Topic 3 - Conservation of energy Flashcards
Change in GPE =
mass(m) x gravitational field strength(g) x change in height(h)
KE =
1/2 x m x v2
Conservation of energy
Energy can be stored, transferred between stores, and dissipated - but it can never be created or destroyed. The total energy of a closed system has no net change.
What does a diagram representing energy transfers look like
Boxes showing energy stores and arrows showing energy transfers
Energy stores and transfers involved in a ball rolling up a slope
Energy is transferred mechanically from the kinetic energy store of the ball to its gravitational potential energy store.
Energy stores and transfers involved in a moving object hitting and obstacle(eg a bat hitting a ball)
Some energy from the bats KE store is transferred mechanically to the balls KE store. Some is transferred mechanically to the thermal energy stores, and some is transferred to the surroundings by heating. The rest is carried by sound.
Energy stores and transfers involved in an object being accelerated by a constant force(eg a rock dropped from a cliff)
Assuming their is no additional forces acting against the object, gravity does work on the rock, so the rock constantly accelerates towards the ground( about 10 m/s2). energy is transferred mechanically from the GPE store to the KE store of the rock.
Energy stores and transfers involved in a vehicle slowing down(without braking)
Energy in the KE store is transferred mechanically due to friction and then by heating to the road and the tyres thermal energy store.
Energy stores and transfers involved in bringing water to a boil in an electric kettle
Energy is transferred electrically from the mains to the kettle, and then by heating to the thermal energy store of the water.
Useful energy
Energy that gets transferred from one store to a useful store.
Explain how mechanical processes produce some energy that isnt useful
Mechanical processes need to overcome fricitional forces. The energy needed to overcome these forces is transferred to the thermal energy stores of the object and surroundings. This energy is quickly dissipated and therefore wasted.
Lubrication
Reduces energy transferred by friction by using lubricants(usually liquids).
Insulation
Reduces the rate of energy transfer by heating. Reduces thermal conductivity.
Thermal conductivity
Shows how well a material transfers energy by conduction. A builiding should have low thermal conductivity walls to prevent it cooling too quickly.
Efficiency =
Useful energy transferred by device(J) / Total energy input to device(J)
