# Physics gcse question need help

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#1
The question goes as follows
An ice cube has a temperature of -15.0 degrees
The total thermal energy needed to raise the temperature of this ice cube to 0.0 degrees and completely melt the ice cube is 5848 J
Specific heat capacity of ice=2100J/Kg C
Specific latent heat of fusion of ice= 334000J/Kg
Calculate the mass of the ice cube
0
10 months ago
#2
Hey,
You should use the equation
Q= m * c * Δθ + m* L
So, if θ1= -15 degrees C, θ2 = 0 degrees C, Q=5848 J, C=2100 J/kg and L=334000J/KG
You have the equation
5848 = m *2100*15 + 334000*m

Which leaves the ice at a mass of 16 grams.

I hope this has helped you.
0
#3
(Original post by ChristianaGa)
Hey,
You should use the equation
Q= m * c * Δθ + m* L
So, if θ1= -15 degrees C, θ2 = 0 degrees C, Q=5848 J, C=2100 J/kg and L=334000J/KG
You have the equation
5848 = m *2100*15 + 334000*m

Which leaves the ice at a mass of 16 grams.

I hope this has helped you.
That's very helpful however could you explain how you got to the equation please?
0
10 months ago
#4
Well, being an international student, our system is different and we just memorised the equation by heart but you can split it in two, and if it helps you to understand you can do so using the units.
So the first part of the equation
Q= m * c * Δθ means that, to get the change in energy, you have to multiply the mass with the heat capacity to get to the energy change per 1 degree celsius and then you multiply it to the change of temperature to get the energy change for that temperature change. So you have
x kg * y J/(kg*C) * z C, you can see the kilograms and the celsius cancel out and you are left with the energy
For the second part (Q=m*L) is the energy required to change the state of the water from solid (ice) to liquid. L is the energy that takes per kilogram, so you just multiply that with the mass and get the energy needed to change state. Here you have x kg * y J/kg, the kilograms cancel out and you are left with the Joules.
And you add the two energies you have found to get the energy required for the increase in temperature and the change of state.
I hope I explained it well, if not you can always look for videos for these two equations online, but it is something you will have to know by heart
0
#5
(Original post by ChristianaGa)
Well, being an international student, our system is different and we just memorised the equation by heart but you can split it in two, and if it helps you to understand you can do so using the units.
So the first part of the equation
Q= m * c * Δθ means that, to get the change in energy, you have to multiply the mass with the heat capacity to get to the energy change per 1 degree celsius and then you multiply it to the change of temperature to get the energy change for that temperature change. So you have
x kg * y J/(kg*C) * z C, you can see the kilograms and the celsius cancel out and you are left with the energy
For the second part (Q=m*L) is the energy required to change the state of the water from solid (ice) to liquid. L is the energy that takes per kilogram, so you just multiply that with the mass and get the energy needed to change state. Here you have x kg * y J/kg, the kilograms cancel out and you are left with the Joules.
And you add the two energies you have found to get the energy required for the increase in temperature and the change of state.
I hope I explained it well, if not you can always look for videos for these two equations online, but it is something you will have to know by heart
I'm sorry for being vague. We have learnt the separate equations it's just I was confused to how you combined it.
0
10 months ago
#6
Oh, ok then. Yes you can just combine them since both energies are added up to figure out the energy needed to melt the ice
0
10 months ago
#7
wait I'm a little bit confused. i didn't use the latent heat of fusion energy anywhere? And i did this:

5845=masss*2100*15
and i got the mass=0.186kg.

Could someone please explain this to me. Tyyy xx
0
#8
(Original post by vix.xvi)
wait I'm a little bit confused. i didn't use the latent heat of fusion energy anywhere? And i did this:

5845=masss*2100*15
and i got the mass=0.186kg.

Could someone please explain this to me. Tyyy xx
Ok, thanks to ChristianaGa I understand this now. So the question says "The total thermal energy needed to raise the temperature of this ice cube to 0.0 degrees and completely melt the ice cube is 5848 J" meaning the energy needed to raise the temperature and melt the ice cube. So we know that the equation of specific heat capacity is the one you just used, this only calculates the energy that is used to just raise the temperature. What about the energy to melt it? Well, the specific latent heat of fusion equation comes into play. This equation is used to calculate the energy required to melt a solid into a liquid. So referring back to the question, we know that 5845J of energy is the sum of both specif heat capacity equation and the specific latent heat equation. Therefore we can use the equation E= m * c * Δθ + m* L. Now from your comment, it can be inferred that you haven't gone over the specific latent heat equation. Well, its "Energy=mass x specific latent heat". Ok, so this is where I primarily was confused, you don't have to calculate mass separately by substituting values into the two equations. Instead, you can combine the equations as shown above, so that you get:
Q= m * c * Δθ + m* L
and then sub the known values
5848 = m *2100*15 + 334000*m
and then consequently solve for m
M=0.016 kg.
1
10 months ago
#9
(Original post by ShimbleBobble)
Ok, thanks to ChristianaGa I understand this now. So the question says "The total thermal energy needed to raise the temperature of this ice cube to 0.0 degrees and completely melt the ice cube is 5848 J" meaning the energy needed to raise the temperature and melt the ice cube. So we know that the equation of specific heat capacity is the one you just used, this only calculates the energy that is used to just raise the temperature. What about the energy to melt it? Well, the specific latent heat of fusion equation comes into play. This equation is used to calculate the energy required to melt a solid into a liquid. So referring back to the question, we know that 5845J of energy is the sum of both specif heat capacity equation and the specific latent heat equation. Therefore we can use the equation E= m * c * Δθ + m* L. Now from your comment, it can be inferred that you haven't gone over the specific latent heat equation. Well, its "Energy=mass x specific latent heat". Ok, so this is where I primarily was confused, you don't have to calculate mass separately by substituting values into the two equations. Instead, you can combine the equations as shown above, so that you get:
Q= m * c * Δθ + m* L
and then sub the known values
5848 = m *2100*15 + 334000*m
and then consequently solve for m
M=0.016 kg.
ohhh I get it tysmmmm!!! good luck with your gcses!
Last edited by vix.xvi; 10 months ago
0
#10
(Original post by vix.xvi)
ohhh I get it tysmmmm!!! good luck with your gcses!
Eyyyy! thanks. You too
0
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