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OCR Gateway 2016 Physics topic 7

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1 OCR Gateway 2016 Physics topic 7
06/12/2018 06/12/2018 Energy OCR Gateway 2016 Physics topic 7 W Richards The Weald School

2 P7.1 Work Done 06/12/2018

3 Gravitational potential
The 9 types of energy 06/12/2018 Type 3 example sources Type 3 example sources Heat Kinetic (movement) Nuclear Sound Light Chemical Electrical Gravitational potential Elastic potential

4 The Laws of Physics 06/12/2018 There are many laws of physics, but one of the most important ones is: Energy cannot be created or destroyed, it can only be converted from one form to another

5 Electricity Light + heat
Energy changes 06/12/2018 To describe an energy change for a light bulb we need to do 3 steps: 1) Write down the starting energy: 3) Write down what energy types are given out: 2) Draw an arrow Electricity Light + heat What are the energy changes for the following…? An electric fire A rock about to drop An arrow about to be fired

6 Energy Transfer Examples
Look at the following objects and write down what their energy changes are: a) Electric Light Bulb b) Battery c) Hairdryer d) Candle Burning e) Model Car (Wind-up) f) Dynamo g) Yo-Yo h) Radio i) Kettle j) Dropping a Golf Ball

7 Specific Heat Capacity
06/12/2018 If we heat this beaker up it’s fairly clear that the liquid will gain internal energy and get hotter. Q. What three things does the increase in temperature depend on? The amount of liquid (i.e. the mass) The amount of heat energy going in to the liquid (i.e. how hot the Bunsen is and how long it’s on for) The substance being heated – e.g. is it water or cooking oil?

8 Putting these into an equation…
06/12/2018 Let’s put the factors from the last slide into an equation: If the rise in temperature depends on the mass of liquid, how much energy it is given and the substance being heated then we can say: ΔE = mcΔθ Where: ΔE = amount of heat energy being supplied (in Joules) m = mass of liquid (in kg) c = “specific heat capacity” (in J/KgOC) Δθ = change in temperature (in OC)

9 Investigating Specific Heat Capacity
06/12/2018 Investigating Specific Heat Capacity How can we measure SHC experimentally? E = VIt and E = mcΔT A V 12V Calculate E if E = voltage x current x time (in seconds) Divide this by the mass of the water in kg Divide this by the change in temperature Write down your answer – specific heat capacity of water = _____J /(kg.0C)

10 Some example questions
06/12/2018 A beaker filled with 0.1kg of water with specific heat capacity 4200J/(kg.0C) is heated from 200C to 800C. Calculate the amount of heat energy gained by the water. Another beaker containing 24g of water starts at 500C. If it loses 2000J of energy what temperature has it dropped to? 25.2 KJ 30.20C

11 Another way… 06/12/2018 E A V 12V A metal

12 Question 1 06/12/2018 If you eat a pizza from a hot oven, the crust might be harmless while the cheese topping scalds your tongue. Use your ideas about specific heat capacity to explain why.

13 Question 2 Using the equation E = m × c × θ…
06/12/2018 Using the equation E = m × c × θ… How much energy would be transferred to raise the temperature of 2 kg of water from 20°C to 30°C? The specific heat capacity of water = 4181 J/Kg/°C

14 Question 3 06/12/2018 The specific heat capacity of water is 4181 J/Kg/°C and that of lead is 128 J/Kg/°C. If you had both the same mass of lead and water which would require the most energy to heat and why?

15 Question 4 06/12/2018 Radiators can either be filled with water or filled with oil. Water has a higher specific heat capacity. What are the advantages and disadvantages of each?

16 Question 1 06/12/2018 If you eat a pizza from a hot oven, the crust might be harmless while the cheese topping scalds your tongue. Use your ideas about specific heat capacity to explain why. Crust has a lower SHC so the cheese holds the heat in for a longer amount of time.

17 Question 2 Using the equation E = m × c × θ…
06/12/2018 Using the equation E = m × c × θ… How much energy would be transferred to raise the temperature of 2 kg of water from 20°C to 30°C? The specific heat capacity of water = 4181 J/Kg/°C 83,620J

18 Question 3 06/12/2018 The specific heat capacity of water is 4181 J/Kg/°C and that of lead is 128 J/Kg/°C. If you had both the same mass of lead and water which would require the most energy to heat and why? Water would need more energy to heat up as it has a higher SHC.

19 Question 4 06/12/2018 Radiators can either be filled with water or filled with oil. Water has a higher specific heat capacity. What are the advantages and disadvantages of each? Water would need more energy to warm up (disadvantage?) but would also take a longer time to cool down again (advantage?).

20 Work done = Force x distance moved
Mechanical Work done 06/12/2018 06/12/2018 When any object is moved around work will need to be done on it to get it to move (obviously). We can work out the amount of work done in moving an object using the formula: Work done = Force x distance moved in J in N in m W s F

21 Example questions 25J 20J, GPE 4m, KE 50N 2MJ
06/12/2018 06/12/2018 Amy pushes a book 5m along the table with a force of 5N. She gets tired and decides to call it a day. How much work did she do? Jodie lifts a laptop 2m into the air with a force of 10N. How much work does she do? What type of energy did the laptop gain? Ronnie does 200J of work by pushing a wheelbarrow with a force of 50N. How far did he push it? What type of energy did the wheelbarrow gain? Julian cuddles his cat and lifts it 1.5m in the air. If he did 75J of work how much force did he use? Travis drives his car 1000m. If the engine was producing a driving force of 2000N how much work did the car do? 25J 20J, GPE 4m, KE 50N 2MJ

22 Work done by a flowing Current
06/12/2018 06/12/2018 The amount of energy that flows in a circuit will depend on the amount of charge carried by the electrons and the voltage pushing the charge around: Energy transferred = charge x voltage (in J) (in C) (in V) W Q V

23 Example questions (1st set)
06/12/2018 06/12/2018 In a radio circuit a voltage of 6V is applied and a charge of 100C flows. How much energy has been transferred? In another circuit a voltage of 10V is applied and 200C of charge flows around the circuit. How much energy is transferred? 48J of electrical work is done by a 6V battery. How much charge flowed around the circuit? Another battery transfers 20C of charge and does 100J of electrical work. What was the battery’s voltage? 600J 2000J 8C 5V

24 Kilowatt hours 06/12/2018 Electricity is measured in units called “kilowatt hours” (kWh). For example… A 3kW fire left on for 1 hour uses 3kWh of energy A 1kW toaster left on for 2 hours uses 2kWh A 0.5kW hoover left on for 4 hours uses __kWh A 200W TV left on for 5 hours uses __kWh A 2kW kettle left on for 15 minutes uses __kWh

25 The Cost of Electricity
06/12/2018 To work out how much a device costs we do the following: Cost of electricity = Power (kW) x time (h) x cost per kWh (p) For example, if electricity costs 12p per unit calculate the cost of the following… A 2kW fire left on for 3 hours A 0.2kW TV left on for 5 hours A 0.1kW light bulb left on for 10 hours A 0.5kW hoover left on for 1 hour 72p 12p 12p 6p

26 Reading Electricity Meters
06/12/2018 How many units (KWh) of electricity have been used? If 1 unit costs 15p how much has this electricity cost? 1 month later…

27 Kinetic energy = ½ x mass x speed squared
06/12/2018 Any object that moves will have kinetic energy. The amount of kinetic energy an object has can be found using the formula: Kinetic energy = ½ x mass x speed squared in J in kg in m/s KE = ½ mv2

28 Example questions 06/12/2018 Lydia drives her car at a speed of 30m/s. If the combined mass of her and the car is 1000kg what is her kinetic energy? Sam rides her bike at a speed of 10m/s. If the combined mass of Sam and her bike is 80kg what is her kinetic energy? Josh is trying to catch a bus and is running at 3m/s. If he has a mass of 60kg how much kinetic energy does he have? A 2000kg car is being driven at a speed of 10m/s. If it doubles its speed to 20m/s what happens to the car’s kinetic energy? 450,000J 4000J 270m/s Increases from 100,000J to 400,000J

29 Example questions (higher)
06/12/2018 Steve is running away from the police and has 100J of kinetic energy. If he is running at 2m/s what is his mass? James is driving in his car and has a combined mass of 1200kg. If he has 540KJ of kinetic energy what speed is he driving at? Dave is running and has a kinetic energy of 750J. If his mass is 60kg how fast is he running? Stuart is spotted walking around Tescos. If he has a kinetic energy of 150J and he’s walking at a pace of 2m/s what is his mass? 50kg 30m/s 5m/s 75kg

30 Force and Extension Consider a mass on a spring:
06/12/2018 Consider a mass on a spring: What happens when a mass is added? When a force is applied to this spring it will change shape and extend. The spring will have “stored elastic potential energy”

31 Elastic Potential Energy
06/12/2018 06/12/2018 Elastic potential energy is the energy stored in a system when work is done to change its shape, e.g: Describe the energy changes when the mass is: At the top of it’s movement In the middle At the bottom

32 Elastic Potential Energy
06/12/2018 Task: Calculate how much stored EPE there is in your springs Stored EPE = ½ke2 F = ke Weight added (N) Extension (m) Stored EPE (J) 1 2 3 4 5 6

33 Gravitational Potential Energy
06/12/2018 To work out how much gravitational potential energy (GPE) an object gains when it is lifted up we would use the simple equation… GPE = mass x grav. field strength x Change in height (Joules) (newtons) (N/Kg) (metres) GPE H mg

34 Some example questions…
06/12/2018 How much gravitational potential energy have the following objects gained?: A brick of mass 1kg lifted to the top of a house (10m), A 1,000kg car lifted by a ramp up to a height of 2m, A 70kg person lifted up 50cm by a friend. How much GPE have the following objects lost?: A 0.2kg football dropping out of the air after being kicked up 30m, A 0.05kg egg falling 10m out of a bird nest, A 1,000kg car falling off its 200cm ramp. 100J 20KJ 350J 60J 5J 20KJ

35 Extension questions 06/12/2018 Jonny decides to use a spring (spring constant = 25N/m) to fire a 20g object straight upwards. He extends the spring by 50cm and fires the object upwards. How far up would it go? In the above example, how fast would the object be moving immediately after leaving the spring? 15.6m 17.7m/s

36 P7.2 Power and Efficiency 06/12/2018

37 The Laws of Physics Recall one of the most important laws of Physics:
06/12/2018 Recall one of the most important laws of Physics: Energy cannot be created or destroyed, it can only be converted from one form to another

38 Conservation of Energy
06/12/2018 In any energy change there is ALWAYS some “waste” energy: e.g. a light bulb: Electricity Light heat In this example HEAT is wasted and it is transferred to the surroundings, becoming very difficult to use. Describe the following energy changes and state the “waste” energy or energies: A vacuum cleaner A TV A dynamo/generator

39 You need to learn this equation!!
Efficiency 06/12/2018 Efficiency is a measure of how much USEFUL energy you get out of an object from the energy you put INTO it. For example, consider a TV: Light (80J) Electrical Energy (200J) Sound (40J) Heat (?) Efficiency = Useful energy out Energy in x100% You need to learn this equation!!

40 Some examples of efficiency…
06/12/2018 5000J of electrical energy are put into a motor. The motor converts this into 100J of movement energy. How efficient is it? A laptop can convert 400J of electrical energy into 240J of light and sound. What is its efficiency? Where does the rest of the energy go? A steam engine is 50% efficient. If it delivers 20,000J of movement energy how much chemical energy was put into it? 2% 60%, given out to the surroundings as heat 40,000J

41 Electrical Power 06/12/2018 06/12/2018 The power of an electrical device is given by this equation: P I V Power = voltage x current in W in V in A

42 Which devices transfer the most power and why?
Example Questions 06/12/2018 06/12/2018 Complete the following table: Appliance Power rating (W) Voltage (V) Current needed (A) Toaster 230 4 Fire 8 Hoover 460 2 Hairdryer 1150 Computer 100 Stereo 80 Which devices transfer the most power and why?

43 Example Questions Complete the following table: Appliance
06/12/2018 06/12/2018 Complete the following table: Appliance Power rating (W) Voltage (V) Current needed (A) Toaster 920 230 4 Fire 1840 8 Hoover 460 2 Hairdryer 1150 5 Computer 100 0.43 Stereo 80 0.35

44 Examples of wasted energy
06/12/2018 Clearly, not all of the energy from the fuel put into this car will become movement energy. Where is energy wasted in this car? The bulb on the left is an “energy efficient” one. Why is the bulb on the right wasting energy?

45 Increasing Efficiency (HT only)
Earlier on in this unit we considered these energy changes. How could we increase the efficiency of these devices? a) Electric Light Bulb b) Battery c) Hairdryer d) Candle Burning e) Model Car (Wind-up) f) Dynamo g) Yo-Yo h) Radio i) Kettle j) Dropping a Golf Ball

46 Ways to reduce heat loss
06/12/2018 Q. How would you reduce energy loss in these objects?

47 Heat loss through walls
06/12/2018 I’m worried about how much heat I’m losing through my walls. What can I do to reduce this? Here’s a new building with cavity wall insulation being installed: How does the use of cavity wall insulation help keep a building warmer? The rate of heat loss through a wall depends on two things: How thick the wall is The thermal conductivity of the wall


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