Today's Energy Mini Test

Here are the answers to today's mini test to help you prepare for the energy NAB.  Numbers in brackets are the marks allocated to each answer/line of working.  Remember that you get marks for working even if the answer is wrong, so make sure that you show clearly the stages you took.

1. State 3 renewable energy sources.
           Choose from geothermal, wind, hydro, solar, tidal/wave        (3)

2.a) Calculate the Spring Stiffness of a spring which was compressed 30mm by a 10N force.
            SS = Force / extension
                 = 0.3N/mm                               (1)

b) Calculate the strain energy of the above spring.
            Es = 1/2 F x                                    (1/2)

                 = 1/2 x 10 x 0.03                       (1/2)

                 = 0.15J                                       (1)

3.a) Calculate the kinetic energy of a ball (mass of 10kg) which is falling, if, at the top of the building it had a potential energy of 12kJ and now has a potential energy of 8kJ.

            Et = Ek + Ep                                   (1/2)

       1200 = Ek + 8000                                (1/2)

           Ek = 4000J (4kJ)                               (1)

b) What height is the ball at now?
           Ep = mgh                                        (1/2)

       8000 = 10 x 9.81 x h                           (1/2)

       8000 = 98.1 x h

            h = 8000/98.1

               = 81.5m                                        (1)

4. What is the acceleration due to gravity (g) on earth?
            9.81 m/s/s                                        (1)     

5. Calculate the speed of a lorry with a weight of 5000N and a kinetic energy of 6500J.
           m = w/g                (1/2)         Ek = 1/2 mv2                (1/2)

               = 5000/9.81      (1/2)      6500 = 1/2 x 510 x v2      (1/2)

               = 510kg              (1)                = 255

                                                           v2 = 6500/255

                                                               = 25.5

                                                           v = 5.05 m/s                   (1)

6. What is the mass of an object which has the potential energy of 7620J at a height of 3m?
         Ep = mgh                        (1/2) 

     7620 = m x 9.81 x 3            (1/2) 

              = m x 29.43

          m = 7620/29.43

              = 260kg                        (1)

7. Calculate the heat energy produced by an electric hair drier if it is 65% efficient and used 240V and 6A over 10 minutes.

     Ee = VIt                                (1/2)       Eh = 0.65 x Ee            (1/2)

          = 240 x 6 x (10 x 600)     (1/2)            = 0.65 x 864000    (1/2)

          = 864kJ                              (1)             = 562kJ                    (1)