1. A photocell changes light into electricity.

       Higher tier only:

       When light (photons) falls on a silicon photocell, electrons are released from its crystal.

       These electrons produce a direct current.

       The larger the surface area of a photocell, the larger its power.

   2. The fuels that cause global warming when burned are the fossil fuels.

   3. Wind turbine changes kinetic energy of the wind into electrical energy.

   4. The missing words are:

       Nuclear power stations use uranium as fuel. This fuel is non renewable. 

       The fuel rods release a lot of energy as heat.