are used to decrease the value of___________ (a )Current (b) voltage (c) resistance (d) potential

XXIII.

Step down transformer are used to increase the value of___________ (a )Current (b) voltage (c) resistance (d) potential

XXIV.

The shape of magnetic lines of forces in case of straight current carrying conductor is_______ (a) Elliptical (b) triangular (c) circular (d) rectangular

XXV.

A transformer has 100 turns in its primary and 500 in its secondary coil. If 6DC volt is applied across its primary, then voltage induced in its secondary would be ________ (a) 0 (b) 30 (c) 45 (d) 6

Q#2: Give short answer of the following questions I.

What is the magnetic effect of an electric current?

Ans: when current passes through a conductor, a magnetic field is produced in the space around that conductor. So we can say that the effect of electric current is to produce magnetic field. OR A magnetic field is induced as an effect of electric current. II.

How can we determine the direction of magnetic field of a current carrying wire?

Ans: we can determine the direction of magnetic field of a current carrying wire by right hand rule. I.e. if we grasp the current carrying wire in our right than rotating fingers indicates the direction of magnetic field, induced due to that current. III.

How we can determine the polarity of solenoid?

Ans: To determine the polarity of solenoid, hold the solenoid in your right hand by curling the fingers in the direction of current; the stretched thumb would indicate toward the North Pole. IV.

How can we indicate the direction of current on a paper?

Ans: the direction of current has been indicated by placing a dot or a sign of multiplication in the circles, a dot indicates that the current is directed out of the paper (plane) or towards us, while the cross sign indicates that the current is directed into the paper (plane) or away from us. V.

How the magnetic lines of forces are formed in a current carrying coil?

Ans: the magnetic lines of force generated by the flow of current in the coil are straight and parallel in a small region near the centre of coil and are directed perpendicular to its plane. They are approximately circular near the wire. VI.

Where the magnetic field of a coil is uniform?

Ans: The magnetic field is uniform in a small area surrounding the centre of coil and In the remaining portion it is non-uniform. VII.

State Faraday’s left hand rule?

Ans: According to this rule, stretch the thumb, forefinger and middle finger of your left hand mutually at right angles to each other. If the forefinger points in the direction of the magnetic field, the middle finger in the direction of current, then the thumb would indicate the direction of force on the conductor. VIII.

What is a motor? How many types of it are there?

Ans: A machine which converts electrical energy into mechanical energy is called motor. There are two types of motors (1) AC motor (2) DC motor IX.

Define electromagnetic induction?

Ans: A phenomenon in which an emf is produced due to relative motion of coil and magnet. X.

Define magnetic flux?

Ans: The number of magnetic lines of forces passing through any surface area is known as magnetic flux through that surface. XI.

How an emf is induced in a solenoid?

Ans: if the magnetic flux through a coil or a solenoid is changing an emf is induced in it. XII.

Define mutual and self induction?

Ans: if a current is induced in a circuit due to a change of current in another circuit, this phenomenon is known as mutual induction, while if the current is induced in a circuit due to a change of current in the same circuit, this phenomenon is known as self induction. XIII.

Write the expression of turn ratio of a transformer?

Es/Ep=Ss/Sp XIV.

How many types of transformer define each?

Ans: There are three types of transformer i.e. 1. Step up transformer 2. Step down transformer 3. Stabilizer XV.

On which principle transformer works?

Ans: The transformer works on the principle of mutual induction. XVI.

On which components does a transformer consist?

Ans: It consists of two coils which are wound on two different sides of a rectangular iron core.

XVII.

Why we use iron core in a transformer?

Ans: The iron core is used in transformer to enhance the magnetic flux to a very large value and also concentrate it into the core that the whole flux generated by the primary coil is also linked with the secondary. XVIII.

How a transformer does works?

Ans: As the flux of primary coil changes in accordance with the principle of mutual induction, an alternating voltage is induced across the secondary coil. XIX.

On which factor does the value secondary voltage depends?

Ans: The value of secondary voltage depends upon the number of turns in the primary and secondary coils. XX.

Explain the step down transformer on the basis of their turn ratio?

Ans: if the number of turns in the secondary would be smaller than the number of turns in the primary. Such a transformer is known as step down transformer. XXI.

Explain the step up transformer on the basis of their turn ratio?

Ans: if the number of turns in the secondary would be larger than the number of turns in the primary. Such a transformer is known as step up transformer. XXII.

How does magnetic field strength vary with the variation in the number of lines of force?

Ans: the number of lines of force in a magnetic field depends upon the strength of the field. A stronger field has more lines than weaker one. XXIII.

Why we call a solenoid to be a bar magnet?

Ans: Because the lines of force inside and out a solenoid Produced by the solenoid are same as of a bar magnet. XXIV.

Write the construction of a DC motor?

Ans: DC motor consist of a rectangular coil mounted on a spindle so that it can rotate between two poles of a permanent magnet, connected to the battery through brushes and a ring divided into two halves. XXV.

How coil continue its rotation when brushes are at vacant space between split rings?

Ans: Coil continues its rotation in between both halves of split rings due to its momentum. XXVI. XXVII. XXVIII. XXIX. XXX. XXXI. XXXII. XXXIII.

Long Questions Q#3: (a) Explain the magnetic field of current carrying solenoid? (b) State Fleming’s left hand rule and explain when we can use it? (c) Define electromagnetic induction. Q#4: (a) explain the working of an AC generator? (b) what is the difference in mutual and self induction? (c) Explain how a DC motor converts electrical energy into mechanical energy. Q#5: (a)write a note on the Transformer. (b) a step down transformer changes 250V AC into 6V AC. If the number of turns in its primary is 1000, find the number of turns in its secondary.