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Heat Energy

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Increasing the temperature of 1 kg of water from 10oC to 15oC takes _________ energy compared to the energy needed to increase its temperature from 15oC to 20oC.

A. moreB. lessC. the same

Increasing the temperature of 1 kg of water from 10oC to 15oC takes _________ energy compared to the energy needed to increase the temperature of 2 kg of water by the same amount.

A. moreB. lessC. the same

Increasing the temperature of 1 kg of water from 10oC to 15oC takes _________ energy compared to the energy needed to increase its temperature from 15oC to 20oC.

A. moreB. less*C. the same

Increasing the temperature of 1 kg of water from 10oC to 15oC takes _________ energy compared to the energy needed to increase the temperature of 2 kg of water by the same amount.

A. more*B. lessC. the same

Latent Heat

3U Physics

Note that the equation

Q = mcDT

can only be applied if the material does not change state while changing temperature.

Changes of state require additional energy:

Additional heat is required to change a material’s state from solid to liquid (at the melting point):

Q = mLf

where Lf is the latent heat of fusion (different for different materials).

This heat is absorbed by the material when it melts and released when it freezes.

Latent heat explains why ice stays on the ground even when the temperature is above 0oC – it takes additional latent heat to do the melting.

Similarly, additional heat is to change a material’s state from liquid to gas (at the boiling point):

Q = mLv

where Lv is the latent heat of vapourization (different for different materials).

This heat is absorbed by the material when it ____________ and released when it ____________.

This heat is absorbed by the material when it evaporates and released when it condenses.

Latent heat explains why sweating cools you down – the sweat absorbs heat energy from your skin when it evaporates.

How much heat must be removed from 0.10 kg of water at 80.0oC to cool it to ice at -30.0oC?

Notecwater = 4200 J/kgoC

cice = 2100 J/kgoC

Lf(water) = 3.3 x 105 J/kg

How much heat must be removed from 0.10 kg of water at 80.0oC to cool it to ice at -30.0oC?

To cool the water to 0oC:

Q = mcDT

= (0.10 kg)(4200 J/kgoC)(80.0oC)

= 33 600 J

How much heat must be removed from 0.10 kg of water at 80.0oC to cool it to ice at -30.0oC?

To cool the water to 0oC:

Q = mcDT

How much heat must be removed from 0.10 kg of water at 80.0oC to cool it to ice at -30.0oC?

To freeze the water at 0oC:

Q = mLf

How much heat must be removed from 0.10 kg of water at 80.0oC to cool it to ice at -30.0oC?

To freeze the water at 0oC:

Q = mLf

= (0.10 kg)(3.3 x 105 J/kg)

= 33 000 J

How much heat must be removed from 0.10 kg of water at 80.0oC to cool it to ice at -30.0oC?

To cool the ice to -30.0oC:

Q = mcDT

How much heat must be removed from 0.10 kg of water at 80.0oC to cool it to ice at -30.0oC?

To cool the ice to -30.0oC:

Q = mcDT

= (0.10 kg)(2100 J/kgoC)(30.0oC)

= 6 300 J

How much heat must be removed from 0.10 kg of water at 80.0oC to cool it to ice at -30.0oC?

33 600 J + 33 000 J + 6 300 J

= 7.3 x 104 J

Homework Set 9: Latent Heat