When a 25.0-Ohm resistor is connected across the terminals of a 12. 0 -V battery

Question

When a 25.0-Ohm resistor is connected across the terminals of a 12. 0 -V battery, the voltage across the terminals of the battery falls by 0. 300 V. What is the internal resistance of this battery? a. 0.43 Ohm b. 0. 51 Ohm c. 0.64 Ohm d.1.11 Ohm e. 1.69 Ohm In the circuit shown in the figure, all the lightbulbs are identical. Which of the following is the correct ranking of the brightness of the bulbs? a. A is brightest, C is dimmest, and B is in between. b. A is the brightest, and B and C have equal brightness but less than A. c. A and B have equal brightness, and C is the dimmest. d. B and C have equal brightness, and A is the dimmest. e. All three bulbs have the same brightness. At t = 0 the switch S is closed with the capacitor unchanged. If C = 30 mu F, e = 30 v, and R = 10 k Ohm. What is the potential difference across the capacitor when I = 2.0 mA? a. 5 V b. 10 V c. 15 V d. 25 V e. 30 V An electron moving in the direction of the +x-axis enters a magnetic field. If the electron experiences a magnetic deflection in the -y direction, the direction of the magnetic field in this region points in the direction of the a. -x-axis b. -y-axis c. +y-axis d. -z-axis e. + z-axis

Explanation / Answer

7.

In RC circuit

V = V0*(1 - exp(-t/RC))

I = I0*exp(-t/RC)

RC = 30*10^-6*10*10^3 = 0.3 sec

I0 = V0/R = 3 mA

I = 2 mA

So,

t = RC*ln (I0/I)

t = 0.3*ln (3/2) = 0.1216 sec

now at that time

V = 30*(1 - exp(-0.1216/0.3)) = 10 V

Correct option is B.

8.

F = q*VxB

direction of force is -y

direction of speed = +x

So,

-j = ix(B)

we know that

ixj = k

jxk = i

kxi = j and ixk = -j

So direction of magnetic field should be = +k = +z-axis

Correct option is E.

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