Topic 3 - conservation of energy

Question 1

Explain what it is meant by conservation of energy

Answer 1

the law of conservation of energy states that the total energy of an isolated system remains constant; it is said to be conserved over time.

Question 2

Explain how to draw and interpret diagrams to represent energy transfer

Answer 2

In all flow diagrams there must be energy stores and energy transfers present to show how energy is transferred from one store to others.

Based on the law of conservation of energy, the total energy present in the original stores must be equal to the energy present is the stores after the transfers have taken place, since you can’t destroy or create energy in any give system.

To draw them, draw boxes with the use and arrows suggesting how its transferred (eg mechanical) and spilt them into their stores

Question 3

Explain the four different ways of energy transfer

Answer 3

MECHANICALLY - by a force moving through a distance

ELECTRICALLY - by the use of electric current

THERMALLY - because of a difference in temperature

RADIATION - by waves such as electromagnetic or sounds

Some of the energy will be usefully transferred, and some will be wasted or DISSIPATED

Question 4

Explain how and why mechanical processes become wasteful

Answer 4

Mechanical processes become wasteful when they lead to a rise in the temperature in the surroundings through heating. In all system changes, energy is stored in less useful ways.

Eg
The light and sound energy from a TV will eventually be absorbed by walls and by people leading to a rise in temperature

Question 5

Explain what happens when there is an energy transfer in a closed system

Answer 5

When there are energy transfers in a closed system, there is no net change to the total energy in the system

Question 6

Explain the eight main energy stores and give examples

Answer 6

CHEMICAL - batteries, food, fuel
KINETIC - moving objects 
GRAVITATIONAL POTENTIAL - raised mass
ELASTIC - stretched spring
THERMAL - hot object
MAGNETIC - two magnets
ELECTROSTATIC - two charges
NUCLEAR - radioactive decay

Question 7

Give an example of where energy is wasted (dissipated)

Answer 7

Camping gas + oxygen (chemical store) goes to useful store as hot water in pan by heating but also goes to pan and surrounding air thermal store by heating as wasted energy

Question 8

Explain ways of reducing unwanted energy transfer

Answer 8

Unwanted energy transfers can be reduced through lubrication and thermal insulation

Question 9

Explain the factors that determine the rate at which thermal energy is transferred through the wall of a house

Answer 9

1) the difference in temperature between the warmer interior and colder exterior
2) the thickness of the walls
3) the material that the walls are made from

Question 10

Describe the effects of the thickness and thermal conductivity of the walls of a building on its rate of cooling qualitatively

Answer 10

A material with a better thermal conductivity is a better conductor of energy than one with a lower thermal conductivity. The rate they transfer energy differs due to different materials having different relative thermal conductivities. Lower thermal conductivity results in higher temperatures.

Thickness also reduces or increases the rate at which the wall transfers energy. Thicker walls result in less energy transferred, resulting in higher temperatures.

Question 11

Explain what efficiency is

Answer 11

All machines waste some of the energy they transfer, most machines waste energy as heat energy. The efficiency of a machine is a way of saying how good it is at transferring energy into useful forms . The higher the efficiency, the better the machine is at transferring energy into useful forms.

Question 12

Explain how efficiency can be increased

Answer 12

This can be achieved by reducing wasted energy transfers so more of the input energy is usefully transferred.

Mechanical devices can be made more efficient through lubrication to reduce the friction between moving parts of a machine and increase the amount of useful energy transferred.

For systems that are designed to transfer thermal energy, the efficiency can be improved by reducing the wasteful dissipation of thermal energy to the surroundings, for example by using insulation.

Question 13

Explain the renewable energy resources on earth

Answer 13

1. BIO FUELS - animal or plant matter used to generate electrical energy, thermal energy or used to power cars
2. HYDROELECTRICITY- generates electricity from water flowing behind a dam flowing down a pipe and turning a turbine to generate electricity
3. WIND TURBINES - use kinetic energy from the wind to generate electricity
4. TIDAL POWER - uses the rise and fall of the tide or tidal currents to generate electricity
5. SOLAR CELLS - convert solar energy Or energy directly from the sun , directly to electricity. Solar energy can be used directly to cook food or to heat water

Question 14

Explain the non renewable resources used

Answer 14

1. Nuclear fuels such as uranium are used to generate electricity and are used as energy sources in spacecraft
2. Fossil fuels are coal, oil and natural gas. They are used to generate electricity, to power transport, and to heat homes and for cooking
3. They are available all the time, unlike some renewable energy sources

Question 15

Explain patterns and trends in the use of energy resources and factors affecting demand for energy

Answer 15

The use of energy resources has changed over the years, this is because of several factors:

1. The worlds population - in the past 200 years, the population of the world has risen from 1 billion to 7 billion people
2. The development of technology - vehicles and other devices have increased in use and these require energy
3. Electrical energy - power stations require fuel to generate electricity

• most of the worlds energy use has been fossil fuels, before about 1900 wood was rte main source of fuel and its use has remained constant over time.
• in most recent years, there has been an increase in the use of nuclear fuel and hydroelectric power

Question 16

Analyse the changes involved in the way energy is stored when a vehicle slows down

Answer 16

To begin with, the vehicle’s energy is in the kinetic store.

The brakes do work slowing the car down. During this process, energy is dissipated (lost) through heat and sound.

Question 17

Analyse the changes involved in the way energy is stored when a moving object hits an obstacle

Answer 17

The object’s energy is in the kinetic store to begin with because it is moving.
When the object collides with the obstacle, energy is converted to:
The kinetic store of the obstacle (making it move),
The thermal store of the object and the obstacle (the particles in the object and the obstacle vibrate more).

Some energy remains in the object’s kinetic store as it moves away after the collision.

Question 18

Analyse the changes involved in the way energy is stored when bringing water to a boil in an electric kettle

Answer 18

Energy transfers from the electrical store of the mains power supply to the thermal store of the water.

Question 19

Analyse the changes involved in the way energy is stored when an object is accelerated by a constant force

Answer 19

Work is done by a force on an object.

This work is converted to the object’s kinetic store

Question 20

Analyse the changes involved in the way energy is stored when an object is projected upwards or up a slope

Answer 20

The object’s energy is initially in the kinetic store as it moves upwards.
The object’s energy is slowly transferred from the kinetic store to the gravitational potential store as it slows down and climbs higher.
Once the object reaches its highest point, all of it’s energy is in the gravitational potential store.
As the object falls again, energy transfers from the gravitational potential store to the kinetic store.