Year 9 AQA GCSE Physics PDF

# Year 9 AQA GCSE Science (Physics) ## Topic 3 Energy Stores and Changes ### 4.1.1 Energy changes in a system, and the ways energy is stored before and after such changes #### 4.1.1.1 Energy Stores and Systems * A system is an object or group of objects. * There are changes in the way energy is...

# Year 9 AQA GCSE Science (Physics) ## Topic 3 Energy Stores and Changes ### 4.1.1 Energy changes in a system, and the ways energy is stored before and after such changes #### 4.1.1.1 Energy Stores and Systems * A system is an object or group of objects. * There are changes in the way energy is stored when a system changes. **Examples:** * An object projected upwards * A moving object hitting an obstacle * An object accelerated by a constant force * A vehicle slowing down * Bringing water to a boil in an electric kettle. **Students should be able to:** * Describe all the changes involved in the way energy is stored when a system changes, for common situations (including the examples above) * Throughout this Energy topic, calculate the changes in energy involved when a system is changed by: heating, work done by forces, work done when a current flows. * Use calculations to show on a common scale how the overall energy in a system is redistributed when the system is changed. #### 4.1.1.2 Changes in energy * **The kinetic energy of a moving object can be calculated using the equation:** $Kinetic Energy = 0.5 \times mass \times (speed)^2$ $E_k = \frac{1}{2}mv^2$ Where: * $E_k$ is kinetic energy in joules ($J$) * $m$ is mass in kilograms ($kg$) * $v$ is speed in metres per second ($m/s$). * **The amount of elastic potential energy stored in a stretched spring can be calculated using the equation:** $Elastic potential energy = 0.5 \times spring constant \times (extension) 2$ $(assuming the limit of proportionality has not been exceeded)$ $E_e = \frac12 ke^2$ Where: * $E_e$ is elastic potential energy in joules, ($J$) * $k$ is the spring constant in newtons per metre, ($N/m$) * $e$ is extension in metres ($m$). * **The amount of gravitational potential energy gained by an object raised above ground level can be calculated using the equation:** $Gravitational potential energy = mass \times gravitational field strength (g) \times height$ $E_p = mgh$ Where: * $E_p$ is gravitational potential energy in joules, ($J$) * $m$ is mass in kilograms ($kg$) * $g$ is gravitational field strength in newtons per kilogram ($N/kg$) * $h$ is height in metres ($m$). **Students should be able to:** * Calculate the amount of energy associated with a moving object, a stretched spring and an object raised above ground level. * Recall and apply the equation for kinetic energy * Apply the equation for elastic potential energy, which is given on the Physics equation sheet * Recall and apply the equation for gravitational potential energy #### 4.1.1.3 Energy changes in systems * **The amount of energy stored in or released from a system as its temperature changes can be calculated using the equation:** $Change in thermal energy = mass \times specific heat capacity \times temperature change$ $\Delta E = mc\Delta \theta$ Where: * $\Delta E$ is change in thermal energy in joules, ($J$) * $m$ is mass in kilograms, ($kg$) * $c$ is specific heat capacity in joules per kilogram per degree Celsius ($J/kg$ °C) * $\Delta \theta$ is temperature change in degrees Celsius (°C). * **The specific heat capacity of a substance is the amount of energy required to raise the temperature of one kilogram of the substance by one degree Celsius.** **Students should be expected to:** * Apply the equation for specific heat capacity, which is given on the Physics equation sheet. **Required Practical: Specific Heat Capacity. AT1 and 5.** #### 4.1.1.4 Power * **Power is defined as the rate at which energy is transferred or the rate at which work is done.** $Power = \frac{energy transferred}{time}$ $P = \frac{E}{t}$ $Power = \frac{work done}{time}$ $p = \frac{w}{t}$ Where: * $P$ is power in watts ($W$) * $E$ is energy transferred in joules ($J$) * $W$ is work done in joules ($J$) * $t$ is time in seconds ($s$). * **An energy transfer of 1 joule per second is equal to a power of 1 watt.** **Students should be able to:** * Recall and apply both of the equations for power. * Give examples that illustrate the definition of power, e.g. comparing two electric motos that both lift the same weight through the same height but one does it faster than the other. ### 4.1.2 Conservation and Dissipation of Energy #### 4.1.2.1 Energy transfers in a system * Energy can be transferred usefully, stored or dissipated, but cannot be create or destroyed. * Whenever there are energy transfers in a system only part of the energy is usefully transferred. * The rest of the energy is dissipated so that it is stored in less useful ways. This energy is often described as being 'wasted'. * Unwanted energy transfers can be reduced in a number of ways, for example through lubrication and the use of thermal insulation. * The higher the thermal conductivity of a material the higher the rate of energy transfer by conduction across the material. **Students should be able to:** * Describe, with examples, where there are energy transfers in a closed system, that there is no net change to the total energy. * Describe, with examples, how in all system changes energy is dissipated, so that it is stored in less useful ways. The energy is often described as being 'wasted'. * Explain ways of reducing unwanted energy transfers, for example, through lubrication and the use of thermal insulation. * Describe how the rate of cooling of a building is affected by the thickness and thermal conductivity of its walls. Students do not need to know the definition of thermal conductivity. #### 4.1.2.2 Efficiency * **The energy efficiency for any energy transfer can be calculated using the equation:** $efficiency = \frac{useful\ output \ energy \ transfer}{total \ input \ energy \ transfer}$ * **Efficiency may also be calculated using the equation:** $efficiency = \frac{useful \ power \ output}{total \ power \ input}$ **Students should be able to:** * Recall and apply both equations for efficiency. * Calculate or use efficiency values as a decimal or as a percentage. * (HT only) Describe ways to increase the efficiency of an intended energy transfer.