The current passing through a choke coil of 5 H is decreasing at the rate of 3 As-1. The emf induced in the coil is
1. 15 V
2. -15 V
3.
4.
If all linear dimensions of an inductor are tripled, then self-inductance will become (keeping the total number of turns per unit length constant)
1. 3 times
2. 9 times
3. 27 times
4. 1/3 times
A coil of resistance 20 and inductance 5H has been connected to a 200 V battery. The maximum energy stored in the coil is
(1) 250 J
(2) 125 J
(3) 500 J
(4) 100 J
Two coils have a mutual inductance of 5 mH. Current changes in the first coil according to the equation I = cos wt, where = 10 A and rad/s. Maximum value of e.m.f. induced in the second coil is
1. 5 volt
2. 2 volt
3. 4 volt
4. volt
Lenz's law is a consequence of the law of conservation of
(1) Charge
(2) Momentum
(3) Mass
(4) Energy
A copper ring is held horizontally and a bar magnet is dropped through the ring with its length along the axis of the ring. The acceleration of the falling magnet while it is passing through the ring is-
(1) Equal to that due to gravity
(2) Less than that due to gravity
(3) More than that due to gravity
(4) Depends on the diameter of the ring and the length of the magnet
A magnet is brought towards a coil (i) speedily (ii) slowly then the induced e.m.f./induced charge will be respectively
(1) More in first case / More in the first case
(2) More in first case/Equal in both case
(3) Less in first case/More in second case
(4) Less in first case/Equal in both case
As shown in the figure, a magnet is moved with a fast speed towards a coil at rest. Due to this induced electromotive force, induced current and induced charge in the coil is E, I, and Q respectively. If the speed of the magnet is doubled, the incorrect statement is
(1) E increases
(2) I increases
(3) Q remains the same
(4) Q increases
The magnetic field in a coil of 100 turns and 40 square cm area is increased from 1 Tesla to 6 Tesla in 2 second. The magnetic field is perpendicular to the coil. The e.m.f. generated in it is
(1) 104 V
(2) 1.2 V
(3) 1.0 V
(4) 10–2 V
A metallic ring connected to a rod oscillates freely like a pendulum. If now a magnetic field is applied in the horizontal direction so that the pendulum now swings through the field, the pendulum will
(1) Keep oscillating with the old-time period
(2) Keep oscillating with a smaller time period
(3) Keep oscillating with a larger time period
(4) Come to rest very soon
An aluminum ring B faces an electromagnet A. The current I through A can be altered. Then :
(1) Whether I increases or decreases, B will not experience any force
(2) If I decrease, A will repel B
(3) If I increases, A will attract B
(4) If I increases, A will repel B
A coil having \(n\) turns and resistance \(R\) \(\Omega\) is connected with a galvanometer of resistance \(4R\) \(\Omega.\) This combination is moved in time \(t\) seconds from a magnetic field \(W_1\) weber/m2 to \(W_2\) weber/m2. The induced current in the circuit is: (Assume area = \(1\) m2)
1. \(-\frac{(W_2-W_1)}{5Rnt}\)
2. \(-\frac{n(W_2-W_1)}{5Rt}\)
3. \(-\frac{(W_2-W_1)}{Rnt}\)
4. \(-\frac{n(W_2-W_1)}{Rt}\)
A rectangular coil ABCD is rotated anticlockwise with a uniform angular velocity about the axis shown in the diagram below. The axis of rotation of the coil as well as the magnetic field B are horizontal. The induced e.m.f. in the coil would be maximum when
(1) The plane of the coil is horizontal
(2) The plane of the coil makes an angle of 45° with the magnetic field
(3) The plane of the coil is at right angles to the magnetic field
(4) The plane of the coil makes an angle of 30° with the magnetic field
A conducting square loop of side L and resistance R moves in its plane with a uniform velocity v perpendicular to one of its sides. A magnetic induction B constant in time and space, pointing perpendicular and into the plane of the loop exists everywhere. The current induced in the loop is:
1. clockwise
2. anticlockwise
3. anticlockwise
4. Zero
The magnitude of the earth’s magnetic field at a place is B0 and the angle of dip is δ. A horizontal conductor of length l lying along the magnetic north-south moves eastwards with a velocity v. The emf induced across the conductor is
1. Zero
2. B0lv sinδ
3. B0lv
4. B0lv cosδ
Two circuits have coefficient of mutual induction of 0.09 henry. Average e.m.f. induced in the secondary by a change of current from 0 to 20 ampere in 0.006 second in the primary will be
(1) 120 V
(2) 80 V
(3) 200 V
(4) 300 V
The inductance of a coil is 60μH. A current in this coil increases from 1.0 A to 1.5 A in 0.1 second. The magnitude of the induced e.m.f. is
(1) 60 × 10–6 V
(2) 300 × 10–4 V
(3) 30 × 10–4 V
(4) 3 × 10–4 V
The core of a transformer is laminated to reduce energy losses due to
(1) Eddy currents
(2) Hysteresis
(3) Resistance in winding
(4) None of these
A power transformer is used to step up an alternating e.m.f. of 220 V to 11 kV to transmit 4.4 kW of power. If the primary coil has 1000 turns, what is the current rating of the secondary ? Assume 100% efficiency for the transformer
(1) 4 A
(2) 0.4 A
(3) 0.04 A
(4) 0.2 A
A step-down transformer is connected to main supply 200V to operate a 6V, 30W bulb. The current in primary is
(1) 3 A
(2) 1.5 A
(3) 0.3 A
(4) 0.15 A
As shown in the figure, P and Q are two coaxial conducting loops separated by some distance. When the switch S is closed, a clockwise current IP flows in P (as seen by E) and an induced current flows in Q. The switch remains closed for a long time. When S is opened, a current flows in Q. Then the directions of and (as seen by E) are
(1) Respectively clockwise and anticlockwise
(2) Both clockwise
(3) Both anticlockwise
(4) Respectively anticlockwise and clockwise
A conducting wireframe is placed in a magnetic field that is directed into the paper. The magnetic field is increasing at a constant rate. The directions of induced current in wires AB and CD are:
1. | B to A and D to C |
2. | A to B and C to D |
3. | A to B and D to C |
4. | B to A and C to D |
A highly conducting ring of radius R is perpendicular to and concentric with the axis of a long solenoid as shown in fig. The ring has a narrow gap of width d in its circumference. The solenoid has a cross-sectional area A and a uniform internal field of magnitude B0. Now beginning at t = 0, the solenoid current is steadily increased so that the field magnitude at any time t is given by B(t) = B0 + αt where α > 0. Assuming that no charge can flow across the gap, the end of the ring which has an excess of positive charge and the magnitude of induced e.m.f. in the ring are respectively
(1) X, Aα
(2) X πR2α
(3) Y, πA2α
(4) Y, πR2α
A loop abcd is moved across the pole pieces of a magnet as shown in fig. with a constant speed v. When the edge ab of the loop enters the pole pieces at time t = 0 sec. , which one of the following graphs represents correctly the induced emf in the coil?
(1)
(2)
(3)
(4)
Some magnetic flux is changed from a coil of resistance 10 ohm. As a result an induced current is developed in it, which varies with time as shown in figure. The magnitude of change in flux through the coil in webers is
(1) 2
(2) 4
(3) 6
(4) None of these
The graph gives the magnitude B(t) of a uniform magnetic field that exists throughout a conducting loop, perpendicular to the plane of the loop. Rank the five regions of the graph according to the magnitude of the emf induced in the loop, greatest first
(1) b > (d = e) < (a = c)
(2) b > (d = e) > (a = c)
(3) b < d < e < c < a
(4) b > (a = c) > (d = e)
A flexible wire bent in the form of a circle is placed in a uniform magnetic field perpendicular to the plane of the coil. The radius of the coil changes as shown in the figure. The graph of induced emf in the coil is represented by
(1)
(2)
(3)
(4)
A square loop of side 5 cm enters a magnetic field with 1 cms-1. The front edge enters the magnetic field at t = 0 then which graph best depicts emf
(1)
(2)
(3)
(4)
A long solenoid of diameter 0.1m has 2 turns per meter. At the centre of the solenoid, a coil of 100 turns and radius 0.01m is placed with its axis coinciding with the solenoid's axis. The current in the solenoid reduces at a constant rate to 0 A from 4A in 0.05s. If the resistance of the coil is , the total charge flowing through the coil during this time is
1. 32
2. 16
3. 32
4. 16
A uniform magnetic field is restricted within a region of radius r. The magnetic field changes with time at a rate . Loop 2 of radius R is outside the region of magnetic field as shown in the figure. Then the emf generated is-
(1) zero in loop 1 and zero in loop 2
(2)
(3)
(4)
A transformer having efficiency of 90% is working on 200 V and 3 kW power supply. If the current in the secondary coil is 6A, the voltage across the secondary coil and the current in the primary coil respectively are
1. 300V,15A
2. 450V,15A
3. 450V,13.5A
4. 600V,15A
A 220V input is supplied to a transformer.The output circuit draws a current of 2.0A at 440V. If the efficiency of the transformer is 80%, the current drawn by the primary windings of the transformer is
1. 3.6A 2. 2.8A
3. 2.5A 4. 5.0A
A conducting circular loop is placed in a uniform magnetic field with its plane perpendicular to the magnetic field. The radius of the loop starts shrinking at The induced emf in the loop when the radius is 2 cm is
1. 2.
3. 4.
a long solenoid has 500 turns. When a current of 2 A is passed through it, the resulting magnetic flux linked with each turn of the solenoid is Wh. The self-inductance of the solenoid is
(a) 2.5 h
(b) 2.0 H
(c) 1.0 H
(d) 4.0 H
A current-carrying wire is placed below a coil in its plane, with current flowing as shown.
If the current increases –
1. no current will be induced in the coil
2. an anticlockwise current will be induced in the coil
3. a clockwise current will be induced in the coil
4. the current induced in the coil will be first anticlockwise and then clockwise
When the current in a certain inductor coil is 5.0 A and is increasing at the rate of 10.0 A/s, the potential difference across the coil is 140V. When the current is 5.0 A and decreasing at the rate of 10.0 A/s, the potential difference is 60V. The self-inductance of the coil is –
1. 2H
2. 4H
3. 8H
4. 12H
The magnetic flux through a coil varies with time as . The ratio of emf at t = 3s to t = 0s will be
1. 1 : 9
2. 1 : 6
3. 6 : 1
4. 9 : 1
A wire of fixed lengths is wound on a solenoid of length and radius r. Its self inductance is found to be L. Now if same wire is wound on a solenoid of length and radius r/2, then the self inductance will be –
1.
2.
3.
4.
PQ is an infinite current carrying conductor. AB and CD are smooth conducting rods on which a conductor EF moves with constant velocity v as shown. The force needed to maintain constant speed of EF is –
1.
2.
3.
4.
A 50 turns circular coil has a radius of 3 cms, it is kept in a magnetic field acting normal to the area of the coil. The magnetic field B increased from 0.10 tesla to 0.35 tesla in 2 milliseconds. The average induced emf in the coil is-
1. 1.77 volts
2. 17.7 volts
3. 177 volts
4. 0.177 volts
The magnetic flux through each turn of a 100 turn coil is , where t is in second. The induced emf at t = 2 s is
1.
2.
3.
4.
A copper rod of length 0.19 m is moving parallel to a long wire with a uniform velocity of 10 m/s. The long wire carries 5 ampere current and is perpendicular to the rod. The ends of the rod are at distances 0.01 m and 0.2 m from the wire. The emf induced in the rod will be-
1.
2.
3.
4.
A long solenoid having 1000 turns per cm is carrying alternating current of one ampere peak value. A search coil of area of cross-section and of 20 turns is placed in the middle of the solenoid so that its plane is perpendicular to the axis of the solenoid. The search coil registers a peak voltage . The frequency of the current in the solenoid is -
1. 1.6 per second
2. 0.16 per second
3. 15.9 per second
4. 15.85 per second
A small square loop of wire of side l is placed inside a large square loop of wire of side . The loops are coplanar and their centers coincide. The mutual inductance of the system is proportional to
1.
2.
3.
4.
A coil of the area and of 50 turns is kept with its plane normal to a magnetic field B. A resistance of 30 ohms is connected to the resistance-less coil. B is Gauss. The current passing through the resistance at t = 5 ms will be-
1.
2.
3.
4.