Is there any device which may control the direct current without dissipation of energy?

A rough incline plane is placed on a cart moving with a constnat velocity u on horizontal ground. A block of mass M rests on the incline. Is any work done by fore of friction between the block and incline? Is there then a dissipation of energy?

State any rule that relates the direction of electric current and the direction of accompaying magnetic field?

Name the device which directly converts solar energy into electrical energy.

What is the meaning of the terms frequency of an altrnating current? What is its value in India? Why is an alternating current considered to be advntageous over direct current for long range transmission of electric energy?