9. The standard unit of energy is the Joule (j) or Kilojoules (Kj). 1 Kj is equal to 1000 joules.
Energy Interconversions
- Electric motor drives pulley on line shaft to raise mass:
A power supply unit (PSU) was used to supply electrical energy to a motor which converts the electrical energy to kinetic energy after some energy loss through electrical resistance and heat, the kinetic energy is then converted to rotational energy at the generator with losses due heat and sound. It then becomes kinetic energy at the belt with losses due to heat, friction and sound, energy is then converted to rotational energy driving the line shaft with losses due to heat and friction whilst applying gravitational potential energy (GPE) to the mass being lifted.
- Falling mass drives dynamo via line shaft to light lamp:
Gravitational potential energy (GPE) is stored in the weight mass, when released this was converted to kinetic energy for the weight mass with some energy loss due to friction, heat and sound. At the same time the GPE was also converted to rotational energy at the line shaft with losses due to heat and friction. The energy is then converted to kinetic energy along the belt with energy loss due to heat, friction and sound, it is then converted to rotational energy at the dynamo with losses due to heat, friction and sound, the dynamo then converted this to electrical energy with losses due to heat and sound the electrical energy was then transmitted through wires and converted to thermal energy at the lamp with losses due to electrical resistance and heat.
- Electric motor drives flywheel:
A PSU was used to supply electrical energy to a motor which then converted it to kinetic energy with energy losses due to heat and sound, this was then converted to rotational energy with losses due to friction and heat, the energy was then converted to kinetic energy along the belt with losses due to heat, friction and sound, it was then converted to rotational energy with some loss due to heat, friction and sound, finally becoming stored rotational energy at the flywheel.
- Flywheel drives dynamo to light lamps:
Stored rotational energy in the flywheel is converted to rotational energy with some loss due to heat, friction, sound and vibration, this is then converted to kinetic energy along the belt with losses due to heat, friction and sound, it is then converted to rotational energy to drive the dynamo with losses due to heat and friction , it then becomes electrical energy which was transmitted through wires to the lamp where it was converted to thermal energy after losses due to electrical resistance and heat.
- Small motor drives dynamo to light lamps
A PSU was used to supply electrical energy to a motor, this was then converted to rotational energy at the pulley with energy loss due to heat, sound and friction, it was then converted to kinetic energy along the band with losses due to heat, friction and sound, this was then converted to rotational energy at the dynamo with losses due to heat and sound, the dynamo then converted it to electrical energy which was then transmitted through wires to a bulb where it was converted to thermal energy after some loss due to electrical resistance and heat.
- Hand wheel drives dynamo to light lamp
Chemical energy from metabolised foods was supplied by a person to the hand wheel this was converted to rotational energy with losses due to heat and friction, it was then converted to kinetic energy along the belt with energy loss due to friction, heat and sound, this was then converted to rotational energy at the dynamo with loss from friction and heat, it was then converted to electrical energy and transmitted through wires to the bulb where it was converted to thermal energy with losses due to electrical resistance and heat.
- Battery drives lamps (stored electrical energy)
A PSU was used to supply electric energy to a lead/ acid battery, this energy was converted to chemical energy in the battery with losses due to electrical resistance and heat. When this supply of electric was cut off and a switch thrown the chemical energy (stored electrical energy) in the battery was converted to electrical energy with losses due to electrical resistance and heat, this was then transmitted through wires to the bulb where it was converted to thermal energy with some loss due to electrical resistance and heat.
- Solar (electromagnetic radiation) panel array and lamp drives electric fan.
Solar and thermal energy was generated through the use of a desk lamp with an enclosed 100 watt bulb as the light source; much of this energy was lost through heat and light diffusion especially in relation to the distance of the energy source from the panel. When held over the solar panel array the light energy is converted to electrical energy with energy loss through heat and light dissipation (as reflected by the speed of the fan, the closer the energy source the faster the fan turned therefore the more efficient the conversion process, the further away the source the slower the fan turned and the more energy is lost). The electrical energy was then transmitted through wires to a fan where it was converted to rotational energy with losses due to electrical resistance, heat and sound.
Unit 1.3 Dewi Hanks
Investigating the efficiency of energy transfer from a burning peanut.
AIM:
To investigate how much energy is transferred from a burning peanut to water and to calculate the efficiency of the energy transfer process.
APPARATUS:
Stand, boss and clamp, Measuring cylinder, Bunsen burner, Mounting needle, Thermometer, Boiling tube, Peanut, Beaker of water.
METHOD:
20cm³ (20g) of water was measured in the measuring cylinder, this was then poured into the boiling tube. The boiling tube was then clamped to the stand and the thermometer inserted into the tube. After allowing the water to achieve room temperature an initial temperature reading was taken. The peanut was then weighed on the top-pan balance and the mass recorded. The peanut was then fixed to the mounting needle. The Bunsen burner was then lit and adjusted to have the hole fully open in order to produce a roaring flame into which the mounted peanut was placed. When the peanut began to burn it was moved from the flame to a point under the boiling tube to heat the water. This continued until the peanut stopped burning whilst ensuring that the water was stirred to distribute the heat evenly. Once the peanut stopped burning a final water temperature reading was taken. ALL measurements were taken to one decimal place.
RESULTS:
Therefore in order to calculate the energy produced by the peanut the following equation will be used:
Mass of water x rise in temperature x 4.2
(Where 4.2 is the figure we know as the amount of energy needed to raise the temperature of 1cm³ of water by 1°C)
Therefore energy produced by the peanut = 20.0 x 44.0 x 4.2
= 3696 Joules
As the energy unit used in describing the energy value of food is the kilo joule:
Energy produced by the peanut is 3.7kJ
Therefore in order to work out how much energy would have been produced in this experiment by 1g of peanuts the figures need to be normalised as follows:
0.8 ⁄0.8 = 1 therefore 3.7⁄0.8 = 4.6kJ
Therefore in this experiment 1g of peanuts would have produced 4.6kJ of energy.
In order to then work out the efficiency of the experiment The Manual of Nutrition (HMSO) was consulted which gave an official figure for the energy content of peanuts as:
1182kJ per 50g of peanuts
Therefore to work out how much energy is officially contained in 1g of peanuts:
1182kJ⁄50g = 23.6kJ
So in order to work out the efficiency of the experiment the following equation was used:
Experiment value kJ X 100%
Official value kJ
Therefore: 4.6⁄23.6 x 100% = 19.5%
CONCLUSIONS:
The experiment showed that the energy transfer was very inefficient with a large difference between the official figure for the energy content of peanuts and the results that were achieved in the laboratory.
Suggested reasons for this inefficiency and discrepancy would be:
- Light produced during burning
- Heating of air around the experiment area
- Heating of the boiling tube
- Heating through conduction of the mounting needle
- Conduction at the clamp
- Energy loss during transfer from Bunsen burner to boiling tube.
In order for this experiment to work more accurately and be more efficient it would be necessary to conduct it in a more controlled environment with many less variables to affect it.
Unit 1.3
Dewi Hanks
Investigating the efficiency of heating a metal by an electrical method.
AIM:
To investigate and calculate the efficiency of heating a block of metal of known mass through the application of an electrical current.
APPARATUS:
A B C D E
F G H
A: Metal block in cladding
B: Immersion heater (Heating coil)
C: Thermometer (0 - 110°C x 0.1° C)
D: Voltmeter (0 – 20v)
E: Ammeter (0 – 10A)
F: Variable Rheostat (0 - 15Ώ)
G: Stop clock
H: d.c. power supply (PSU)
Also used not shown top pan balance. Optional and not shown a Joule meter.
METHOD:
Using banana cables we completed a circuit as follows; the PSU was connected to the Ammeter which was in turn connected to the Rheostat. The Rheostat was then connected to the heating coil. (All these connections were made using RED banana cables. A BLACK banana cable was then used to connect the second terminal of the immersion heater back to the PSU. We then connected the Voltmeter to the PSU using one BLACK and one RED banana cable.
Note the connection from the PSU to the Ammeter should be made as follows:
FROM the PSU: into the
TO the RHEOSTAT: into the
The connections to the Voltmeter should be made as follows:
RED banana cable into the
BLACK banana cable into the
We next weighed the first metal block using the top pan balance and recorded its mass in Kg. the immersion heater was placed into a hole in the metal block after applying a little Vaseline to aid thermal contact and the thermometer was placed in the other hole with Vaseline to improve the thermal contact between the thermometer bulb and the metal. After allowing the temperature to settle we recorded the initial temperature of the metal block in ˚C.
The current was switched on and the stop clock started, we recorded the temperature at 1 minute intervals (shown in the table below) and switched on the Ammeter and the Voltmeter to get readings of them.
After 10 minutes we switched off the current and recorded the final temperature achieved along with the Ammeter and Voltmeter readings all these were then set into the table below.
The experiment was then repeated with a different block of metal.
MEASUREMENTS:
TABLE A: Temperature rise in metal block over 10 minutes.
TABLE B: Experimental measurements.
TABLE C: Specific heat capacity of metal blocks available.
CALCULATIONS:
If c is the specific heat capacity of the metal, then, assuming no heat losses:
Energy supplied, I V t = mc (θ2 - θ1)
Energy supplied to the block = I V t
BRASS = 2.9 x 11.2 x 600 = 19488
MILD STEEL = 2.9 x 11.2 x 600 = 19488
Thermal energy gained by the block = mc (θ2 - θ1)
BRASS = 1 x 388 x (48 – 22) = 1 x 388 x 26 = 10088
MILD STEEL: 1 x 525 x (41 – 22) = 1 x 388 x 19 = 9975
Efficiency of energy transfer in the experiments =
Energy output / energy input x 100 %
BRASS = 10088 / 19488 x 100 % = 51.8 %
MILD STEEL = 9975 / 19488 x 100 % = 51.2 %
CONCLUSIONS:
The heating of the metal block provided us with a much more efficient model of energy transfer than did the burning peanut experiment, this was most probably due to there being less variables that could not be controlled in the metal block experiment and due to the fact that we had a continuous, regulated source of power rather than being reliant on the steadiness of the human hand for the heating process.
It also leads to the conclusion that electricity provides a much more efficient means for the transfer of thermal energy.
References:
Publications:
Azzopardi and Stewart (1995) Accessible Physics, Macmillan Press
Internet sites:
(accessed 27/09/08)
(accessed 27/09/08)
(accessed 10/10/08)
(accessed 10/10/08)
science.howstuffworks.com (accessed 10/10/08)
