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Uni physics problem! i can't figure this one out!?

Answers:27   |   LastAnswerAt:2018.10  

Asked at 2011.04.21 21:08:01
What is the minimum amount of energy required to completely melt a 7.25-kg lead brick which has a starting temperature of 18.0 °C? The melting point of lead is 328 °C. The specific heat capacity of lead is 128 J/(kg · C°); and its latent heat of fusion is 23 200 J/kg.
answer jcherry_99  Answered at 2011.04.21 21:08:01
In a way, you can do this by unit analysis. Just break it down into steps.

Step one
Find the heat needed to get the brick up to it's melting point.
m = 7.5 kg
t1 = 18oC
t2 = 328oC
Specific heat = 128J/kg oC

Note the units of the specific heat. Really important. It's joules / (kg oC) That means you need you mass in Kg and your temperature in Celcius. You have those things. No conversions needed.

Also ...

You want the units to cancel so that only Joules are left. That means the temperature and specific heat and mass aare multiplied.

Q = m * c * ( t2 - t1 )
Q = 7.25 kg * 128 J/(kg oC) * (328 - 18 )oC
Q = 287700 J

Step Two
Melt the stuff.

Add enough heat to overcome the heat of fusion.

Q = H * m

H = 23200 J/kg
m = 7.25 kg

Again, no concersions and you are multiplying.
Q = 23200 J / kg * 7.25 kg
Q = 168 200 J

Step three
Find the total

Q_total = 287700 J + 168 200 J

and I'll let you get the total.

Note: I've rounded.
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