Electric Charges and Fields — Physics STD 12 Science — Question

A radioactive material decays by simultaneous emission of two particles with respective half lives 1620 and 810 years. The time (in years) after which one- fourth of the material remains is

(a) 1080

(b) 2430

(c) 3240

(d) 4860

Impedance of L-R circuit is :

In a triode valve the amplification factor is 20 and mutual conductance is 10^–3 mho. The plate resistance is

(a) 2 × 103 W

(b) 4 × 103 W

(c) 2 ×104 W

(d) 2 ×104 W

A metallic surface with work function of 2 eV, on heating to a temperature of 800 K gives an emission current of 1 mA. If another metallic surface having the same surface area, same emission constant but work function 4 eV is heated to a temperature of 1600 K, then the emission current will be

(a) 1 mA

(b) 2 mA

(c) 4 mA

(d) None of these

Copper and germanium are cooled from room temperature to 30 K. In this process, the resistance:

Assertion  : Microwave propagation is better than the sky wave propagation.

Reason      :  Microwaves have frequencies 100 to 300 GHz, which have very good directional properties.

(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.

(b) If both assertion and reason are true but reason is not the correct explanation of the assertion.

(c) If assertion is true but reason is false.

(d) If the assertion and reason both are false.

According to Coulomb’s law, the coulomb force measured between two point charges is directly proportional to:

Copper has face centered cubic (fcc) lattice with interatomic spacing equal to 2.54Å. The value of the lattice constant for this lattice is

(a) 1.27 Å

(b) 5.08 Å

(c) 2.54 Å

(d) 3.59 Å

Two resistance wires on joining in parallel the resultant resistance is .  One of the wire breaks, the effective resistance is 2 ohms.  The resistance of the broken wire is

(a)

(b) 2 ohm

(c)

(d) 3 ohm

As the mass number A increases, the binding energy per nucleon in a nucleus: