Physics Exam Paper 2011 Hons Q.7 (Heat and Temp.)

Dwellingdweller

This question has me stumped, I just did the whole exam paper and I could not for the life of me figure this one out (I'm stupid, I know). Anyways, I looked at the SEC marking scheme for it and here's the solution they have for it.

"When making a hot drink steam at 100*C is added to 160g of milk at 20*C. If the final temperature of the drink is to be 70*, what mass of steam should be added? You may ignore energy losses to sorroundings."

( m c dt ) of m (I don't know what m stands for here ) = ( m l ) of steam + (mass of steam)(4.18x10^3)(30)
they then work it out and get:

mass of steam = 31.2x10^3/2.4654x10^6 => 12.66g

I really don't get this Could anyone please explain to me how they worked this out? Thank you

finality

Dwellingdweller wrote: »

This question has me stumped, I just did the whole exam paper and I could not for the life of me figure this one out (I'm stupid, I know). Anyways, I looked at the SEC marking scheme for it and here's the solution they have for it.

"When making a hot drink steam at 100*C is added to 160g of milk at 20*C. If the final temperature of the drink is to be 70*, what mass of steam should be added? You may ignore energy losses to sorroundings."

( m c dt ) of m (I don't know what m stands for here ) = ( m l ) of steam + (mass of steam)(4.18x10^3)(30)
they then work it out and get:

mass of steam = 31.2x10^3/2.4654x10^6 => 12.66g

I really don't get this Could anyone please explain to me how they worked this out? Thank you

Basically, most of these heat questions work on the basis that heat in = heat out.

Heat is mc(dt), i.e. (mass)(specific heat capacity)(change in temperature)

So on one side, the milk is gaining heat, and on the other side, the steam is losing heat.

The heat gained by the milk = The heat lost by the steam

mc(temperature change)milk = mc(temperature change)water + mlsteam

(.16)(3.90x10^3)(70-20) = (m)(4.18x10^3)(100-70) + (m)(2.34x10^6)

Dwellingdweller

finality wrote: »

Basically, most of these heat questions work on the basis that heat in = heat out.

Heat is mc(dt), i.e. (mass)(specific heat capacity)(change in temperature)

So on one side, the milk is gaining heat, and on the other side, the steam is losing heat.

The heat gained by the milk = The heat lost by the steam

mc(temperature change)milk = mc(temperature change)water + mlsteam

(.16)(3.90x10^3)(70-20) = (m)(4.18x10^3)(100-70) + (m)(2.34x10^6)

Ah, I get it now! I do feel a bit stupid for asking about something so simple though, but hey. :P Thanks!

finality

Dwellingdweller wrote: »

Ah, I get it now! I do feel a bit stupid for asking about something so simple though, but hey. :P Thanks!

Haha no problem, I actually did that question on Wednesday and those heat questions always take quite a bit of thinking about at first.