Practical Circuits€¦ · Web view07/10/2018 · Question Practice on Specific Heat Capacity....

EE 7-1EE 7-1 Heat EnergyHeat EnergyHeat Energy, Mass and Change in Temperature (Specific Heat Heat Energy, Mass and Change in Temperature (Specific Heat Capacity)Capacity)

EXPERIMENT 28 - E and ΔT Copy the following table into your rough/example jotter

Connect up the circuit as shown,

Ask which materials are available to use. These will probably be metal blocks and a liquid such as water - select one of these.

Measure the starting temperature of the material. Switch on the heater until a temperature rise of at least 5 oC is shown on

the thermometer. Note the reading on the Joulemeter and the temperature. Switch the power supply back on and repeat the measurements every 5 oC

as the block heats up. Plot a graph of Energy supply against change in temperature. What relationship does your graph show between Energy and change in

Temperature?

Westhill Academy MJR Electricity & Energy (National 5) Page 93

Temperature (oC) TotalEnergy used E(Joules)

InitialTi

FinalTf

Change

ΔT

JouleMeter

Power Pack

1 kg Metal block (or beaker of

water)

Thermometer

Immersion Heater

- +

EXPERIMENT 29 - E and m

Repeat the experiment we have done previously, however you will change the mass of material and attempt to obtain the same temperature rise for each mass of water

Copy the following table into your rough/example jotter

Select a mass of water. Measure the start temperature of the water. Switch on the electrical supply. Observe the thermometer unit the temperature of the water rises by 5oC. Switch off the electrical supply and measure the energy needed to do this Record your results in your table. Repeat this for 5 different masses of water. Plot a graph of Energy supplied against mass. What is the relationship between Energy supplied and mass of material

used?


Mass(kg)

Temperature (oC) TotalEnergy used E(Joules)

InitialTi

FinalTf

Change

ΔT55555

JouleMeter

Water

Thermometer

Immersion Heater

- +

Heat EnergySpecific Heat Capacity

When supplying heat energy to a substance we observe the following relationships

From graphing E against ΔT we obtained: <WRITE YOUR RELATIONSHIP HERE>

From graphing E against m we obtained:<WRITE YOUR RELATIONSHIP HERE>

These can be combined to give

Where c is a constant called the SPECIFIC HEAT CAPACITY. It is unique to each substance and each state a particular substance is in eg. For ice (solid) c = 2108 J kg-1 oC-1

For water (liquid) c = 4187 J kg-1 oC-1

For steam (gas) c = 1996 J kg-1 oC-1

If we rearrange this equation it is easier to see what ‘specific heat capacity’ is :

DEFINITION: Specific Heat Capacity is the energy supplied to a substance to change the temperature of 1 kg by 1 oC.

To consider it another way,

AFTER HEATING, this is the energy STORED in the substance per kg per oC, hence the ‘Heat Capacity’ part of the name. The ‘specific’ part of the name really means ‘specific to one kilogram’ (i.e. per kilogram).

Substances also cool down, so the specific heat capacity is also the heat RELEASED per kilogram per oC of temperature loss by the substance during the cooling process.

<AFTER THIS NOTE INSERT YOUR E - ΔT AND E - m GRAPHS>


NOTES

J kg-1 oC-1J kg

oC or K (see gas laws section)

Question Practice on Specific Heat Capacity1. A 20 kg block of steel cools down from 45°C to 20°C. How much heat energy does it give out?2. During an experiment, a girl supplies 12 000 J of energy to 0.25 kg of water in a polystyrene

container. (a) By how much does the temperature of the water rise?(b) She repeats the experiment in exactly the

same way, but this time she uses 0.25 kg of methanol instead. What is the rise in temperature this time?3. How much heat energy is needed to heat up 2 kg of sea water by 1°C?4. How much heat energy is needed to heat up 3 kg of aluminium by 4°C?5. Calculate the amount of heat energy required to raise the temperature of 2 kg of

water by 10°C.6. Calculate the amount of heat energy required to raise the temperature of 4 kg of

steel by 15°C.7. How much heat energy is given out when 1 kg of water cools down from 80°C

to 20°C?8. An electric cooker has a 500 W cooking element.

(a) How much heat energy can it give out in 5 minutes?

(b) If the element is used to heat 1 kg of cooking oil for 5 minutes, what would be the rise in

temperature?

9. The specific heat capacity of fire-clay is about 800 joules per kilogram degree Celsius.

(a) containing 60 kg of fire-clay, if the clay's

temperature rises by 100°C?

( b) If the heater takes 4 hours to heat up, and the heating element works at 230 V, what is the current in the

element?

10. Oil-filled radiators are a useful means of providing background heating in a room. An electrical element warms up oil sealed inside the radiator.


MaterialSpecific

Heat Capacity(J kg-1 oC-1)

water 4 200sea water 3 900methanol 2 500

ice 2 100aluminiu

m900

concrete 800granite 800glass 700steel 500

copper 400mercury 150Cooking

oil1970

(a) How much energy is required to raise the temperature of the oil from 15°C to

40°C? The radiator contains 2 kg of oil and the oil used has a specific heat capacity of 2 500 joules per kilogram degree Celsius.

(b) If the element works at 230 V and takes 10 minutes to heat the oil through the above temperature change,

what is the current in the element?

EE7-2 Energy and Mass when Changing State (Latent Heat)

EXPERIMENT 30 - Cooling Curves and Change of State (fusion)Collect the following equipment

2 test-tubes (one containing some solid cetyl alcohol, the other an equal volume of water), 1 beaker, kettle, 2 thermometers, stopwatch

Perform the experiment1. Put a kettle of water on to boil

2. Place a thermometer into each tube. Place the tubes into the beaker of boiling water and leave them until the cetyl alcohol melts

and temperature of the liquids in the tube reads about 65-80°C.3. Start the stop clock and immediately measure the temperature of each

liquid.4. Note the temperatures at 1 minute intervals for 30 minutes in your

rough jotter.5. Plot a graph of temperature against time for both liquid

samples, on the same graph. Draw the results in 2 different colours on a single set of axes,

These are known as cooling curves(keep your graph safe for including with the notes that

follow this activity).


water

beaker boiling water

cetyl alcohol

boiling tube

thermometer

Answer these questions as a group or a class in your rough jotter or a mini whiteboard1. What is the difference between the two cooling curves?

2. Is the water a liquid at the start?

3. Is the water a liquid at the end?

4. Is the cetyl alcohol a liquid at the start?

5. Is the cetyl alcohol a liquid at the end?

6. Did the temperature of the water (a) remain constant for a time or (b) fall all through the 20 minutes?

7. Did the temperature of the alcohol (a) remain constant for a time or (b) fall all through the 20 minutes?

8. At all times, the water and alcohol must have been losing heat because they were hotter than their surroundings.

When the alcohol was at a constant temperature, it must still have been losing heat. Extra heat must have been produced in the alcohol to keep the temperature up. What was happening to the alcohol at this time?9. What change of state did the alcohol go through?10. A heating curve shows what happens to the temperature of an object as heat

energy is supplied (usually at a constant rate). Sketch in your rough jotters what you would expect these curves to look like, for water and cetyl alcohol over the temperature range, 20°C until they convert to a gas

11. What has to be supplied to change the state of a solid into a liquid or a liquid into a gas?


T T

timetime

Water Cetyl Alcohol

Latent Heat

When a substance changes its state, from solid to liquid or from liquid to gas, energy is taken in to break and alter the bonds between the particles.

The heat required to change the state of 1 kg of a substance without change in temperature is called latent heat . This is clearly shown on a cooling curve as the temperature of the curve remains steady for a short time.

<INSERT GRAPH>

When a substance changes state from gas to liquid or liquid to solid, the latent heat is given out, again altering the bonding between the particles.

Specific latent heat of fusion (lf) The energy needed to change 1 kg from solid to liquid (or liquid to a solid) without change in temperature.

Specific latent heat of vaporisation (lV) The energy needed to change 1 kg from liquid to gas (or gas to a liquid) without change in temperature

We can now define the Latent heat of an object as the energy supplied or removed per kilogram to change the state of a substance


NOTES

J

kg

J kg-1

EE7-3 Using Specific Heats and Latent Heats

Night Storage HeatingNight Storage radiators use cheaper ‘off peak’ electricity supplied by Electrical Companies at night. The heating elements are fixed into fire-clay blocks.During the night, when the electricity is on, the blocks are supplied with heat

at a rapid rate by electrical heating elements running at a high temperature. Fire-clay stores heat very well. In the morning, the electricity goes off but the blocks lose this heat at a gradual (slower) rate for the rest of the day. Some storage radiators have a fan built into them to aid the flow of air between the blocks. When heat is required the fan can be switched on, however this will deplete the heat stored in the blocks more quickly. The whole system relies on the blocks having a large Specific Heat

Capacity - they also make the heaters very heavy!

CoolantWater is an exceptionally useful substance for transferring heat from one location to another. It is easily available, cheap and has a reasonably high Specific Heat Capacity so it will absorb a high amount of heat energy for every oC temperature rise. Because it is a liquid it is then easy to move it to another location using a pump and pipes, two examples are given below:

Vehicle Engine cooling system

Domestic Heating system

Write a short note on the use of specific heat capacities in your jotter

cooling jacket for engine

water pump

Hot air

Cold air flow

Radiator

Engine block

burning fuel(oil, gas, wood pellets)radiates heat which is absorbed by the water

water pump

radiator

radiator

radiator

HEATre-radiated to

living environment

HEATre-radiated to

living environment

HEATre-radiated to

living environment

pipework

NOTES

RefrigerationThe coolant is a liquid as it enters the expansion valve (small hole). As it passes through, the sudden drop in pressure makes it expand, cool, and turn into a gas (just like a liquid aerosol turns into a cool gas when you spray it out of a can). As the coolant flows around the chiller cabinet (usually around a pipe buried in the back wall), it absorbs and removes heat from the food inside. The compressor then squeezes the coolant, raising its temperature and pressure. It's now a hot, high-pressure liquid. The coolant flows through thin pipes on the back of the fridge, giving out its Latent Heat and cooling back into a liquid as it does so. The coolant flows back into the expansion valve and the cycle repeats itself. So heat is constantly picked up from inside the refrigerator and put down again outside it

Cool boxes

Cool boxes are used to keep food cool for picnics.The sides are well insulated, which cuts down the heat getting in. But the most important feature of the box is a plastic pack of chemicals.The pack is first kept in a freezer until the chemicals have cooled down and changed from a liquid to a solid.You put the food in the bottom of the cool box, place one or two frozen packs on top, and then firmly close the lid. At first the cold frozen pack keeps the box cool, because it takes heat energy (see specific heat capacity formula) from the food. Even so, the temperature will slowly rise and the chemicals start to melt. But the chemicals need even more (latent) heat to melt, and so the food is kept cool for even longer.

Write a short note on the use of Latent heats in your jotterNOTES

HOMEWORK 4 - Choose one of the following topics to researchUsually we tell you to stick to about 150 words, but this may not be possible for this one, so you decide how much you need to write, but keep it compact and to the point - more than two typed pages you really need to think about cutting back a bit!

A reasonably recent type of heating is ground source heating, which can be considered

Geothermal energy. While probably not suitable for large commercial heat production it is very suitable for including when building new homes.Research ground source heating and how it extracts heat from the ground

One type of injury that most people dread is burns or scaldings, though first aiders and medical professionals have to deal with these frequently. while both hot water and steam cause

damage, steam can potentially cause a much worse burn than hot water So why is a steam burn potentially much worse than a hot water burn?

Well kind of…….but all we want you to do is consider this simple question;

Why do some foods seem to be much warmer than others when consumed - sometimes to the point of burning your tongue? eg. in a cheese and tomato toastie, why do the tomatoes seem much hotter than the cheese? (Assuming there are no chillies/wasabi/cayenne pepper/curry powder etc. present!)You may want to think about the differences between heating up the bricks in a night storage heater and releasing the heat during the day when considering this question or the role that steam has to play in this!

Astronauts or cosmonauts have to wear special protective clothing due to the extreme nature of the environment in

space, not simply because of the vacuum of space, but also due to the extremes in temperature…….HOWEVER THEY ARE NOT ALONE!!

………Even on earth some professions rely on specialist clothing to deal with extreme temperature environments, examples are fire fighters, mountain climbers, Arctic/Antarctic workers, deep sea divers etc..Select an extreme environment profession and research the clothing they wear, commenting on how heat energy is conserved and what this means about the specific heat capacity of the materials used.

EE7-4 Conservation of Energy and Heat Transfer

We have already discussed the link between particles being supplied with heat energy and the particles moving during the processes of Conduction and convection in previous S1/2 and S3 Energy courses. This means there must be a direct link between the KINETIC ENERGY of particles and the TEMPERATURE of the substance.

Kinetic Energy and Heat Energy

The MORE HEAT ENERGY a substance is supplied with the hotter it becomes and the FASTER the particles move, so the KINETIC ENERGY of the particles increases.In a solid this means they vibrate more vigorously in their fixed positions within the solid lattice of atoms.In a fluid (liquid and gas) where particles can move and change places this means they move with greater Kinetic Energy and collide with objects with more force.

Going Further - an example from Gas Laws

This idea is illustrated by this formula, used when physicists deal with gases.

where kB is just a special constant called Boltzmanns constant

This formula is not in any way part of the National 5 course, and you will never be asked to use it this year, however it does show that there is a link between Kinetic Energy and the Temperature of the particles - everything else in the formula creates a constant(or a ‘constant of proportionality’).

NOTES

Question Practice on Specific Heat Capacity and Latent Heat

Specific heat capacity of water = 4 200 J kg-1 oC-1

Specific latent heat of fusion of ice = 3.34 x 1 05 J kg -1

Specific latent heat of vaporisation of water = 22.6 x 1 05 J kg - 1

1. 5 000 J of energy raises the temperature of 2 kg of a liquid by 1°C. Howmuch energy will be required to raise the temperature of 4 kg of the

liquidby 1°C?

2. For 2 kg of the same liquid, how much energy is needed to raise thetemperature by:(a) 2°C, (b) 3°C, (c) 4°C, (d) 25°C (e) 0.1°C?

3. The specific heat capacity of concrete is about 800 J kg -1 °C -1. How much heat is

stored in a storage heater containing 60 kg of concrete when it is heated through

1 0 0°C?

4. Water in an aluminium beaker is heated by a bunsen burner. The temperature-time

graph is shown below.

Explain why the graph does not continue to rise after it reaches A.

5. Joe put 1 litre (1 kg) of water at 15°C into his 1 500 W kettle. He switched it on and then forgot about it for 5 minutes. Unfortunately, it did not have an

automatic cut-out, and when he came back, the kitchen was full of steam.(a) How much energy was required to bring the water to boiling point?(b) How much electrical energy had been used altogether?(c) How much energy had been used to make steam?(d) How much water had been turned into steam?(e) Which of your answers are approximate and why?

6. It is often said that a scald from steam is much worse than a scald from boiling water.(a) Calculate the energy given out by 0.1 g of boiling water as it cools to

30°C (skin temperature). (b) Calculate the energy given out by 0.1 g of steam as it changes

to water at 100°C.(c) The water in (b) must still cool down to 30°C, so what is the total amount of energy released by the steam?(d) Explain clearly why a scald from steam at 100°C is worse than a scald from the same amount of boiling water.

7. A heating coil carries an electric current of 2 A for 50 s at a voltage of 10 V. If this is sufficient to boil away 5 g of liquid nitrogen at its boiling point, what is the specific latent heat of vaporisation of nitrogen?

8. A freezer cools 0.4 kg of water from 20°C to its freezing point in 30 minutes. Calculate:(a) the energy removed per minute from the water, and(b) how much longer it would take for the water to be changed into ice, if

heat continued to be removed at the same rate.

9. Why does the air at the back of a fridge become warm?

10. Which properties would you look for when choosing a refrigerant liquid?

EE7-5 Conservation of Energy to Determine Heat Transfer

As we have already mentioned in the conservation of energy earlier in this section, often wasted energy is in the form of heat. For example if an object travels down a slope, moving freely without propulsion other than gravity, the Potential energy it possessed at the top of the slope is not equal to the energy lost at the bottom. The ‘missing’ energy has been converted to heat in the bearings of the wheels, or between the road and the tyres (especially important to motorcyclists to improve the grip of their soft rubber tyres). So most of the work done against friction is converted to heat.

In many circumstance other energy types are deliberately converted to heat, e.g. electrical heaters and gas/oil boilers in our homes, brakes on vehicles, generation of light by either fluorescent (energy saving) illumination or filament lamps/bulbs. This is where we need to reconsider the Law of Conservation of Energy again

i.e. The total energy in any system always remains constantOR

Energy is never created or destroyed it is merely converted from one form to another.

Look back at the Specific Heat experiments.

Did we actually measure the heat energy that was in the metal block?

What type of energy did we ACTUALLY measure?

What were we assuming about energy transfer in this situation?

Is this the case?

Why would there be a difference between the values of the two energy types

What could we do to prevent this difference being so large?

JouleMeter

Power Pack

Water

Thermometer

Immersion Heater

Vehicle Braking - Kinetic to Heat

If we consider a vehicle slowing down we see another important conversion of energy

Calculate the kinetic energy of the vehicle at the start of its journey assuming the mass of the motorcycle is 195 kg and the rider is 85 kg.

How much kinetic energy is left at the end of its journey?

What form has the ‘energy lost’ been converted into?

Where does this happen?

If this item is made of Stainless steel with a specific heat capacity of

What eventually happens to the heat created in this item ?

Electrical to Heat

Consider the following problem where a kettle is being used to boil a mass of water. The water in the kettle is about room temperature (20 oC)

The kettle is connected to the mains supply running a t 230 V, and takes a current of 12.5 Amps

What is the Power used by the kettle’s heating element

If it takes 3 minutes 18 seconds to reach the boiling point of water, what is the maximum Energy the element can supply to heat the water?

If the mass of water heated is 1.7 kg, calculate the value that this gives for the specific heat capacity of water.

v = 0 ms-1

AT START AT END

Concealed Element

v = 20 ms-1

The time taken to reach boiling point is actually a little longer than this - can you explain why?

Industrial laser cutting - Light to Heat

Industrial lasers can be used for cutting any kind of material, titanium, steel, plastic, wood, etc.

To cut holes in stainless steel a company is using a 80 watt CO2 laser which takes 10 seconds to vaporise the material, a gas jet is then used to blow away the molten material to create a hole. It has to raise the temperature of 0.05 g of stainless steel to its melting point to allow a gas jet to blow the material out of a single hole.

The output power of the cutting laser is only 5 % of the power it’s used, so how much power actually reaches the surface of the stainless steel?

What is the energy supplied to the steel while this laser is cutting the hole?

If the specific heat capacity of the stainless steel is 510 J kg-1 oC-1, what is the theoretical rise in temperature produced in this process (assuming no change of state)?

Is this sufficient to raise the stainless steel to its melting point (1510 oC)?

In practice it takes a little longer to create the hole. Using what you know about metal explain why this happens.

Question Practice on Heat

1. A solar furnace consists of an array of mirrors which reflect heat radiation onto a central curved reflector.

A heating container is placed at the focus of the central curved reflector.Metals placed in the container are heated until they melt.The diagram below shows the heat rays after reflection by the mirrors on thehillside.

(a) Copy and Complete the diagram to show the effect of the central curved reflector on the heat rays.

(b) 8000 kg of pre-heated aluminium pellets at a temperature of 160 °C are placed in the container. Aluminium has a specific heat capacity of

How much heat energy is required to heat the aluminium to its melting point?

(c) (i) How much extra energy is required to melt the aluminium pellets?

(ii) The power of the furnace is 800 kW. How long will it take for this extra energy to be supplied?

(iii) Explain why it takes longer, in practice, to melt the aluminium.©SQA 2009 SG Credit exam paper

Data Tables

2. An electric kettle is used to heat 0.4 kg of water.(a) The initial temperature of the water is 15 °C.

Calculate how much heat energy is required to bring this water to its boiling point of 100 °C.

(b) The automatic switch on the kettle is not working. The kettle is switched off 5 minutes after it had been switched on.

The power rating of the kettle is 2000W.(i) Calculate how much electrical energy is converted into heat

energy in this time.(ii) Calculate the mass of water changed into steam in this time.

3. A technician tests an electric kettle. The kettle is filled with water and switched on for 3 minutes.

The technician records the following information.

(a) (i) Show that 517 500 J of electrical energy is supplied to the kettle in

(ii) Calculate the mass of water in the kettle.(iii) Explain why the mass of water will be less than calculated in (a)

(ii).

(b) The technician tests a second kettle. When the water boils this kettle does not switch off and continues to heat the water.

(i) State what happens to the temperature of the water when it boils.

While the water is boiling, the kettle supplies 565 000 J of heat energy to the water.

(ii) Calculate the mass of water changed into steam.

©SQA 2007 SG Credit exam paper


4. A steam cleaner rated at 2 kW is used to clean a carpet. The water tank is filled with 1.6 kg of water at 20 °C. This water is heated until it boils and produces

steam. The brush head is pushed across the surface of the carpet and steam is released.

(a) Calculate how much heat energy is needed to bring this water to its boiling point of 100 °C.

(b) After the steam cleaner has been used for a period of time, 0.9 kg of boiling water has changed into steam.

(i) Calculate how much heat energy was needed to do this. (ii) Calculate how long it would take to change this water into steam.

5. A steam wallpaper stripper is used on the walls of a room.

Water is heated until it boils and produces steam. The plate is held against the wall and steam is released from the plate.

The tank is filled with water. The water has an initial temperature of 20 °C.(a) (i) Calculate the energy required to bring the water to its boiling

point.(ii) Calculate the time taken for this to happen.(iii) The actual time taken for this to happen was found to be longer

than that calculated in (a) (ii). Explain why.(b) Calculate the current required by the wallpaper stripper.(c) After using the wallpaper stripper for some time, 1.2 kg of water is

converted into steam. Calculate the energy used to do this.©SQA 2007 Int 2 exam paper


6. A fridge/freezer has water and ice dispensers as shown.

(a) Water of mass 0.1 kg flows into the freezer at 15 °C and is cooled to 0 °C.

Calculate the energy removed when the water cools.(b) Calculate how much energy is released when 0.1 kg of water at 0 °C

changes to 0.1 kg of ice at 0 °C.(c) The fridge/freezer system removes heat energy at a rate of 125 J/s.

(i) Calculate the minimum time taken to produce 0.1 kg of ice from 0.1 kg of water at 15 °C.

(ii) Explain why the actual time taken to make the ice will be longer than the value calculated in part (i).

7. On the planet Mercury the surface temperature at night is –173 °C. The surface temperature during the day is 307 °C. A rock lying on the surface of the planet has a mass of 60 kg.

(a) The rock absorbs 2·59 × 107 J of heat energy from the Sun during the day.

Calculate the specific heat capacity of the rock.(b) Heat is released at a steady rate of 1440 J/s at night.

Calculate the time taken for the rock to release 2·59 × 107J of heat.(c) Energy from these rocks could be used to heat a base on the surface

of Mercury.How many 60 kg rocks would be needed to supply a 288 kW heating

system?

©SQA 2009 Int 2 exam paper

©SQA 2010 Int 2 exam paper

8. An experiment was carried out to determine the specific heat capacity of water.

The energy supplied to the water was measured by a joulemeter.

The following data was recorded.Initial temperature of the water = 21 °C.Final temperature of the water = 33 °C.Initial reading on the joulemeter = 12 kJ.Final reading on the joulemeter = 120 kJ.Mass of water = 2·0 kg.Time = 5 minutes.

(a) (i) Calculate the change in temperature of the water.(ii) Calculate the energy supplied by the immersion heater.(iii) Calculate the value for the specific heat capacity of water

obtained from this experiment.(b) (i) The accepted value for the specific heat capacity of water is

quoted in the Data Table at the start of these questions. Explain the difference between the accepted value and the value obtained in the experiment.

(ii) How could the experiment be improved to reduce this difference?

(c) Calculate the power rating of the immersion heater.©SQA 2011 Int 2 exam paper

9. A coolant pack is used to treat an injured player at a hockey match.

Before use the coolant pack is stored in a refrigerator at 2 oC.The coolant inside the pack changes state

from liquid to solid.The coolant has a melting point of 7 oC and a mass of 0.5 kilograms.The coolant pack is removed from the refrigerator and placed on the injured

ankle of a player.(a) (i) Calculate the energy required to raise the temperature of the

coolant pack from 2 oC to its melting point.(Specific heat capacity of coolant = 2100 J kg-1 oC-1)

(ii) Where does most of the energy required to raise the temperature of the coolant pack come from? (b) Having reached its melting point the coolant pack then remains at the same temperature for 15 minutes.

What is happening to the coolant during this time? (c) One of the other players suggests insulating the coolant pack and

ankle with a towel.Why should this be done? ©SQA 2008 SG General exam

paper

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