Current Electricity — Physics STD 12 Science — Question

A cubical block of mass 1.0kg and edge 5.0cm is heated to 227^°C. It is kept in an evacuated chamber maintained at 27^°C. Assuming that the block emits radiation like a blackbody, find the rate at which the temperature of the block will decrease. Specific heat capacity of the material of the block is 400Jkg^-1K^-1.

Two cells of voltage 10V and 2V and internal resistances 10Ω and 5Ω respectively, are connected in parallel with the positive end of 10V battery connected to negative pole of 2V battery (Fig). Find the effective voltage and effective resistance of the combination.

A U-tube having unequal arm-lengths has water in it. A tuning fork of frequency 440Hz can set up the air in the shorter arm in its fundamental mode of vibration and the same tuning fork can set up the air in the longer arm in its first overtone vibration. Find the length of the air columns. Neglect any end effect and assume that the speed of sound in air = 330m/s.

Seema's uncle was advised by his doctor to have an MRI (Magnetic Resonance Imaging) scan of his brain. Her uncle felt it to be expensive and wanted to postpone it. When Seema learnt about this, she took the help of her family and also approached the doctor, who also offered a substantial discount. She then convinced her uncle to undergo the test to enable the doctor to know the condition of his brain. The information thus obtained greatly helped the doctor to treat him properly.
Based on the above paragraph, answer the following questions:

1. What according to you are the values displayed by Seema, her family and the doctor?
2. What could be the possible reason for MRI test to be so expensive?
3. Assuming that MRI test was performed using a magnetic field of 0.1 T., find the minimum and maximum values of the force that the magnetic field could exert on a proton (charge = 1.6 x 10^-19 C) moving with a speed of 10⁴ m/s.

A capacitor has some dielectric between its plates, and the capacitor is connected to a DC source. The battery is now disconnected and then the dielectric is removed. State whether the capacitance, the energy stored in it, electric field, charge stored and the voltage will increase, decrease or remain constant.

A sound source, fixed at the origin, is continuously emitting sound at a frequency of 660Hz. The sound travels in air at a speed of 330m/s. A listener is moving along the line x = 336m at a constant speed of 26m/s. Find the frequency of the sound as observed by the listener when he is:

1. At y = - 140m.
2. At y = 0.
3. At y = 140m.

A double convex lens has focal length 25cm. The radius of curvature of one of the surfaces is double of the other. Find the radii, if the refractive index of the material of the lens is 1.5.

Determine the 'effective focal length' of the combination of the two lenses having focal lengths 30 cm and -20cm if they are placed 8.0 cm apart with their principal axes coincident. Does the answer depend on which side of the combination a beam of parallel light is incident? Is the notion of effective focal length of this system useful at all?

A bullet of mass 20g moving horizontally at a speed of 300m/s is fired into a wooden block of mass 500g suspended by a long string. The bullet crosses the block and emerges on the other side. If the centre of mass of the block rises through a height of 20.0cm, find the speed of the bullet as it emerges from the block.