Worked example A kettle works on the UK mains (230 V) and a current of 12 A flows when it is switched on. What is the power rating of the kettle? How much energy would the kettle transform if it was switched on for 2 minutes? What is the maximum mass of 20 water which could be heated to 99 in this ⁰C ⁰C time? What assumptions did you make in part c? a. V = 230 V P = IV I = 12 A P = 12 × 230 P = ? P = 2760 W b. P = 2760 W P = E/t t = 2 × 60 = 120 s 2760 = E/120 E = ? E = 120 × 2760 E = 331200 J E = 331 kJ c. m = ? Eh = cmΔT T1 = 20 331 × 103 = 4180 × m × 79 ⁰C T2 = 99 m = 331 × 103/330220 ⁰C = T2 - T1 = 79 m = 1.002 ΔT ⁰C c m = 1 kg = 4180 J/kg⁰C Eh = 331 kJ = 331 × 103 J d. Assuming all energy supplied to kettle heats the water, none is lost to the surroundings. 3. Gas laws and the kinetic model Pressure When an object exerts a force on another object, this force is spread across the entire surface area of contact. For example a wooden block sitting a table will exert a force on the table top due to the weight of the block. If the box is turned on its end, it will exert the same force but over a smaller surface area. The amount of force exerted on a unit area is defined as pressure
Worked example A kettle works on the UK mains (230 V) and a current of 12 A flows when it is switched on. What is the power rating of the kettle? How much energy would the kettle transform if it was switched on for 2 minutes? What is the maximum mass of 20 ⁰C water which could be heated to 99 ⁰C in this time? What assumptions did you make in part c? a. V I P = 230 V = 12 A = ? P P P = IV = 12 × 230 = 2760 W b. P t E = 2760 W = 2 × 60 = 120 s = ? P 2760 E E E = E/t = E/120 = 120 × 2760 = 331200 J = 331 kJ c. m T1 T2 ΔT c Eh = ? = 20 ⁰C = 99 ⁰C = T2 - T1 = 79 ⁰C = 4180 J/kg⁰C = 331 kJ = 331 × 103 J Eh 331 × 103 m m m = cmΔT = 4180 × m × 79 = 331 × 103/330220 = 1.002 = 1 kg d. Assuming all energy supplied to kettle heats the water, none is lost to the surroundings. 3. Gas laws and the kinetic model Pressure When an object exerts a force on another object, this force is spread across the entire surface area of contact. For example a wooden block sitting a table will exert a force on the table top due to the weight of the block. If the box is turned on its end, it will exert the same force but over a smaller surface area. The amount of force exerted on a unit area is defined as pressure
energy_summary_book.docx