1. P.1 Book E3 Section 1.3 Cooking without fire Microwave oven accident Types of cookers Check-point 7 Choosing cookers Check-point 8 1.3Cooking without fire

2. P.2 Book E3 Section 1.3 Cooking without fire Microwave oven accident Do you know why this accident happens? A microwave oven is convenient to use. However, care should be taken in using it. A man boiled an egg in a microwave oven and the egg exploded when he took it out. His eyes and shoulders got burned and he was sent to the hospital…

3. P.3 Book E3 Section 1.3 Cooking without fire 1 Types of cookers Some common types of electric cookers: Electric hotplates Induction cookers Microwave ovens They all generate heat to cook, but according to different principles.

4. P.4 Book E3 Section 1.3 Cooking without fire 1 Types of cookers a Electric hotplates Electric hotplate: Has a coil of resistance wire  heating element  Usually encased in a metallic tube filled with an electrically insulating material  The tube shaped into a coil and flattened  Good thermal contact with cookware

5. P.5 Book E3 Section 1.3 Cooking without fire a Electric hotplates Current flows through the resistance wire.  The wire heats up.  Electrical energy  heat (internal energy) Rate of energy conversion: P = I 2 R P : power of heat dissipation I : electric current through the resistance wire R : resistance of the wire The current can be adjusted by changing the voltage across the wire.

6. P.6 Book E3 Section 1.3 Cooking without fire 1 Types of cookers b Induction cookers In an induction cooker, a high frequency a.c. flows through a horizontal coil of wire under the non-metallic top plate. a horizontal coil of wire inside

7. P.7 Book E3 Section 1.3 Cooking without fire 1 Types of cookers b Induction cookers  An e.m.f. is induced in the base of the cooking pot.  A large current (eddy current) is induced.  Heating effect is produced. 1.3 The induction cooker The a.c. sets up a fast changing magnetic field. The base of the cooking pot is made of metal. Video pot non-metallic top plate a.c. in coil metallic base magnetic field produced by a.c. in coil

8. P.8 Book E3 Section 1.3 Cooking without fire b Induction cookers Cooking pots suitable for induction cookers: Metallic base  Produce heating effect Made of ferromagnetic materials (e.g. iron)  Help strengthen the induction effect and heating effect

9. P.9 Book E3 Section 1.3 Cooking without fire b Induction cookers The plastic top plate of an induction cooker:  An insulator  Cannot provide any current path  Does not heat up An induction cooker will start cooking only when a suitable pot is put on it. No current will pass through the coil when an unsuitable container is put on the oven, even if it is turned on.

10. P.10 Book E3 Section 1.3 Cooking without fire b Induction cookers Electric hotplateInduction cooker With smooth top plate  Can be cleaned more easily than gas stoves Lower power  Need longer cooking time than gas stove Higher power and efficiency  Can cook as fast as gas stove Comparison between electric hotplate and induction cooker:

11. P.11 Book E3 Section 1.3 Cooking without fire 1 Types of cookers A microwave oven cooks by exposing the food to microwaves. c Microwave ovens Microwaves  f = 2.45 GHz, = 0.12 m  Produced by rapidly accelerating e – in the magnetron tube Distributed to the food in the cooking chamber

12. P.12 Book E3 Section 1.3 Cooking without fire c Microwave ovens Water molecules are polar.  Food containing water can be heated up by microwaves effectively. The oscillating E-field of the microwave makes the water molecules vibrate.  KE of the molecules   Food temperature 

13. P.13 Book E3 Section 1.3 Cooking without fire c Microwave ovens Microwaves can penetrate deeply into food.  Heat up different parts of food at the same time  Short cooking time Microwave ovens Heat food without getting hot themselves Very convenient in defrosting and reheating food Cannot brown the surface of food

14. P.14 Book E3 Section 1.3 Cooking without fire Check-point 7 – Q1 Which of the following types of pots works best on an induction cooker? A Ceramic B Glass C Copper D Iron

15. P.15 Book E3 Section 1.3 Cooking without fire Check-point 7 – Q2 The coils of an electric hotplate are flattened to increase the contact area with the pot. This is mainly to enhance the ___________ of heat to the pot. A conduction B radiation C convection D all of the above

16. P.16 Book E3 Section 1.3 Cooking without fire 2 Choosing cookers a Energy efficiency CookerEnd-use energy efficiency Electric hotplate74% Induction cooker84% Microwave oven56% Typical values of end-use energy efficiency for different kinds of cookers:

17. P.17 Book E3 Section 1.3 Cooking without fire a Energy efficiency Microwave oven  Can only convert ~60% input energy into microwaves  energy is also used to operate other devices  Less efficient than induction cookers Induction cooker  Does not get hot during operation  No heat loss directly from the cooker  More efficient than electric hotplate

18. P.18 Book E3 Section 1.3 Cooking without fire a Energy efficiency Finding end-use energy efficiency Note: The efficiencies do not tell us which cooker uses more or less energy when we cook a particular dish of food. The actual energy used depends on: The cookware used Method (fry, boil, steam, etc.) Type of food Example 7

19. P.19 Book E3 Section 1.3 Cooking without fire Example 7 Finding end-use energy efficiency (Specific heat capacity of water = 4200 J kg –1  C –1 ) A 1500-W electric hotplate boils 2 kg of water at 20  C in 10 min. End-use energy efficiency = ? Energy output = mc∆T = 2  4200  (100 – 20) = 672 000 J

20. P.20 Book E3 Section 1.3 Cooking without fire Example 7 Finding end-use energy efficiency Power output = energy output time = 672 000 10  60 = 1120 W End-use energy efficiency power output power input  100% = = 1120 1500  100% = 74.7%

21. P.21 Book E3 Section 1.3 Cooking without fire a Energy efficiency Boiling water Example 8

22. P.22 Book E3 Section 1.3 Cooking without fire Example 8 Induction cooker: 1000 W, 80% efficient Microwave oven: 1000 W, 55% efficient The cookers are used to boil 1 kg of water at 25  C. Boiling water Specific heat capacity of water = 4200 J kg –1  C –1 Cost of electricity = $0.9 per kW h (a)Find the time needed for the two appliances to boil the water. Energy needed = mc∆T = 1  4200  (100 – 25) = 315 000 J

23. P.23 Book E3 Section 1.3 Cooking without fire Example 8 Boiling water Induction cooker: Power output = power input  efficiency = 1000  80% = 800 W Time needed = energy needed power output = 315 000 800 = 394 s

24. P.24 Book E3 Section 1.3 Cooking without fire Example 8 Boiling water Microwave oven: Power output = power input  efficiency = 1000  55% = 550 W Time needed = energy needed power output = 315 000 550 = 573 s

25. P.25 Book E3 Section 1.3 Cooking without fire Example 8 Boiling water (b)Find the cost of using the two appliances in (a). Induction cooker: Total energy input = 1000  394 Cost of electricity = 0.9  0.109 = $0.0981 Microwave oven: Total energy input = 1000  573 Cost of electricity = 0.9  0.159 = $0.143 = 394 000 J = 0.109 kW h = 573 000 J = 0.159 kW h

26. P.26 Book E3 Section 1.3 Cooking without fire 2 Choosing cookers b Comparing different types of electric cookers Factor Electric hotplate Induction cooker Microwave oven Selling price Running cost Safety Convenience lower higher lower higher, heat loss to surroundings lower, no direct heat loss higher, inefficient energy conversion hot plate may be unnoticed top plate does not get hot case does not get hot take time to warm up coil no warm up time ideal for reheating or defrosting food

27. P.27 Book E3 Section 1.3 Cooking without fire Check-point 8 – Q1 The end-use energy efficiency of a gas cooker is half that of a hotplate. Which one cooks a steak faster? A Gas cooker B Hotplate C Both use the same time D Cannot be determined

28. P.28 Book E3 Section 1.3 Cooking without fire Check-point 8 – Q2 Which of the following cookers is the most suitable to use in an office for reheating noodles in a heat-resisting plastic container? A Conventional oven B Electric hotplate C Induction cooker D Microwave oven

29. P.29 Book E3 Section 1.3 Cooking without fire Check-point 8 – Q3 A 1200-W induction cooker boils away 0.4 kg water at 100  C in 15 min. End-use energy efficiency = ? (Specific latent heat of vaporization of water = 2.26  10 6 J kg –1 ) Energy output = ml v = 904 000 J = 0.4  2.26  10 6 Power output = energy output time = 904 000 15  60 = 1004 W End-use energy efficiency  100% power output power input = = 1004 1200  100% = 83.7%

30. P.30 Book E3 Section 1.3 Cooking without fire The End