Energy

Energy stores and transfers

System: something in which we are studying changes e.g. an electrical kettle & its surroundings

Law of conservation of energy = energy can’t be created or destroyed.

Meaning total energy put into a system = total energy transferred out of that system

Joules (J) = units for energy

Energy transfer diagrams

We represent energy stores and transfers in diagrams like this:

Sankey Diagrams

A Sankey diagram shows the amount of energy transferred and where to.

Different thickness of arrows represents how much energy is transferred in that way.

Percentages are put in brackets after the energy transfer.

This is a Sankey diagram for a light bulb:

Energy efficiency

Dissipated: spread out (for energy)

Mechanical processes become wasteful when they cause a rise in temperature because energy dissipates through heating into the surroundings

Efficiency describes how good a machine is at transferring energy into useful forms. It is given in a number between 0 and 1, or sometimes a percentage.

0 (0%) = machine wastes all its energy

0.5 (50%) = machine wastes half its energy, usefully transfers half of its energy

1 (100%) = all energy is transferred usefully

Efficiency = useful energy transferred by device total energy supplied to device

Reducing unwanted energy transfer (increasing energy efficiency)

Friction can be reduced by lubrication - making a surface smooth means things can move on it easier so there is less friction

Thermal insulation helps slow the rate at which energy is transferred out of a place e.g. a house

Keeping warm

Insulation slows the rate at which energy is transferred out of a place (e.g. a house)

Energy can be transferred by heating in different ways:

Conduction: vibrations passed between particles in a solid

Convection: part of a fluid is warmer (less dense) and rises up, on the other side the colder (more dense) fluid sinks down. This creates a convection current going around in a circle

Radiation: energy transferred through waves. Infrared radiation can pass through some solid objects

Thermal conductivity is how well a material allows heat to move through it. It can depend on thickness and the temperature difference across it. A good insulator needs low thermal conductivity

Energy resources

Solar Energy

Heating and lighting from the sun are either used with solar cells to generate electricity, or radiation from the sun is used to heat water for homes, using solar panels.

Renewable: yes

Wind energy

Using wind turbines, we can turn the kinetic energy of the wind to turn a turbine and generate electricity.

Renewable: yes

Hydroelectric energy

Water flowing in a river turns a turbine to generate electricity. Usually need to build a dam and let water flow through it gradually.

Renewable: yes

Wave energy

The sea’s waves have kinetic energy, and using machines we can turn that into energy.

Renewable: yes

Nuclear energy

The energy in uranium nuclei is transferred into heating, which is used to create steam that turns a turbine and generates energy.

Renewable: no

Fossil fuels

Chemical energy in coal, oil and gas is transferred into heating, which is used to create steam which turns a turbine and generates electricity

Renewable: no

Biomass energy

Chemical energy in things that were once alive e.g. trees is transferred to heating when they are burned.

Renewable: yes, as long as we keep planting new plants

Geothermal energy

Rocks deep underground are very hot, and we can use this heat to generate electricity by producing steam to turn a turbine.

Renewable: yes

Tidal energy

At high tide, water is trapped behind a dam, and at low tide it is released, turning a turbine which is used to generate electricity.

Renewable: yes

Waves

A wave is an oscillation (vibration) that transfers energy from one place to another. Waves transfer energy and information, but not matter.

Wavelength: length of a full cycle e.g. from crest to crest (m)

Amplitude: size of wave (1/2 of crest to trough)

Frequency: number of complete waves to pass a point per sec

Period: time taken to move through one complete cycle

Velocity: speed of an object in a particular direction

Speed = frequency x wavelength (Hz)

Transverse waves (e.g. light waves): vibrations are perpendicular to direction of transfer

Longitudinal waves (e.g. sound waves): vibrations are parallel to direction of transfer

Crest: highest point of wave

Trough: lowest point of wave

Equilibrium: middle point of wave

Wavefront: all locations where the wave is at the same phase e.g. where all the troughs are in the same phase

Speed = distance time

Period = 1/frequency

Frequency = 1/period

Speed (m/s) = frequency (Hz) x wavelength (m)

Reflection, refraction, transmission & absorption

Reflection: when a wave bounces off a surface at an angle and changes direction

Refraction: when light changes speed between different media

Transmission: when the wave passes through the material

Absorption: when the wave is taken in and the energy is transferred to the material

Refraction

When a wave enters a block, it bends towards the normal

The frequency of the wave remains the same, and the wavelength gets smaller

When the wave gets faster, it bends away from the normal

When it gets slower, it bends towards the normal

The ear

Humans can hear 20Hz-20,000Hz

Ultrasound: very high sounds humans can’t hear (over 20,000Hz)

Infrasound: very low sounds humans can’t hear (under 20Hz)

Parts of the ear

Auditory nerve: electrical signals carry messages along this to the brain

Eustachian tube: tube that connects ear and nose

Ear canal: tube that carries sound to the inner ear

Pinna: visible part of the ear, collects sound waves and funnels them into the ear

Eardrum: thin membrane that vibrates when sound waves hit it

Ossicles: made up of the stirrup, anvil and hammer, they help transmit vibrations to the inner ear

Semi-circular canals: help you balance

Cochlea: snail shaped and full of cilia (tiny hairs) that convert vibrations into electric signals (aka nerve impulses)

The base of the cochlea is thicker and stiffer than the apex, and vibrates at higher frequencies.

Measuring waves in a ripple tank (practical)

1. Count how many waves are formed in 10secs

2. Put a ruler against the tank and use it to measure the length of the waves (easier to take a photo and measure off that)

3. Measure the distance between two points and see how long it takes waves to go from one part to another

To get frequency:

Divide the number of waves in 10s by 10 (step 1)

To get speed:

Divide the distance by the time (both from step 3)

OR Multiply the wavelength (step 2) by frequency (above)

Ultrasound & Infrasound

Ultrasound

Ultrasound uses waves to see inside the body. They are high frequency sounds that humans can’t hear above 20,000 Hz

Ultrasound sonar:

1. Ship emits ultrasound wave downwards

2. Wave reflects off seabed and returns to the ship

3. Speed = distance/time is used to calculate depth of seabed (distance) as instruments on the ship measure the speed & time of the wave

Ultrasound scanning:

1. Transducer (or probe) is placed against skin

2. Gel is used to stop waves reflecting off skin

3. Transducer emits ultrasound waves and measures how long it takes for them to bounce off something and come back

4. Shorter time = closer object

5. Speed = distance/time used for exact distance calculations

6. An image can also be made if you know how far away lots of things are

Infrasound

Infrasound waves are very low sounds that humans can’t hear with frequency lower than 20Hz

Infrasound and the earth’s core:

• The properties of the earth change as you go deeper

• The S wave shadow zone means that there is something liquid in the centre of the earth stopping the S waves going through, as they can’t travel through liquids (the mantle)

• When P waves pass in and out of the liquid mantle, the refract and change direction

• The few weak waves received in the P wave shadow zone are because of the solid inner core

• This happens no matter where the earthquake is

Seismic waves: waves produced by earthquakes

Seismometers: instruments that detect/measure seismic waves

Motion

Scalars and vectors

Scalars just have a magnitude (size)

Vectors have a magnitude and a direction

→ need to learn this:

Speed & velocity

speed = distance time

velocity = displacement time

Velocity is speed in a stated direction

NB: velocity and displacement have a direction, so you use compass points e.g. 3m/s NE

Acceleration

Acceleration is the rate of change of velocity (it doesn’t have to be getting faster)

Equation with time

a = v - u t

aka change in velocity / time

a = acceleration

t = time

u = starting velocity

v = ending velocity

Equation with distance

a = (v² - u²) ÷ 2 x

a = acceleration

x = distance

v = end velocity

u = start velocity

You would be acceleration if you go around a corner because you are changing direction and acceleration (and velocity) has a direction.

Acceleration in a free fall is 10m/s²

Graphs

Instantaneous speed: speed at any given moment in a journey e.g. at 10 secs

Average speed: speed averaged taking into account the whole journey

average speed = total distance total time

Distance/time

Straight line = constant speed

Horizontal line = no movement (stationary) (constant speed of 0)

The gradient of the line represents the speed. Steeper gradient = more speed

Direction doesn’t matter for distance/time graphs as distance is a scalar quality. Even if the object goes backwards, the line will still go up.

Velocity/time

The gradient gives the acceleration, so a line sloping up means it is accelerating. A downwards line shows deceleration / negative acceleration

Horizontal line = constant velocity

If the line is at 0 velocity there is no movement

Direction does matter, if the velocity is a negative number (moving backwards) the line will go below 0

Comparisons

Gradients

Gradient = rise / run

1. Pick two points on a line

2. Find the vertical distance between points (rise)

3. Find the horizontal distance between points (run)

4. Divide the rise by the run