HEAT

Test
Click here for answer

Yes - that's how it works! Click on "Click here for answer" and the answer will be revealed.

Question One

One way of preventing hypothermia is by using emergency blankets when a tramper gets cold.  Emergency blankets are thin sheets with one side made of shiny silver coloured foil. The emergency blanket is wrapped around the tramper with the shiny side facing inside, towards their body.  Explain why a person will be warmer with the silver side of the blanket facing towards your body rather than facing outwards.

Click here for answer

When the shiny side faces towards the body, the radiation the person emits is reflected by the shiny side back into their body because shiny surfaces are poor absorbers of radiation. This reduces heat loss from the person’s body.

Question Two

A good way of keeping warm and preventing hypothermia is to drink hot sweet drinks. A gas stove is used to boil hot water in a metal pot to make hot chocolate.   Name the process by which heat energy is transferred through the bottom of the pot to the water. 

Click here for answer

Conduction

Question Three

Name the process by which heat is then spread throughout the water. 

Click here for answer

Convection

Question Four

Discuss how this process spreads the heat throughout the water.

Click here for answer

The water at the bottom of the pot heats up, and as it heats up it expands. It becomes lighter/less dense than the cooler water and therefore rises to the top of the pot where it cools. Cooler, denser water at the top sinks to the bottom where it then heats up. This starts convection currents where the water cycles throughout the pot, spreading the heat throughout the pot.

Question Five

Explain why the base of the pot is made of metal, but the handle is covered in plastic.

Click here for answer

The base of the pot is a metal which is a good conductor of heat so the water can be boiled.   The handle is covered in plastic which is a poor conductor of heat, so you do not burn yourself when lifting the pot.

Question Six

A fridge keeps food and drink cold, even on a very hot day. A fridge cools its contents by pumping cool liquid (called coolant) through pipes in the top of the fridge.  In a fridge the coolant flows through pipes in the ice box, cooling the air around the ice box. The icebox is in the centre top of the fridge. The rest of the air in the fridge is cooled by convection currents.  Discuss how convection currents cool all the air in the fridge.

Click here for answer

Air at the top of the fridge is cooled by the cold liquid. As it gets colder the air gets denser and the cold air sinks to the bottom of the fridge. Away from the cooling pipes it gets warmer because it picks up heat from the food. The air becomes less dense and rises to the top where it gets cooled again.

Question Seven

Discuss how the design of a fridge (white outer casing, thick walls lined with polystyrene, rubber seals around the door) prevents the transfer of heat from the surrounding areas into the fridge.

Click here for answer

There are five ways the fridge features prevent heat transfer.  The white outer casing prevents heat being transferred into the fridge by radiation. This is because white surfaces reflect radiation well. The rubber seals around the door keep the cold air in and the heat air out. This prevents convection currents taking the hot air from the van into the fridge. The rubber seals around the door reduce the heat being transferred into the fridge by conduction as rubber is a poor conductor of heat/insulator.  The thick walls reduce the heat transferred by conduction. The thicker the walls the slower the conduction. The polystyrene walls reduce the heat transferred by conduction as polystyrene is a poor conductor of heat/insulator.

Question Eight

Fiona parks her car is in full sunshine. When she returns to the car she finds that it is quite hot inside. To cool it down she winds the windows down a little.
She decides that heat has entered through the glass of the closed windows. State the type of heat transfer that happens through clear glass.

Click here for answer

Radiation

Question Nine

Discuss how the colour of a car and slightly open windows could affect the temperature inside the car.

Click here for answer

A light colour would reflect radiation while a darker colour would absorb radiation. The heat could then be conducted to the interior by the metal roof.  Open windows would allow rising hot air (convection) to leave the interior to be replaced by cooler air from below. Closed windows would prevent circulation.

Question Ten

It is the start of the Winter Sports season and Ruth has been working out at the gym. After her workout she goes to the changing room, which is at 25 °C and well ventilated. While sitting on a bench recovering, she loses heat to the surroundings in several ways,
Complete the table below by filling in the blank spaces.


Heat Transfer

Percentage of total
heat lost

By

To the

evaporation

(3)

25%

(1)

bench

5%

convection

(4)

10%

(2)

surroundings

60%

Click here for answer

(1) conduction   (2) radiation (3) air/sweat  (4) air currents

Question Eleven

Suddenly the door to the changing room is opened by another sportswoman.  Which type of heat transfer would increase the most? Discuss your reasoning.

Click here for answer

Convection/evaporation.  Opening the door allows air to move freely removing hot air from around body by convection (also increased loss by allowing greater evaporation).

Question Twelve

A student also is using a gas heater to keep the room warm.  Use the following diagram of her study to answer the questions that follow.

 

She notices that when she studies late at night with the heater on, that her feet get cold yet when she stands up, the air feels quite warm.

Which method of heat transfer best describes the warmth from the heater rising to the top of the room? 

Click here for answer

Convection

Question Thirteen

Explain how insulation in the ceiling helps to keep the room warm.

Click here for answer

Heat cannot travel through the insulation easily as air pockets in it act as a good insulator.

Question Fourteen

After leaving the chair next to the heater for a long time, the student notices that it also feels warm.  Discuss the effect of the colour and texture of the chair on the amount of radiant energy it absorbs when sitting close to the heater.

Click here for answer

The darker the colour of the chair (eg black), the more heat energy will be absorbed.  A rough texture will increase this further.

Question Fifteen

Explain why it is important that she should keep the curtains shut when trying to keep the room warm.

Click here for answer

When warm air particles come into contact with cold glass, heat energy is transferred to the glass by conduction.  Pulling the curtains creates an insulating layer and prevents warm air coming into contact with the window.

Question Sixteen

After turning the heater off the room remained warm for another hour.  Use scientific ideas to discuss three methods that may be used to keep your house warm.  

Click here for answer

Carpet to provide an insulating layer on the floor.  Blocking drafts at the bottom of doors prevents cold air entering by convection.  Thick curtains provide an insulating layer around windows.  Insulation (eg pink bats) in the ceiling reduces the heat that is lost at the top of the room.  Ceiling fan to drive warm convected air back down.