Investigating Methods of Heat Transfer (WJEC GCSE Physics)

Investigating Convection

Aim of the Experiment

• The aim of the experiment is to make observations about convection currents

Variables:

• Independent variable = Initial temperature of water
• Dependent variable = Time taken 
• Control variables:
  • Size of potassium manganate (VII) crystal
  • Size of flame used

Equipment List

List of Equipment Used to Investigate Convection

Equipment	Purpose
2 × 250 cm3 beakers	One for cold water, one for hot water
2 × crystals of potassium manganate (VII) crystals of similar size	To colour the water to observe the convection currents
Glass tube	To allow deliberate placement of the potassium manganate (VII) crystal
Tripod & gauze	To position the beaker above the Bunsen burner
Heat proof mat	To protect the worktop
Bunsen burner	For heating the water to produce a convection current
Forceps	To transport the potassium manganate (VII) crystal to the beaker without contact
Stop watch	To measure the time taken for the water to reach uniform colour

Method

Equipment for Potassium Manganate (VII) Convection Investigation

The Bunsen flame should be placed beneath the potassium manganate (VII) crystal so that the convection current can be seen 

1. Fill one beaker with 200 cm³ of cold tap water
2. Place the beaker on the gauze on top of the tripod with the heat proof mat underneath
3. Pick up the crystal potassium manganate (VII) using the forceps
4. Place the glass tube into the beaker and drop the potassium manganate crystal through the tube to the bottom of the beaker (this provides an air corridor for the crystal to be placed at the bottom of the beaker, otherwise it would begin to dissolve as it falls to the bottom) 
5. Place your finger over the top of the glass tube and remove it from the beaker
6. Light the Bunsen burner and slowly turn down the gas to get the smallest blue flame possible
7. Place the Bunsen flame directly beneath the crystal and start the stop watch
8. Record the time at which the water reaches a uniform colour
9. Record your observations focussing on the convection current
10. Repeat the experiment using hand-hot tap water

Observation Table for Convection Investigation

Analysis of Results

• The observations should be that:
  • The purple coloured water rises above the flame
  • Then spreads across the beaker
  • Then sinks down on the other (colder) side
  • The hot tap water takes less time than the cold tap water to reach a uniform colour
• The explanation of the observations are that:
  • Warmer water is less dense so it rises and the denser cooler water takes its place
  • The warmer water cools and becomes denser and sinks back down
  • The resulting motion is a convection current
• For the water with a higher initial temperature, the water particles will have a higher kinetic energy
  • This means that the water particles will move faster than those in the cold water
  • Therefore, convection will happen more quickly in the hot tap water than in the cold tap water

Evaluating the Experiment

Random Errors:

• The crystal of potassium manganate can be difficult to place in exactly the same place twice, so the shape of the convection currents may not be identical
• The size of the crystals may not be exactly the same, a larger crystal will take less time to reach a uniform colour in the water
• The flame size may not be identical if the flame was relit at any point. A larger flame will speed up convection

Safety Considerations

• Potassium manganate (VII) can be harmful to the skin so forceps should be used to avoid contact 
• Potassium manganate (VII) can stain clothes so be careful to avoid residue from the crystals, splashes from the solution, and clean up any splashes or spills immediately
• Lowering the gas feed via the tap could cause the flame to go out. If this happens, turn the gas off completely until you light the Bunsen again
• Any glass, metal, water or potassium manganate solution will become very hot when heated and will cause burns if touched
• Allow the beaker, its contents, the gauze, tripod and Bunsen burner to cool before dismantling the equipment

Investigating Radiation

Aim of the Experiment

• The aim of the experiment is to investigate which colour paper or foil is the best absorber of heat

Variables:

• Independent variable = colour of paper or foil
• Dependent variable = temperature
• Control variables:
  • Bulb / lamp used
  • Distance from the lamp to the thermometer
  • Room temperature - make sure windows and doors are closed to prevent draughts
  • Time taken

Equipment list

List of Equipment Used to Investigate the Density of a Range of Objects

Equipment	Purpose
Filament lamp	To provide a heat source
2 × bulb thermometers	To measure the temperature change
Small piece of black paper	To cover the bulb of one thermometer
Small piece of silver foil	To cover the bulb of the other thermometer
Sellotape	To attach the paper / foil to the thermometer
Stop watch	To measure the time taken
2 × clamp stand, clamp & boss	To hold the thermometers at set distance from the heat source
Ruler	To measure the distance between the heat source and the thermometers

Method

Equipment for Thermal Radiation Investigation

The thermometers should be clamped at the same distance from the lamp to ensure a fair test

1. Attach a 2 cm strip of black paper to one thermometer using Sellotape
2. Attach a 2 cm strip of silver foil to the other thermometer using Sellotape
3. Clamp both of the thermometers 10 cm away from the filament lamp
4. Record the initial temperature readings on both thermometers
5. Start the stopwatch as you switch on the lamp
6. Record the temperature on each thermometer after 10 minutes

Example Results Table for Radiation Investigation

A results table should include spaces for each measurement taken arranged in a logical way

Analysis of Results

• The colour of an object affects how good it is at emitting and absorbing thermal radiation
• The black paper should reach a higher temperature than the silver foil
  • This is because black objects are better absorbers (and emitters) of thermal (heat) energy
  • Silver coloured objects are poor absorbers (and emitters) of thermal energy
    • This is because shiny surfaces reflect thermal radiation

Evaluating the Experiment

Systematic Errors:

• Read the thermometer from eye level to ensure you are reading the correct value
• Make sure the surface area of the paper and foil are equal
• Make sure the Sellotape has the same coverage on both the paper and the foil

Random Errors:

• Try to control for any changes in temperature that may occur within the room
  • Close doors and windows to prevent drafts
  • Close blinds to keep out intermittent strong sunlight

Safety Considerations

• The lamp may become hot so allow it to cool before touching it to avoid burns

Investigating Conduction: Metal Rods Experiment

Aim of the Experiment

• The aim of the experiment is to determine the order of different metals based on their conductivity

Variables:

• Independent variable = type of metal
• Dependent variable = time taken for drawing pin to fall
• Control variables:
  • Length of metal rods
  • Type of drawing pins used
  • Position of flame on rod
  • Amount of Vaseline used on each rod

Equipment list

List of Equipment Used to Investigate Conduction in Metal Rods

Equipment	Purpose
4 metals rods of different metals (aluminium, brass, copper, iron)	To determine the order of conductivity of the metals
4 drawing pins	To attach to the ends of the metal rods
Vaseline	To attach the drawing pins to the metal rods
Tripod	To position the metal rods on
Heat proof mat	To protect the worktop
Bunsen burner	For heating the the metal rods
Stop watch	To measure the time taken for the drawing pins to fall

Method

Equipment for Conduction in Metal Rods Investigation

The end of each rod must be placed equally in the flame to ensure a fair test

1. Lay the metal rods across the tripod with the ends together so that they will all fit into the flame
2. Add a small amount of Vaseline to the other end of each rod and attach a drawing pin
3. Bring the Bunsen flame to the ends of the rods so they are heated equally and start the stopwatch
4. Record the time at which each drawing pin falls
5. To increase the accuracy of the results, repeat the investigation three times and calculate an average time for each metal

Example Results Table for the Metal Rod Conduction Investigation

A results table should include spaces for each measurement taken and for any calculations required (such as averages)

Investigating Conduction: Conductive Ring Experiment

Aim of the Experiment

• The aim of the experiment is to determine the order of different metals based on their conductivity

Variables:

• Independent variable = type of metal
• Dependent variable = time taken for match to fall
• Control variables:
  • Type of matches used
  • Position of metal strips in flame
  • Amount of Vaseline used

Equipment list

List of Equipment Used to Investigate Conduction in Metal Rods

Equipment	Purpose
Conducting ring with 4 strips of different types of metal (aluminium, copper, brass, steel)	To determine the order of conductivity of the metals
4 wooden matches	To attach to the ends of the metal strips
Vaseline	To attach the matches to the metal strips
Clamp stand, clamp and boss	To position the conductive ring above the flame
Heat proof mat	To protect the worktop
Bunsen burner	For heating the the metal strips
Stop watch	To measure the time taken for the matches to fall

Method

Equipment for Conductive Ring Investigation

The flame should be placed directly under the centre of the conductive ring to ensure all metal strips are heated equally

1. Clamp the conductive ring, taking care to keep the clamp away from the centre of the ring
2. Attach a match to each metal strip using Vaseline
3. Place the Bunsen flame beneath the centre point of the conductive ring ensuring equal exposure to the flame for each metal strip and start the stopwatch
4. Record the time at which each match stick falls
5. To increase the accuracy of the results, repeat the investigation three times and calculate an average time for each metal

Example Results Table for Conductive Ring Investigation

A results table should include spaces for each measurement taken and for any calculations required (such as averages)

Analysis of Results

• You should find that copper falls first because copper is the best conductor
• Next should be aluminium, brass and then steel / iron
• You may find that aluminium falls first because the time between aluminium and copper is small
• Repeating the investigation, taking care with the control variables, should give copper a smaller average time

Evaluating the Experiments

Random Errors:

• The base of the rods / metal strips may not have equal contact with the flame
• Having more Vaseline on one pin / match will take longer to melt

Safety Considerations

• The rods will be hot and may roll off the tripod. Use heat-protective gloves to handle them and place them on a heat-proof mat after use
• The conducting ring will become hot, allow it to cool completely before touching
• The Vaseline may stain clothes, wash your hands after use and clean up any mess
• Vaseline on the floor can cause slips. Clean up any spills immediately and alert your teacher 
• Over heating aluminium may cause it to melt which could cause burns - if the aluminium begins to melt, remove the Bunsen flame from the metal and alert the teacher
• When the Bunsen is not in use, ensure it is set to safety flame (yellow) or turn the gas off completely