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# Heat Energy PowerPoint PPT Presentation

Heat Energy. Increasing the temperature of 1 kg of water from 10 o C to 15 o C takes _________ energy compared to the energy needed to increase its temperature from 15 o C to 20 o C. A. moreB. lessC. the same

Heat Energy

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### Heat Energy

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

### Heat Energy

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

### A Caveat

Note that the equation

Q = mcDT

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

### The Heating Curve

Changes of state require additional energy:

### Latent Heat of Fusion

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).

### Latent Heat of Fusion

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

### Latent Heat of Fusion

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.

### Latent Heat of Vapourization

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).

### Latent Heat of Vapourization

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

### Latent Heat of Vapourization

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

### Latent Heat of Vapourization

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

### Latent Heat: Example

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

### Latent Heat: Example

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

### Latent Heat: Example

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

### Latent Heat: Example

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

### Latent Heat: Example

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

### Latent Heat: Example

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

### Latent Heat: Example

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

### Latent Heat: Example

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

### More Practice

Homework Set 9: Latent Heat