# bashas post: help on MCAT physics problems

#### J.Pearlman

##### Member
7+ Year Member
15+ Year Member
Sup guys, take a look at these mcat physics problems. See if you guys can figure out why the answer is what it is and help out these mcat kiddies.

1)a) A student connects three 2-microfarad capacitors in parallel (Total Cap. = 6 microfarads), connected to a 6 V battery. What is the current in the circuit after the transients have disappeared?
The answer is: Zero amps.
b) Now, he replaces the 6 V with 4 Vrms, oscillating at a freq. of 1 MHz. What is the root mean square current in the circuit?
The answer is: 48(pie)A

2)a) 50 cm long hollow glass cylinder open at both ends is held in the air. A device that produces pure tones is placed adjacent to one end. The frequency of the sound is initially set very low then increased gradually. At a frequency of 320 Hz, the tube resonates. What is the speed of sound in air during this experiment?
The answer is: 320 m/s
b) When helium replaced with air, the resonant frequency increases. How come????
c) When the temp is increased, the the freq decreases. How come???

3)a)When the plates of a capacitor are moved farther while voltage source remains attached...
The answer is: The voltage remains the same and the charge decreases. (According the EK Physics
book, the voltage should increase with increasing distance, and the charge should subsequently stay
the same since the capacitance will decrease as V increases)
b) When the capacitance increases, Electric field intensity remains the same. How come?? (Also, is
Electric field same as electric field intensity)

4)a)An astronaut onboard the space shuttle takes a hollow metallic ball and by means of electrostatic
induction, gives it a net positive charge of 2 C. The ball is placed between the plates of a capacitor. 12V is applied across the plates. The capacitance is 1.5 farads, charge is 18 C. What is the electric field intensity at a point 4 m away from the center of the ball. k represents the const in Coloumbs law.
The answer: k/8 N/C (I have no idea how this is so, I even plugged it in the formula for E = kq/r2)
b) An astronaut adjusts the capacitor plates so that they are 3 m apart. The voltage is 12 V. The
hollow metallic ball that has a negligible radius, a net positive charge of 2 C, and a mass of 100 g. It is held inside the capacitor against the positive plate. It is then released. What is the kinetic energy of the ball when it reaches the negative plate?
The answer is: 24 J ( I did this one using the U = 1/2 QV formula, but got 18 J instead)

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