Heat and Phase change: latent heat

[scw287]

Homework Statement

A woman finds the front windshield of her car covered with ice at -12.8°C. The ice has a thickness of 4.60 10-4 m, and the windshield has an area of 1.25 m2. The density of ice is 917 kg/m3. How much heat is required to melt the ice?

Homework Equations

Q=mL or change temp=Q/constant*mass

The Attempt at a Solution

Volume = Area * thickness<<then found the density
Q=mct Energy = mass* specific heat* cahnge in temp
heres the numbers that I plugged in: Q=(917)*(5.75*10^-4)*(12.8)*(22.6*10^5)<this last number is the latent fusion for water

the answer I got was wrong. not sure if I'm make a simple mistake or completely on the wrong track!

thanks in advance

 

[Doc Al]

Try this. Think in terms of two separate steps:
(1) You must raise the temp of the ice to the melting point
(2) You must melt the ice

Find the heat needed for each step and add them up.

 

[scw287]

1.if i raise the temp of the ice to the melting pt. it would need to be at its boiling pt 100 degrees celsius. heat rises

2. now must melt the ice at 0 degrees celsius

I'm sorry I really don't know what I'm doing just learned this for the first time today along with four other physics chapter.

could I use m*L<heat gained
and c*m*change in temp.< heat lost
and set the two equal?
?

 

[Doc Al]

1.if i raise the temp of the ice to the melting pt. it would need to be at its boiling pt 100 degrees celsius. heat rises

What temperature must the ice be in order to melt? Just melt, not boil!

 

[scw287]

well the melting point is at 0. degrees celsius

 

[scw287]

is the latent fusion the same thing as the temperature for which ice needs to be in order to melt?

 

[Doc Al]

well the melting point is at 0. degrees celsius

Right. So how much heat is needed to raise the temperature of the ice? (Hint: Specific heat)

is the latent fusion the same thing as the temperature for which ice needs to be in order to melt?

The latent heat of fusion tells you how much energy you need to melt something when it's already at the melting point.

 

[scw287]

Specific heat for water=4186

 

[scw287]

So I need to use the formula Q=mass*specific heat*change in temp for the first part right?

 

[Doc Al]

Specific heat for water=4186

What's the specific heat of ice?

So I need to use the formula Q=mass*specific heat*change in temp for the first part right?

Right.

 

[scw287]

good after I find that I need add that to melting the ice. How does the area and the density come into play?

 

[Doc Al]

How does the area and the density come into play?

It will allow you to find out how much ice you need to melt.

 

[scw287]

I'm still not sure how to find the melting of the ice, do i need to use latent heat constant?
ahhh so add the two values together and then multiply by the area*the density?