A circular coil having N turns and radius r carries a current I. It is held in the XZ plane in a magnetic field B$${\mathop i\limits^ \wedge }$$ . The torque on the coil due to the magnetic field is :

A boy's catapult is made of rubber cord which is 42 cm long, with 6 mm diameter of cross-section and of negligible mass. The boy keeps a stone weighing 0.02kg on it and stretches the cord by 20 cm by applying a constant force. When released, the stone flies off with a velocity of 20 ms^–1. Neglect the change in the area of cross-section of the cord while stretched. The Young's modulus of rubber is closest to:

Voltage rating of a parallel plate capacitor is 500V. Its dielectric can withstand a maximum electric field of 10⁶ V/m. The plate area is 10^–4 m². What is the dielectric constant is the capacitance is 15 pF? (given $$\varepsilon $$₀ = 8.86 × 10^–12 C²/Nm²)

Two identical beakers A and B contain equal volumes of two different liquids at 60°C each and left to cool down. Liquid in A has density of 8 × 10² kg/m³ and specific heat of 2000 J kg^–1 K^–1 while liquid in B has density of 10³ kg m^–3 and specific heat of 4000 J kg^–1 K^–1. Which of the following best describes their temperature versus time graph schematically? (assume the emissivity of both the beakers to be the same)