Specific Heat calculations
Specific Heat calculations
(OP)
I see people caculating heat Quantity transferred as Q=Mass * Cp * delta T. All is good with this. But...
But they use weight in lieu of mass when they calculate.
In imperial units Cp of water is 1 BTU/lb-F. Isn't tis really lbm (pounds mass)? They really do as follows:
Q = lbf * 1 BTU/lbm-f * delta T.
(where "lbf" is lb force or weight)
Shouldn't the weight of the water be divided by 32.2 to get lbm?
Is this actually correct:
Q = lbf/32.2 * 1 BTU/lbm-F * Delta T ?
But they use weight in lieu of mass when they calculate.
In imperial units Cp of water is 1 BTU/lb-F. Isn't tis really lbm (pounds mass)? They really do as follows:
Q = lbf * 1 BTU/lbm-f * delta T.
(where "lbf" is lb force or weight)
Shouldn't the weight of the water be divided by 32.2 to get lbm?
Is this actually correct:
Q = lbf/32.2 * 1 BTU/lbm-F * Delta T ?





RE: Specific Heat calculations
Yes it should be mass not weight I agree with you, we use metric units here but the equation above we would kg's for mass but weight would be kg's*g.
desertfox
RE: Specific Heat calculations
Desertfox is right. Weight is the name for a force W exerted by Earth on the mass m of a body and is equal to W = mg.
With my 75-kg mass near Earth's surface, my weight is (75 kg)(9.8 m/s2) or 735 N. (N=Newton).
On the moon, where acceleration of gravity is only 1.6 m/s2, I would weigh only 120 N.
RE: Specific Heat calculations
Lbf/g gives you slugs, which is the proper unit of mass in our lovely system.
No.
RE: Specific Heat calculations
What is wrong with the equation as written using the Cp of water and assuming we are on earth?
(I need to keep it in the imperial unit system.)
Q = lbf/32.2 * 1 BTU/lbm-F * Delta T
I think this is correct.
RE: Specific Heat calculations
RE: Specific Heat calculations
No.
F=ma
1 slug under the influence of Earth's gravity (32.2 ft/s/s) has a weight of 32.2 pounds (force)
A pound (mass) is the mass of an object that has a weight of 1 pound (force). So 1 pound (mass) = 1/32.2 slug
When we say the Cp of water is 1 BTU/lb-F we mean an amount of water that weights a pound on Earth.
RE: Specific Heat calculations
TTFN
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RE: Specific Heat calculations
Q= Mass * Cp * delta T
Dimensions:
[BTU] = [Lbm] * [Btu/(lbm*°F)]* [°F]
[Lb m] is a mass (equivalent to grams; given by a scale)
[Lb f] is a force (equivalent to Newtons; Weight= Mass* Gravitity constant)
[Lbf] = [Lbm]*32.174 ft/s^2
[Troll] Use SI and convert in US units after ;) [\Troll]
good online converter (also goo to check the units): http://online.unitconverterpro.com/
RE: Specific Heat calculations
Scales measure force, not mass.
RE: Specific Heat calculations
Adding to MintJulep's last sentence: ...however, since mass and weight are "proportional" in a given location, scales are calibrated to measure mass.
Although a scale measures force, being calibrated in mass units, under the assumption that g = 9.8 m/s2, it whould read more if the weighing is done on an upward accelerating elevator.
RE: Specific Heat calculations
RE: Specific Heat calculations
Even balance-type scales (not only spring scales) compare "weights" labelled in mass units.
RE: Specific Heat calculations
RE: Specific Heat calculations
- Steve
RE: Specific Heat calculations
The British unit of force is the pound-force, 1 lbf ≈ 4.45 N
The British unit of mass is the slug; 1 slug ≈ 14.6 kg.
A supermarket hanging spring scale measures the object's weight, but since mass and weight are proportional at a given location, it also provides a measure of the object's mass.
RE: Specific Heat calculations
I think scales with springs are measuring forces (weight), balances are giving the equilibrium between two torques (force x distance), and piezo-electrics scale are measuring a pressure (force / surface)... Isn't it?
In all case the display is giving the mass.
RE: Specific Heat calculations
Getting back to your original question.
The formula is actually Q=ṁcpΔT.
The little dot over the m means that it is a mass flow rate -- not just a mass. For working in English system, I use pounds-mass/hour. Yes you have to make sure that you are using pounds mass and not pounds force.
Patricia Lougheed
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RE: Specific Heat calculations
Without dots over either the equation is perfectly suitable to figure how much heat was added to a fixed mass to raise the temperature by dT.