1.2 Conservation & Dissipation of Energy

What is the law of conversation of energy?

Energy can be transferred usefully, stored or dissipated, but cannot be created or destroyed.

• In all system changes energy is ___________, so that it is stored in less ________ ways.

• This energy is often described as being ‘________’.

• dissipated

• useful

• wasted

How does a pendulum show the conservation of energy?

• At its highest point of its swing it has maximum gravitational potential energy.

• As it falls it gains kinetic energy which is its greatest at the mid part of its swing.

• As it moves past the midpoint it decreases in kinetic energy and increases again in gravitational potential energy.

What energy transfers happen to a bungee jumper after jumping off the platform?

• When the rope is slack, energy is transferred — _____________ __________ energy store → _________ energy store as the jumper _____________ towards the ground (due to the force of ________).

• When the rope tightens, it ________ the jumper’s fall — the force of the rope reduces the _________ of the jumper.

• Jumper’s __________ energy store decreases, rope’s __________ ____________ energy store increases as rope ____________.

• Eventually the jumper comes to a stop — energy in __________ energy store of the jumper all transferred to the ___________ __________ energy of the rope.

• After reaching the bottom, rope recoils and pulls jumper back up — as jumper rises energy in ___________ ___________ energy store of rope decreases and jumper’s ___________ energy store increases (until rope becomes ________).

• After rope becomes ________ (and at the top of the ascent), jumper’s __________ energy store = 0 — jumper’s ______________ __________ energy store increases through the ascent.

• gravitational potential

• kinetic

• accelerates

• gravity

• slows

• speed

• kinetic

• elastic potential

• stretches

• kinetic

• elastic potential

• elastic potential

• kinetic

• slack

• slack

• kinetic

• gravitational potential

What happens when there are energy transfers in a closed system?

There is no net change to the total energy.

What are two ways to reduce unwanted energy transfers?

• Lubrication

• Thermal insulation

How can lubrication reduce unwanted energy transfers?

• Oil/grease can be used — slightly separates moving parts

• Reduces friction when objects rub against each other

• Decreases the amount of energy wasted as heat to the surroundings

How can thermal insulation reduce unwanted energy transfers?

• Uses materials with low thermal conductivity — e.g. double glazing or loft insulation

• Often works by trapping pockets of air — very poor thermal conductor

• Slows down the rate of energy transfer — reduces the amount of useful thermal energy lost from a system

The higher the __________ _________________ of a material the higher the rate of energy transfer by ______________ across the material.

• thermal conductivity

• conduction

What is the rate of cooling of a building affected by?

• Thickness of its walls

• Thermal conductivity of its walls

How is the rate of cooling of a building affected by the thickness and thermal conductivity of its walls?

• Higher thermal conductivity of the walls = higher rate of energy transfer — building will cool down faster

  • Using materials with low thermal conductivity slows down this heat loss

• Thicker walls reduce the rate of thermal energy transfer — lowers the rate of cooling

What is the independent variable in RP 2 (Activity 1)?

Type of insulating material

What is the independent variable in RP 2 (Activity 2)?

Thickness of material

What is the dependent variable in RP 2?

Temperature decrease over time

What are the control variables in RP 2 (Activity 1)?

• Volume of water

• Initial temperature of the water

• Thickness of the insulation

What are the control variables in RP 2 (Activity 2)?

• Volume of water

• Initial temperature of the water

• Type of insulating material

Describe a method for investigating the effectiveness of different materials as thermal insulators. (6)

• Place a small beaker inside a larger beaker and pack the gap with an insulating material — leave one setup with no insulator as a control

• Pour a fixed volume of hot water into the small beaker

• Place a cardboard lid on top with a thermometer placed through a hole

• Record the initial temperature (this should be the same for each material) and start a stopwatch

• Record the temperature every 3 minutes for 15 minutes

• Calculate the total temperature drop for each material (initial temperature - final temperature) — the one with the lowest value is the best thermal insulator

• This graph shows results for the practical investigating the effectiveness of different materials as thermal insulators.

What does this graph show? (4)

• The curve which takes the longest time for the water temperature to drop (the shallowest) should be the material that is the best insulator.

• The temperature falls quickly at high temperatures and slowly at low temperatures.

• When the beaker is at a high temperature, there is a big difference between the temperature of the beaker and the temperatures of the surrounding air — this means that there is a high rate of transfer.

• When the beaker is at a lower temperature, there is less difference between the temperature of the beaker and the temperatures of the surrounding air — this means that there is a lower rate of transfer.

Describe a method for investigating how the thickness of a material affects the thermal insulation. (6)

• Wrap a beaker in a single layer of an insulator — e.g. newspaper

• Pour a fixed volume of hot water into the beaker

• Add a cardboard lid and insert a thermometer into the water

• Record the initial temperature (this should be the same for each round of the experiment) and start a stopwatch

• Record the temperature every 3 minutes for 15 minutes

• Repeat the experiment, adding more layers of the same material — compare the overall temperature drops (initial temperature - final temperature)

• This graph shows results for the practical investigating how the thickness of a material affects the thermal insulation.

What does this graph show? (5)

• The curve which takes the longest time for the water temperature to drop (the shallowest gradient) shows the amount of layers that provide the best insulation.

• The curve for no insulation has the steepest gradient at any given time interval.

• As the number of layers increases, the gradient of each curve decreases at any given time interval.

• Having more layers increases the insulation which means temperature drops more slowly — the thickest insulation has the lowest rate of cooling.

• The temperature falls quickly at high temperatures and slowly at low temperatures.

What is the equation for the energy efficiency for any energy transfer (energy)?

efficiency = (useful output energy transfer / total input energy transfer) × 100

What is the equation for the energy efficiency for any energy transfer (power)?

efficiency = (useful power output / total power input) × 100