P7 Flashcards
(150 cards)
1
Q
What is energy?
A
Energy is a quantity (a number in joules) that tells you what is possible, but does not tell you what will happen.
2
Q
What are energy stores?
A
Energy stores are used to keep track of energy, and each store is associated with an equation for calculations.
3
Q
What is the law of conservation of energy?
A
The Law of Conservation of Energy states that energy cannot be created or destroyed; it can only be transferred between stores.
4
Q
What is an example of a chemical energy store?
A
A pile of coal with oxygen or glucose in your muscles.
5
Q
What is the equation for thermal energy change?
A
Change in energy = mass x specific heat capacity x temperature difference.
6
Q
What is the equation for work done mechanically?
A
Work done = force x distance.
7
Q
What is the equation for energy transferred electrically?
A
Energy transferred = electrical power x time.
8
Q
What happens in a closed system regarding energy?
A
In a closed system, there is no net change in energy; no external forces act on it, and energy is conserved.
9
Q
How can energy be transferred?
A
Energy can be transferred mechanically, electrically, by heating through particles, or by radiation.
10
Q
What is an analogy used to explain energy?
A
Energy is often compared to money; both are conserved and can be calculated based on initial amounts and changes.
11
Q
What is the significance of thermal energy?
A
Thermal energy is shorthand for energy in a thermal store.
12
Q
What are the learning outcomes of P7.1 Work done?
A
After studying this lesson you should be able to: describe changes in energy storage during system changes, describe energy changes due to work done by forces, calculate energy changes using the mechanical work equation, use a common scale for energy redistribution, and calculate energy associated with a moving body.
13
Q
How do you analyze a situation using energy?
A
To do an energy analysis you need to: choose two points in a process, identify which stores have more or less energy at those points, and determine the type of transfer that occurred.
14
Q
What is the energy analysis for a drag racer at the start of the race?
A
At Point 1 (start of the race): stationary car with more fuel and oxygen, no energy in the kinetic store, and more energy in the chemical store.
15
Q
What is the energy analysis for a drag racer at the end of the race?
A
At Point 2 (end of the race): moving car with less fuel and oxygen, some energy in the kinetic store, and less energy in the chemical store.
16
Q
What is the equation for transferring energy mechanically?
A
You transfer energy mechanically from a chemical store to a kinetic store.
17
Q
What is the work done by a drag racer exerting a force of 4 kN over a distance of 300 m?
A
Work done = F x d = 4000 N x 300 m = 1.2 x 10^6 J (or Nm).
18
Q
How do you calculate the final speed of a drag racer?
A
Make speed the subject of the equation for kinetic energy: v = √(2E/m). For E = 1.2 x 10^6 Nm and m = 300 kg, v = 90 m/s.
19
Q
What happens to the temperature of the air as the drag racer moves through it?
A
Some energy is transferred to the thermal store of the surroundings due to friction, sound, or air resistance.
20
Q
What is a study tip for recalling equations for work done and kinetic energy?
A
Use the units to check that you have changed the subject of the equation correctly.
21
Q
What is the relationship between Newtons and Joules?
A
1 N = 1 kg m/s² and 1 J = 1 N m.
22
Q
What should you be able to describe after studying P7.1?
A
You should be able to describe all the changes involved in the way energy is stored when a system changes for common situations, such as a vehicle slowing down.
23
Q
What calculations should you be able to make in P7.1?
A
You should be able to make calculations of the energy changes associated with changes in a system using the equation for mechanical work.
24
Q
What is the equation for work done by forces?
A
Work done by brakes = F x d
25
How does braking affect energy transfer in a car?
The work done by the brakes can slow a car down, transferring energy from the kinetic store of the moving car to the thermal stores of the brake pads and surroundings.
26
What is the energy analysis for a braking car?
Point 1: a moving car (kinetic store) transfers energy to Point 2: a stationary car (thermal store).
27
How do you calculate the force exerted by the brakes?
Force = energy in kinetic store / distance
## Footnote
Example: For an energy of 1.2 × 10^6 J and a distance of 50 m, Force = 24000 N.
28
What happens when a gymnast lands on a springboard?
The gymnast compresses the springboard, transferring energy from kinetic and gravitational stores to the elastic store of the spring.
29
What is the equation for energy stored in a spring?
E = 1/2 k x^2
30
How do you calculate the energy transferred to a spring?
Use the spring equation E = 1/2 k x^2.
31
What is the energy transferred to the elastic store if a spring compresses by 30 cm with a spring constant of 35 kN/m?
E = 1/2 × 35 × 10^3 N/m × (0.3 m)^2 = 1600 J.
32
Why might the gymnast not reach the same height after jumping on the springboard?
Some energy is transferred to the thermal store of the surroundings due to sound and air resistance.
33
What factors affect the force needed to stop a cyclist?
Air resistance and friction affect the force needed to stop the bicycle.
34
Why is it not possible to calculate the increase in temperature of the surroundings when a car brakes?
The energy transfer involves multiple variables that are difficult to quantify precisely.
35
What should you be able to describe after studying P7.1?
You should be able to describe all the changes involved in the way energy is stored when a system changes for common situations, such as a vehicle slowing down.
36
What calculations should you be able to make in P7.1?
You should be able to make calculations of the energy changes associated with changes in a system using the equation for mechanical work.
37
What is the equation for work done by forces?
Work done by brakes = F x d
38
How does braking affect energy transfer in a car?
The work done by the brakes can slow a car down, transferring energy from the kinetic store of the moving car to the thermal stores of the brake pads and surroundings.
39
What is the energy analysis for a braking car?
Point 1: a moving car (kinetic store) transfers energy to Point 2: a stationary car (thermal store).
40
How do you calculate the force exerted by the brakes?
Force = energy in kinetic store / distance
## Footnote
Example: For an energy of 1.2 × 10^6 J and a distance of 50 m, Force = 24000 N.
41
What happens when a gymnast lands on a springboard?
The gymnast compresses the springboard, transferring energy from kinetic and gravitational stores to the elastic store of the spring.
42
What is the equation for energy stored in a spring?
E = 1/2 k x^2
43
How do you calculate the energy transferred to a spring?
Use the spring equation E = 1/2 k x^2.
44
What is the energy transferred to the elastic store if a spring compresses by 30 cm with a spring constant of 35 kN/m?
E = 1/2 × 35 × 10^3 N/m × (0.3 m)^2 = 1600 J.
45
Why might the gymnast not reach the same height after jumping on the springboard?
Some energy is transferred to the thermal store of the surroundings due to sound and air resistance.
46
What factors affect the force needed to stop a cyclist?
Air resistance and friction affect the force needed to stop the bicycle.
47
Why is it not possible to calculate the increase in temperature of the surroundings when a car brakes?
The energy transfer involves multiple variables that are difficult to quantify precisely.
48
What should you remember about energy transfer in real situations?
The system is not closed and energy will be transferred to thermal stores by friction, sound, and air resistance.
49
What equations do you need to remember for energy calculations?
The equations for kinetic energy and energy in a gravity store.
50
What are the learning outcomes for energy analysis with forces?
1. Describe energy storage changes in common situations.
2. Describe energy changes when a moving object hits an obstacle.
3. Describe changes in energy due to work done by forces.
4. Use a common scale for energy redistribution.
5. Calculate energy associated with a moving body.
6. Calculate energy associated with an object raised above ground level.
51
How can you analyze the energy of a ball thrown upwards?
Use the energy analysis for throwing a ball upwards, considering the ball's motion and forces acting on it.
52
What is the energy analysis for a ball at Point 1 and Point 2?
Point 1: A moving tennis ball (energy in kinetic store).
Point 2: A stationary tennis ball (energy in gravitational store).
53
What are the two ways to calculate energy transferred when throwing a ball?
1. Work done = force x distance.
2. Change in energy (in a gravitational store) = mass x gravitational field strength x change in height.
54
How do you calculate the height a ball reaches when thrown?
Use the equation v² = 2gAh, where v is the initial velocity, g is the gravitational field strength, and Ah is the change in height.
55
What is the height reached by a ball thrown at 5.0 m/s with g = 10 m/s²?
The height above your hand that it reaches is 1.3 m (2 significant figures).
56
Why must you use the vertical height when analyzing objects rolling up a slope?
You need to use the vertical height, not the distance up a slope, for accurate energy calculations.
57
What happens when a moving object hits an obstacle?
The object might bounce or stop, depending on the situation.
58
What is the energy analysis for falling sunglasses?
Point 1: Just before landing (energy in kinetic store).
Point 2: When they have stopped moving (energy in thermal store).
59
What is a key study tip regarding energy stores?
For most stores, the energy associated with the store is not zero at the start or at the end, except for the kinetic store.
60
How does air resistance affect the height a ball thrown upwards reaches?
A ball thrown upwards will reach a smaller height if there is air resistance.
61
How do you calculate the energy transferred to the thermal store of sunglasses with mass 25 g traveling at 2 m/s?
Use the equation E = 0.5mv², where m is the mass in kg and v is the velocity.
62
Why is the energy analysis of the falling sunglasses considered realistic?
It accounts for energy transfer to the thermal store upon impact.
63
How do you calculate the speed of a trolley that reaches a height of 40 cm when pushed up a ramp?
Use the conservation of energy principles to find the initial speed.
64
Why do you not need to know the mass of the trolley to calculate its speed?
The mass cancels out in the energy equations, making it unnecessary for the calculation.
65
What percentage of energy transferred globally is used to run the Internet?
About 2% of the energy transferred globally is used to run the Internet.
66
How does an electric current transfer energy?
Energy is transferred when charge moves from a chemical store of a battery or from a chemical or nuclear store of the fuel in a power station.
67
What does the power rating of an appliance indicate?
The power rating in watts (W) or kilowatts (kW) indicates the rate at which the appliance transfers energy between stores.
68
What is the unit of energy transferred electrically for domestic appliances?
The kilowatt-hour (kWh) is the unit of energy transferred electrically.
69
What is the equation for electrical work done?
(electrical) work done (kWh) = power (kW) x time (h)
70
How many joules are in 1 kWh?
1 kWh = 3,600,000 J.
71
How do you calculate the energy transferred by a 10 kW shower for 15 minutes?
Convert time to hours: 15 minutes = 0.25 h. Use the equation: energy transferred = power x time.
72
What do you pay for when you pay your electricity bill?
You are charged for each kWh, or unit, that you use.
73
How do you calculate the cost of using a 10 kW shower for 15 minutes?
Cost = energy transferred x cost per kWh. If energy transferred is 4 kWh and cost is 10p per kWh, then cost = 4 kWh x 10p/kWh = 40p.
74
What is a common mistake to avoid when discussing power and energy?
Do not confuse kW (power) and kWh (energy transferred).
75
What term should be used instead of 'electrical energy'?
Use 'energy transferred electrically' instead.
76
What is the equation for calculating energy transferred electrically?
E = P × t
77
What do electrical appliances do?
An electrical appliance transfers energy electrically from chemical stores.
78
How do appliances change the energy in stores?
Appliances transfer energy between stores.
79
What happens when a current flows in a wire?
The wire heats up, meaning energy is always transferred to a thermal store.
80
Identify the energy stores involved when using a battery-operated fan.
Kinetic (of fan), chemical (of battery), thermal (of surroundings).
81
What is the power of a kettle that uses 2 kW?
2000 W
82
How much energy is required to heat 0.5 kg of water from 20°C to 100°C?
168000 J
83
How long does it take for a 2 kW kettle to boil 0.5 kg of water?
80 seconds
84
What is the specific heat capacity of water?
4200 J/kg K
85
What is the temperature difference when heating water from 20°C to 100°C?
80 °C
86
What is the cost of running a 1 W LED indicator light for a week if 1 kWh costs 10p?
Estimate the savings by turning off the kettle at the mains.
87
Does turning off lights when not in use save energy?
Yes, it saves energy.
88
What should you be able to describe after studying P7.2.3 Energy analysis - heating?
You should be able to describe the changes in energy involved when a system is changed by heating, make calculations of the energy changes, use a common scale to show energy redistribution, describe energy dissipation, and explain ways to reduce unwanted energy transfer.
89
How is energy transferred to or from a thermal store?
Energy can be increased in a thermal store by heating, which usually involves burning a fuel or using an electric current to transfer energy from a fuel.
90
What does 'in equilibrium' mean?
It means that energy transfer continues until the objects are at the same temperature.
91
How do storage heaters work?
Storage heaters contain a large piece of concrete that heats up during the night and cools down during the day to heat your house.
92
What is energy dissipation?
Energy dissipation occurs when energy is transferred to the surroundings and is no longer in a useful store.
93
What happens to energy after using appliances like televisions?
The energy used is dissipated into the surroundings, heating them up, and is no longer in a useful store.
94
How can you reduce energy dissipation due to friction?
You can reduce energy dissipation due to friction by using lubrication, such as oil.
95
How can you reduce energy dissipation due to heating?
You can reduce energy dissipation due to heating by using insulation, such as foam.
96
Why is the thermal store of concrete in a storage heater useful?
It is useful because it retains energy for heating the house, unlike the thermal store of the surroundings which is not easily usable.
97
What is the specific heat capacity of water?
The specific heat capacity of water is 4200 J/kg K.
98
How do you calculate energy transferred in a storage heater?
Use the equation E = mcΔT, where m is mass, c is specific heat capacity, and ΔT is the temperature change.
99
What is the energy transferred when a 100 kg concrete block changes from 80°C to 20°C?
E = 100 kg x 880 J/kg K x 60 K = 5.3 x 10^5 J.
100
What are the learning outcomes for P7.2.4 Walls and insulation?
After studying this lesson you should be able to describe how the rate of cooling depends on the thickness of walls and the thermal conductivity of walls.
101
How does the thickness of the wall affect the rate of cooling?
The rate of temperature drop is greater if the walls are thinner.
102
What happens to a house when the heating is turned off?
The walls, floor, windows, and roof transfer energy to the thermal store of the surroundings, causing the house to cool down.
103
What is the relationship between thermal conductivity and rate of cooling?
The room with walls of a higher thermal conductivity will cool down faster.
104
What should be considered when choosing building materials for walls?
You need to consider the thermal conductivity of the material.
105
What is the effect of modern insulation materials?
Modern insulation materials have a low thermal conductivity, reducing energy transfer.
106
How can you investigate the thermal conductivity of materials?
Use materials to insulate a hot object and measure the temperature drop over a fixed period of time.
107
What factors must be controlled in an investigation of thermal conductivity?
Thickness, area, and starting temperature.
108
What is the thermal conductivity of concrete?
0.96
109
What is the thermal conductivity of foam (wall insulation)?
0.03
110
What is the thermal conductivity of aerogel?
Aerogel has a very low thermal conductivity and is used to insulate space vehicles.
111
How can living in an igloo be warm?
Ice has a low thermal conductivity, allowing igloos to retain heat.
112
What should you be able to do after studying efficiency?
You should be able to calculate energy efficiency for any energy transfer, explain ways of reducing unwanted energy transfer, and describe how to increase efficiency.
113
What is the equation for efficiency?
Efficiency = useful output energy transfer / input energy transfer
114
What does efficiency tell us?
Efficiency tells us how good the appliance is at doing its job and is expressed as a ratio or percentage.
115
Why can't an appliance be 200% efficient?
No appliances are 100% efficient because energy is always dissipated, meaning some energy is wasted.
116
How do you calculate wasted energy?
Wasted energy = input energy transfer - useful output energy transfer.
117
What is a Sankey diagram?
A Sankey diagram visually represents energy efficiency, where the width of the arrows shows the amount of energy transferred.
118
How can you increase efficiency?
You can increase efficiency by reducing wasted energy through insulation, using better materials, and utilizing technology like LEDs.
119
What is the efficiency of an electric motor that transfers 340J of energy for every 400J supplied?
Efficiency = 340J / 400J = 0.85 or 85%.
120
What is a CFL?
A CFL is an energy-saving light bulb.
121
What should you remember about efficiency calculations?
Efficiency cannot be more than 100%. If you get a number greater than 100, it is incorrect.
122
What is the significance of energy dissipation?
Energy cannot be destroyed, so it is better to say 'dissipated' rather than 'lost'.
123
What are the types of energy stores?
Chemical, thermal, kinetic, gravity, elastic, nuclear, electromagnetic.
124
What is the formula for thermal energy change?
AF = mCAT.
125
What is the formula for gravitational potential energy?
E = mgh.
126
What is the formula for force in terms of mass and acceleration?
F = Amc.
127
How does energy transfer occur?
A store empties through physical processes, filling other stores.
128
How is energy related to money?
Energy can be calculated using equations, similar to money.
129
What principle does energy follow?
Energy is conserved; if it seems lost, check for a thermal store filling.
130
What happens when a ball is dropped?
Before: lots of energy in gravity store. After: lots of energy in kinetic store.
131
What happens when a spring is pulled?
Before: energy in chemical store. After: energy in elastic store.
132
What happens when a car stops or slows down?
Before: energy in kinetic store. After: energy in thermal store.
133
What happens when lifting an object or projecting up a slope?
Before: energy in chemical/kinetic store. After: energy in gravity store.
134
What happens when using a kettle?
Before: energy in chemical store. After: energy in thermal store.
135
What causes current to flow?
Current flows due to mains a.c. or battery (chemical store).
136
What happens to energy in wires?
Some energy goes to thermal store, causing wires to heat up.
137
How does power rating affect chemical stores?
Appliances with a higher power rating deplete chemical stores faster.
138
What is the formula for power?
Power (kW) = (electrical) work done (kWn) / time (h).
139
What is the formula for cost?
Cost (p) = power (kW) x time (h) x unit cost (p/kWn).
140
What do forces do?
Forces change shape/motion and perform mechanical work.
141
How do particles behave?
Particles move/vibrate through conduction and convection.
142
What happens with current flows?
Wires heat up and motors spin, performing electrical work.
143
What is emitted or absorbed in energy transfers?
EM radiation and sound.
144
What factors affect temperature in houses?
Temperature over time depends on temperature difference and material thickness/area.
145
How can energy transfer be reduced?
Use insulation with low thermal conductivity, reduce area, and lower temperature difference.
146
What does efficiency show?
Efficiency shows the ratio of desired energy transfer to total energy transferred.
147
How do efficient appliances work?
They transfer less energy to undesired stores, usually thermal stores of surroundings.
148
How is efficiency expressed?
Usually expressed as a percentage.
149
What is represented by a Sankey diagram?
Input energy transfer, useful energy transfer, and not useful energy transfer.
150
What is the formula for efficiency?
Efficiency = useful output energy transfer / input energy transfer.
