shahjahan brain teaser problems on electricity

Shahjahan brain teaser problems on electricity…

Shahjahan-Brain teaser problems on electricity….

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 23. 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?

(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 5bulb,resistor and15are

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

battery remain unchanged):-

(a) 7.5V and 24mA (b)9.9V and 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

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 100C and 20at 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 100resistor if the power dissipated in the 100resistor

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....

(a) l1+l2l1+l (b) l+2l1 /l1-l(c) ll1/ l+l (d)l-l2ll

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.d5.c 6.c

7.a 8.b

shahjahan brain teaser problems on electricity

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