Download CLASS XII EMI and AC Sure shot Questions 2015-16

CLASS XII
EMI and AC
Sure shot Questions 2015-16
2 marks Question
1. State the law that gives the polarity of induced emf. Which other law is alternative to this law?
2. What are the advantages and disadvantages of a.c. over d.c.?
3. Two identical coils carry similar current s and approach each other. How the current in each
coil is affected? ( Any questions from applications of Lenz’s law to determine the direction of
induced current)
3 marks Question
4. What are eddy currents? How can they be minimized? Write any two applications of eddy
Current.
5. Deduce an expression for the average power associated with an inductor(or, Capacitor, LR or
CR circuit).
6. When a circuit element X is connected across an a.c. source , a current of 2 A flows through
it and this current is in phase with the applied voltage. When another element Y is connected
across the same source , the same current flows in the circuit but it leads the voltage by π/2
radians.
7. Name the circuit elements X and Y.
8. Find the current that flows in the circuit when the series combination of X and Y is connected
across the same a.c. voltage.
9. Plot a graph showing variation of the net impedance of this series combination of X and Y as a
function of angular frequency ω of the applied voltage.
5 marks Question
10. Give the principle , construction ,theory and working of an A.C. Generator. What is the source
of energy generation in this device?
11. Define mutual induction. Derive an expression for the mutual inductance of two long co-axial
solenoids having different number of turns.
12. Explain the underlying principle and working of a step-up transformer with the help of a labeled
circuit arrangement . Write any two sources of energy loss in this device .
13. Derive an expression for the motional or back emf across the movable arm of a loop kept in a
uniform magnetic field. The arm is moving with velocity v.
14. Find (i) the external force required to move the arm and (ii) the power dissipated as heat.
MARKING SCHEME
2 marks question
1. Statement of Lenz’s law
1
Alternative Law – Fleming’s Right Hand Rule
1
2. Advantages- (i) It can be easily varied using transformer
(ii) It can be easily converted into d.c.
1
Disadvatages- (i) Shock of a.c. is attractive while that of d.c. is repulsive.
(ii) a.c. cannot be used in the phenomenon like electroplating, electrorefining,
electrotyping etc.
1
3. The coils approaching each other means the magnetic flux strengthen each other. According to
Lenz’s law the induced current be such that the flux is decreased, i.e, the original current in
each coil is reduced.
2
3 marks Question
1. Definition of eddy Current.
It can be minimized using laminated soft iron core.
Applications- Magnetic Brakes, Induction Furnace, Induction motor, Speedometer
2. Expression for average Power
3. (i) X – Resistance R and Y- Capacitor C.
(ii)
I=1A
(iii)
Graph
1
1
1
3
1
1
1
5 marks Question
1. Diagram
Principle
Construction
Theory
Working
Source
2. Definition
Expression : M = µ0 n1 n2 A l
Or, M = µ0 N1 N2 A/ l
3. Principle
Labeled diagram
Working
Sources of energy loss- Copper loss, iron loss, Hysteresis loss and magnetostriction
4. Expression for motional emf: E = Blv
(i)
Force F = B2 l2 v / R
(ii) Power P = Fv = B2 l2 v2 / R
1
1
1
1
1
1
4
1
1
2
1
2
3