 "> | For heat capacity and latent heat, there is no need to use formula, it will make you even confuse. In fact, there is only 1 formula needed to tackle a difficult question. First, you have to understand wat is the unit is talking. Whatelse, the units are the one that give you the defination of heat cacpty, speceific heat capacity and specific latent heat. Whenever you see the word 'specific', it involves with mass. Therefore, Heat capacity = J / K Specific heat capacity = J/ Kg K Specific latent heat = J/Kg these units are simply telling the amount of energy PER 1Kg and 1K if it is not 'specific' it will only have 1k The first problem is tackle questions that require u to find J,K or Kg Note: the C below means degree celcius Skill 1 : Calculating Energy A 2kg copper object is heated from 30 C to 35 C. How much increase in thermal energy is required, given that the specific heat capacity of copper is 400J/Kg K first step, use the heat capacity as your guide. write it as 400J/ 1Kg 1Know the qns asked for 2kg, so simply multiply both side by 2 400J / 1Kg 1K[when 1side has be multiplied by 2, another side will be the same]800J / 2Kg 1K Note: please do not multiply by 2 such that it became 2Kg 2K, this means you are actually multiplying by 4! it works like this, 2 x 2 = 4 see the link? now the next time is the find the temperature difference, 35 - 30 = 5 so do the same thing, multiply both sides by 5 4000J / 2Kg 5K noticed that the 2kg remains unchange. so thats your ans, you need 4000J ! Skill 2 : Calculating mass 4000J of energy is used to heat up a copper block from 50 C to 70 C. What is the mass of the copper block, given that the specific heat capacity of copper is 400J / Kg K? this part is a little bit different, its about finding the mass but just do the same thing, write it down first 400J / 1Kg 1K it mention 20 C so lets just multiply by 20 8000J / 1Kg 20k now you have 8000J, its way of course of wat the question is asking which is only 4000J so now jus bring it down, by dividing it by 2, however, u cannot divide temperature change as the qns said that it increases by 20 C. so simple, jus divide Kg by 2 4000J/0.5Kg 20K hence, the answer is 0.5kg. easy? Skill 3 : Calculating temperature change 4000J of energy is used to heat up a 5Kg copper block from an initial temperature of 20 C. What is the final temperature of the copper block, given that the specific heat capacity of copper is 400J / Kg K 400J / 1Kg 1K copper is 5Kg, so multiply by 5 2000J / 5Kg 1K u need 4000J so to change from 2000J to 4000J you have to muliply 2 again, Kg should not be change 4000J / 5Kg 2K Hence, it raises 2 K, final temperature is 22 C thats half of capacity covered. Simple eh? time for latent heat no need formula, how calculate latent heat etc. its the same method as heat capacity wats more, there isn't any temperature to calculate! well but i think i better go through 1 last time Eg: what is the amount of energy needed to melt 10kg of ice when the specific latent heat of fusion is 336000J/Kg To get 10kg, multiply by both sides, 3 360 000J/10Kg so your ans will be 3360000 Another type of question that you have to learn how to tackle what is the total energy required to heat 2kg of pure ice at -5C to 10C of water? Specific heat capacity of ice : 2100J/Kg K Specific heat capacity of water : 4200J/Kg K Specific latent heat of fusion of ice : 336 000J/Kg This question may seems a complicated question but it is still only by one simple concept. This process has three stages. As everyone know, every substnaces and states have different specific heat capacity hence, we cannot directly take the temperature change as 15. In this question, there are 3 things you have to find. 1. Amount of energy to raise the temperature of ice 2. Amount of energy to melt 3. Amount of energy to raise the temperature of water why do i have to split it up? this is concern about different amount energy in different states, thats is y, total of 3 different heat capacity/latent heat has to be used 1st, you have to calculate the amount of energy needed in the first stage, raising temperature of the ice. The maximum temperature of ice can reach is of coz 0C as it has to undergo the change of state from solid to liquid now 1st stage: 2100J/Kg K --> x2 each side for its mass 4200J/2Kg K --> x5 for each every K change 21000J/2Kg 5K --> the ice has now reached 0C Therefore, energy needed = 21000 in the 1st stage 2nd stage: since its changing state, you have to use latent heat now 336 000J/Kg ---> x2 for mass 672000J/2Kg Therefore, energy needed to melt the ice = 672 000J 3rd stage: Now it has changed into water, bear this in mind, the water is still at 0C 4200J/Kg K ---> x2 for mass again 8400J/2Kg K ---> x10 for temperature 84000J/2Kg 10K Therefore, energy needed in this stage is 84000J now final step, ADD EVERYTHING TOGETHER FOR TOTAL ENERGY! 84000J + 672000J + 21000J = 777 000J (Answer) you hafta practise more urself :D FAQ: why do we still have to multiply the mass again when we had alr multiply it in the first place? the mass never change wat A: thats y i say, heat capacity is only base on 1 concept, when 2kg of ice melted, there is still 2kg of water remains! the same thing has to be repeated FAQ: What about Watts leh? A: take the last qns for eg: how long does it take for a heater wif 200W to complete this process? 200W can jus simply covert it to 200J/s so you can do proportion easier back to the qns, since it is 200J/s ask primary sch qus to urself, how many '200' do i need to reach 777 000J? so jus simply 777 000 divide it by 200 and tada! ans is 777 000 / 200 = 3885seconds! last part of heat capacity this is recommanded not to know wats the formula is talking about. understand it when ur heat capacity is very stable. so at the moment, jus study it w/o knowing wats happening the formula is : mc(temp change) = mc(temp change) m = mass, c = specific heat capacity this formula is used when 2 substances are mixed together at different temperature and will give u a final temperature Example 2kg of copper ball at 120C is thrown in 10Kg of water at 40C. Assuming that no heat is lost to the surrounding, what is the final temperature of the mixture? Specific heat capacity of copper = 400J/Kg K Specific heat capacity of water = 4000J/Kg K lets take copper for the left side and water at the right side X is the final temperature (2)(400)(X-120) = (10)(4000)(40-X) NOTE!!!: the temperature change part, cannot be in the same sequence means when x is on the left side of the bracket, the another side of the eqn CAN NOT hav X on the left side of its bracket too 800(x-120) = 40000(40-x) 800x-96000 = 1600000 - 40000x x = 41.6C thats the ans, if you were given final temp and find mass of specific heat capacity, jus put the final temperature into X and find the other unknown anything gotta do wif finaly temperature can use this formula to solve THE END! PRACTISE AND WORK HARD! |
