good mcqs for preparation of effective examination of different kinds...such as entrance,iit-jee...etc

1. Shahjahan brain teaserproblemson electricity…
Shahjahan-Brainteaserproblemsonelectricity….
1. Two wires of equal lenghts, equal diameters and having resistivities 1 and 2 are
2. connected in series,The equivalent resistivity of the combination is....
(a) (1+2) (b) ½(1+2) (c) /(1+2) (d) 1 2
3. An electric kettle has two coils. when one of them is switched on, the water in the kettle
4. boils in 6
minutes. When the other coil is switched on, the water boils in 3 minutes If the two coils
are connected in series the time taken to boil water in the kettle is:-
(a) 3 minutes (b) 6 minutes (c) 2minutes (d)9 minutes
3. An electric kettle has two coils when one of these is switched on. the water in the kettle
boils in 6
minutes. When the other coil is switched on, boils in 3 minutes If the two coils are
connected in parallel,the time taken to boil water in the kettle is:-
(a)7 minutes (b) 8minutes (c) 6minutes (d) 2minutes
4. Two resistances R1 and R2 have effective resistance Rs when connected in series
combination and Rp when connected in parallel combination,if Rs Rp=16 and R1/R2=4 the
values of R1 and R2 are:- (a) 2 and 0.5 (b) 1 and 0.25(c) 8 and 2 (d)  and 1
Two sources of equal emf are connected to an external resistance R the internal
resistance of the two
sources are R1 and R2 ( R2 > R1 ) if the potential difference across the source having
internal resistance R2 is Zero, then
(a)R=R1R2/(R2-R1) (b)R=R1R2/(R1+R2) (c) R=R2-R1 (d) R=R2(R1+R2)/(R2-R1)
6. Two electric bulbs whose resistances are in the ratio of 1:2 are connected in parallel to a
constant
voltage source the power dissipated in them have the ratio:-
(a) 1:2 (b) 1:1 (c) 2:1 (d) 1:4
7. If the above two bulbs are connected in series, the power dissipatd in them have the ratio:-
(a) 1:2 (b) 2:1 (c) 1:1 (d) 1:3 (e) 2:3
8. The drift velocity of free electrons in a conductor is v, when a current. I is flowing in it If both
the radius and current are doubled, then drift velocity will be:-
(a) V/4 (b) V/2 (c) 4V (d) 2V (e) V
9. A bulb of 300W and 220V is connected with a source of 110V. What is the % decrease in
power?

2. Shahjahan brain teaserproblemson electricity…
(a)100% (b)75% (c) 70% (d)25% (e)30%
10. A heater boils 1kg of water in time t1 and another heater boils the same water in time t2 If
both are connected in series, the combination will boil the same water in time:-
(a)t1t2/t2+t (b) t1t2/t1-t2 (c) t1+t2 (d) 2(t1+t2) (e)1/2(t1-t2)
11.Temperature of a conductor increases by 5C passing electric current for some time.
The increase
in its temperature when double current is passed through the same conductor for the
same time
is……C. (a) 10 (b) 12 (c) 16 (d) 20 (e) 25
12. Two conductors have the same resistance at 0C but their temperture coefficients of
resistonces are  and The respective temperture coefficients of their series and
parallel combinations are nearly....
(a)  (b)  (c)  (d) 
13.find the equivalent resistance of a series circuit containing 15 heater,30
computer , 12 electric fan and 10 washing-machine. What will be the
current passing through the circuit if it is driven by a voltage source of 200
volts?
(a) 2.99A (b) 4.66A (c) 7.88A (d )9.96A
14.find the equivalent resistance when 5bulb,resistor and15are
connected in parallel.
(a) 20 (b) 25(c) 30(d) 35 (e) 40
a car has a fresh storage battery of EMF 12V,internal resistance 0.05W. if the
starter motor draws a current of 90A, what is terminal voltage of battery when starter
is on and after a long use,the internal resistance of the storage battery increases to
500W. what maximum current can be drawn from the battery?(assume e.m.f of the

3. Shahjahan brain teaserproblemson electricity…
battery remain unchanged):-
(a) 7.5V and24mA (b)9.9Vand 30mA (c)20V and 8A (d)11.2V and
5A
16.what is the drift velocity of electrons in a copper conductor having a cross-sectional
area of 5^-6m^2 if current is 10A?(Assume that there are
8^28electrons/m^3A.
(a)60m/s (b)70m/s (c)55.6 10^-5m/s (d)
1.56^-4m/s
17.How many electrons flow through the filament of a 120V, 60W electric lamp in
one second?( Given: electric power is the product of voltage and power):-
(a) 7.77 (b) 6.77 (c) 9 10^9 (d) 3.125^18
18.calculate the drift velocity of electrons in a silver wire having cross-sectional area
of 3.14^-6m^2 and carrying a of 20Amp.(Given avogadro’s
no:6.023^23,atomic weight of siver=108. Density of silver=10.5^3kg/m^3)
(a) 10.88 (b) 14.99 (c) 6.798 10^-4m/s (d) 9.997 10^-7m/s
19.two identical cells of e.m.f of 1.5V each joined in parallel provide supply to an
external circuit consisting of two resistors of 17W each joined in a parallel. A very high
resistance voltmeter reads the terminal voltage of the cell to be 1.4V. what is the
internal resistance of each cell?
(a) 0.25(b)3.8(c)1.2(d)7.2
in case of hydrogen atom, an electron moves in an orbit of radius 5^-11m with a
speed of 2.2^6m/s. calculate the equivalent current.
(a) 1.12mA (b) 2.31ma (c) 20 10^2A (d) 7MA

4. Shahjahan brain teaserproblemson electricity…
21.a battery tester measures the current supplied when the battery is conneted to a
resistor of 100 . if the current is less than 50mA, the battery is “flat ”(it needs to be
replaced). Calculate the maximum internal resistance of a 6V battery that will pass the test.
(a) 30(b)40(c)10 (d)20
a piece of wire has a resistance R. Another wire of same length and material but thrice
in diameter has resistance:-
(a) 1/3R (b)1/9R (c) 3R (d) 9R
23.a platinum wire has a resistance of 100C and 20at 273C. find the value of
temperature coefficient of resistance of platinum.
(a) 3.66 10^-3/k (b) 7.55 10^-4/k (c) 30/C (d) 40/C
24.what resistance would have to be added in a parallel with a 80 electric hair dryer to
reduce the equivalent resistance to 16
(a) 50(b)40 (c)30 (d)20
calculate the current in the 100resistor if the power dissipated in the 100resistor
is 0.81 W.
(a) 90mA b)80mA (c) 70mA (d) 60mA
26.When a cell is connected to a resistances R1 the rate at which heat is generated in it
is the same as when cell is connected to a resistance R2<R1 the internal resistance of
the cell is…
(a) (R1-R2) (b) ½(R1-R2) (c) R1R2/(R1+R2) (d) R1R2
27. Two wires of equal diameters of resistivities R1 and R2 are joined in series. The
equivalent resistivity of
The combination is....

5. Shahjahan brain teaserproblemson electricity…
(a) l1+l2l1+l (b) l+2l1 /l1-l(c) ll1/ l+l (d)l-l2ll

Two heater wires of equal length are first connected in series and then in parallel The
ratio of heat
produced in the two cases is....
(a) 2:1 (b) 1:2 (c) 4:1 (d)1:4
1.b 2.d 9.b 10.c 11.d 12.b 13.a 14.b 15.a 16.d 17.d 18.c 19.c
3.c 4.c 20.a 21.d 22.b 23.a 24.d 25.a 26.d 27.a 28.d
5.c 6.c
7.a
8.b