# Tutor profile: Ravi S.

## Questions

### Subject: Physics (Newtonian Mechanics)

An automobile of mass m accelerate starting from rest with supply of constant power P. Find it’s instantaneous velocity as a function of time.

Fv = P m.dv/dt = P Integration (Pdt/m) = integration (vdv) By integrating V^2 /2 = Pt/m + C Using initial conditions At t= 0 , v=0 so C=0 Solving v = (2Pt/m)^(1/2)

### Subject: Physics (Electricity and Magnetism)

A heater operate with power of 1200 W in 100 V line.It is connected in combination with a resistance of 10 ohm and a resistance r, to a 100 V mains as shown in figure. What will be the value of r so heater operates with power 62.5W.

P=V^2/R Resistance of heater R = 100^2/1200 R = 8.33ohm For second case P =62.5 W I=(62.5/8.333) i= 2.73A Main current in circuit I = 100/(10+(10r/10+r)) This current will distribute at parallel combination i = I(r/10+r) Solving r = 6 ohm

### Subject: Physics

A flat disc of radius r has surface charge density s. The disc rotated about an axis perpendicular to its plane and passing through its centre with angular speed w.find the torque on the disc if it is placed in a uniform magnetic field B directed parallel to plane of disc.

We consider a ring of radius x and very small width Dixon the disc. Charge on the ring dq = s.2 pi x dx Current due to this charge I = dq . Frequency I = s 2 pi x dx . w /2.pi I = swxdx Magnetic moment of ring M = I.A M = swxdx . Pi . X^2 Torque on ring dT = MB sin theta Theta = 90 degrees dT = swpiBx^3 dx Torque T = integration(dT) from 0 to r T = (swpiB r^4)/4

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