Energy Practice NAB

Today we did a practice NAB for your NAB on 27/9/11.  The questions are below and the answers in the pictures at the bottom with the mark allocation drawn on.

Energy Practice NAB

1. A sprung train barrier stops a 2000kg train traveling at 10m/s.  On impact the train exerts a force of 400kN on the barrier.  Calculate the following:
  a) The kinetic energy of the train before it hits the barrier
  b) The compression of the spring in the barrier once the train has stopped moving
  c) The speed of the train once it has collided with the barrier and continued to move after 100mm.

2. An energy audit is carried out on an electrical kettle connected to a 240V supply. The heating element has a resistance of 50Ω and the kettle took 3 minutes to boil 2 litres of water from a starting temperature of 80°c.  Calculate:
  a) The electrical energy going into the kettle
  b) The heat energy produced by the kettle
  c) The energy "lost"
  d) The efficiency of the system

These questions are of the same standard as the NAB, if you are happy with these and how to answer them you will be prepared for the NAB.  Revision of the homework is also a good way to ensure a thorough understanding.  If you were off today at FutureWise then I strongly suggest you come and get your homework marks back!

Here are the answers:

 

Energy Revision

Energy is the ability to do "work".  Which means that it is what makes everything around us happen.  There are different types of energy.  We have been looking at some of the different types of energy and how energy can be transferred from one form to another.  This is a quick revision guide to what we have been doing in class.

Generally energy (or work done) can be found by multiplying the force by the distance it was exerted over, using the equation:
               E (wd) = F d (s)

Power is the rate at which energy is converted. This is another general energy question you need to be aware of. So power is the energy converted over time, or energy is power multiplied by time.
               E = Pt

Potential Energy is the energy a body possesses when it is positioned above the ground (datum) level, it is dependent on the weight (mass x g) x height:
               Ep = mgh

G is the acceleration due to gravity, so the rate at which we are being pulled towards earth.  It is a constant at 9.81 m/s/s on earth.


Kinetic Energy is movement energy.  The greater the mass and speed of a boy, the greater the kinetic energy.
             Ek = 1/2 mv2

Thermal Energy is heat energy.
             Eh = cmΔt 

C is the specific heat capacity of a material and it is constant.  For water C = 4190 J/KgK

Electrical Energy is based on the equations for electrical power, and the general energy power equation.
              E = Pt             P = VI    Therefore  Ee = VIt

Strain energy is the energy generated by stretching or compressing a spring.  First of all you may need to find the force required by multiplying the spring stiffness, in Newtons per millimetre, by the extension or compression.  This will give you the total force to use in the equation:

           Es = 1/2 Fx

Remember that although you need to use millimetres to find the force, x is measured in metres.

To find the efficiency of a system you must look at the energy in and the useful energy out.  Some energy will be "lost" i.e. the heat energy produced by a light bulb, or heat energy produced by friction in moving parts.
            n = Output Energy/Input Energy   (to make it a percentage, multiply by 100.

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)