Physics 2 (Energy, Heat and Temperature)

What is energy

the ability to do work. Measured in Joules (J) or calories (cal). Less is used less often, we only tend to measure energy is calories when we are looking at living organisms.

Energy must be transferred in order to do work. Energy is a conserved quantity, it is always conserved, energy can't be created or destroyed.

Work =

force (newtons, N)) x distance (metres, m)

Kinetic energy definition

the kinetic energy of an object is the energy that it possesses due to its motion. (if something is moving that it has a certain amount of kinetic energy, if something is stationary it has no kinetic energy)

Kinetic energy equation

KE= 1/2mv^2.

So half x mass x velocity squared

Potential energy definition

the stored energy that can be used to do work. Has many different forms but we mainly see gravitational potential energy.

Gravitation potential energy equation

PEg = mgh

So mass x gravitational field strength x heigh

Elastic potential energy defintion

Potential energy stored as a result of deformation of an elastic object, such as the stretching of a spring.

Elastic potential energy equation

PEe = 1/2kx2

So 1.2 x K (spring constant, measured in Nm) x X2 (displacement squared,, measured in metres)

Restoring force equation (also known as Hookes Law)

F = + or - kx

So restoring force = + or - spring constant x displacement


It is + or - depending on when the force is exerted so:
1. For example, if we took a spring that was already compressed and we were calculating the energy that the spring would exert as it extends back to its original size, it would be a -
2. For example, if we were calculating the amount of energy needed to compress a spring it would be +. Think of it as it is a positive number because you need energy to compress the spring.

Thermal energy definition

A form of energy transferred between bodies and different temperatures

calorie units

1Cal = 1kcal = 1000cal

1 calorie definition

The amount of energy used to raise the temperature of 1kg of water by 1 degree celcius.

Temperature

Objects do not "contain" heat, they have thermal energy. Thermal energy is transferred from hotter objects to colder ones. Heat transfer will keep happening from hot to colder objects until both objects reach thermal equilibrium. Hotter = more thermal energy. Colder = less thermal energy.

Kelvin scale

The temperature scale on which zero is the temperature at which no more energy can be removed from matter. If something is at 0 kelvin, it has no thermal energy. This point happens at -273.15 degrees celcius. Kelvin scale doesn't use degrees celcius it uses kelvins but 1 kelvin is the same as 1 degree celcius.

Fahrenheit also exists

0 degrees celcius = 32 fahrenheit
100 degrees celcius = 212 fahrenheit

Internal thermal energy

the grand total of all the energy stored in the motions of the atoms and molecules within a substance.

Internal thermal energy can be described as the sum of kinetic and potential energy. Bonds between the atoms essentially behave like springs. Potential energy is stored in the bonds between the atoms. Kinetic energy is due to the vibration of the atoms about their position.

The absolute temperature is proportional to the absolute energy (the sum of potential and kinetic).

Specific heat definition

the energy needed to raise the temperature of a 1kg or the substance by 1 degree celcius.

specific heat equation

C = Q x m x ΔT

Specific heat capacity = energy transferred x mass x change in temp

Thermal energy (heat) transfer

Thermal energy is transferred in 3 ways:

1. Conduction
Through physical contact e.g a pan base heating up on a fire. The fire is in physical contact with the pan base which heats the pan base up.

2. Convection
The transfer of heat through a liquid, either liquid or gas. Thermal energy is distributed through the movement of a medium. Because it relies on the flow, convection only occurs in liquids and gases. e.g The water in the pan heats up due to the fire. Once the water at the base of the ban is heated by the fire, the rest of the water in the pan becomes heated through convection within the water. The water is the medium.

3. Radiation
The transfer of thermal energy through electromagnetic radiation. Vibrating particles emit electron magnetic radiation. Electro magnetic radiation is emitted by a source and absorbed by a medium. Because it doesn't rely on flow, it is the only one that can occur in a vacuum. E.g the fire emits EM radiation which the pan base receives.

Thermal conduction (within a solid)

Different from the 3 ways above because this method is WITHIN a solid rather than from one solid to another.

Thermal Conduction (also called thermal energy flow) Is the process of heat transfer through a solid by transmitting kinetic energy from one molecule to the next. The main method of thermal transport in solids. Thermal energy transfer always go from the hot end to the cold end.

SI units for thermal conductivity = Js-1 m-1 K-1
Joules per second per metre per Kelvin

Thermal conduction (energy flow) equation

q = (kxsa/L) (T1-T2)

Q = energy flow per unit of time per unit area
K = thermal conductivity - is how easy a material can transfer thermal energy by conduction, the higher the K value the quicker the thermal energy flows from the hot to cold region. sa = surface area
l = length
T = time

K is a constant depending on the medium (the material). K is generally positive. If the flow goes from the cold to to the hot area then K will be negative so if we have a flow in the opposite direction we add a minus sign.

Thermal Convection

Energy transferred through the flow of a medium. Because it relies on flow, convection only occurs in liquids and gases.

How convection occurs: e.g a pan of water placed on a fire
1. Liquid at bottom of pan begins to heat
2. As liquid heats, it expands and therefore the density reduces. The liquid with a lower density rises
3. Liquid at the bottom is replaced with cooler liquid from the top
4. This leads to a constant flow in a circular direction

Thermal radiation

EM radiation is absorbed by other objects.

The efficiency of the transfer depends on:
1. Emission spectrum
2. Absorption spectrum
Basically how much radiation one object is emitting and how readily this emitted radiation can be absorbed by the other object.

Radiation doesn't require a medium (a liquid or gas) like convection or physical contact like conduction and therefore radiation can transmit heat through a vacuum.

Thermal radiation - black body radiation

A black body is an idealised emitter that is in thermal equilibrium with its environment. A black body is a hypothetical body that completely absorbs all radiant energy falling upon it, reaches some equilibrium temperature, and then reemits that energy as quickly as it absorbs it

It absorbs and emits all radiation, regardless of wave length and frequency. Which means that the spectrum (wavelength) is determined by temperature not size or shape. So if we know what wave length something is emitting, we know what the temperature will be. This is planks law. For example, this is how we know what the temperature of the sun is, by measuring the waves it emits.

Emissivity

The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Emissivity is on a scale of 0-1. 1 being a perfect emitter.

Emission rate/heat loss equation =

E/t = eoAT4

emission rate per second = emissivity x stefan botzman constant (which is 5.67 x 10^-8) x surface area of the emitter x temperature cubed

Example question

Calculate heat loss for a cube with a height of 10cm at 27 degrees. E = 1.

Convert 10cm into metres so surface area = 0.1 x 0.1 which equals 0.01m2. Cube has six sides so 0.01 x 6 = 0.06m2.

E/t = 1 x stefan constant x 0.06 x (27+273)4
= 27.6Js-1.

How do we loose heat from our bodies into the environment?

All 3 ways play a role (and 1 extra way) in how we loose heat:

1. Conduction - through things we touch e.g our feet touching the ground

2. Convection - moving air removes heat e.g the part of our skin which is in contact with the outside moving air, looses some energy to the air through conduction. But as the warm energy disappears, the air also brings cool air back to us so we have constant air flowing around us.

3. Radiation - EM emission e.g when we are hot we emit so infrared protons

4. Evaporation e.g sweat. When we sweat or body is transferring our thermal energy into water which is then evaporated. When the water evaporates it carries away the thermal energy.

Factors affecting the thermal energy lost from our body

1. The AREA where the thermal energy is because some areas loose heat more easily
head, armpits, groin

2. TEMPERATURE difference with the environment

3. SURFACE AREA of the skin. Larger sa means heat is lost more efficiently. E.g fingers have a large sa and tend to get cold first.

4. THERMAL CONDUCTIVITY of the tissues

5. THICKNESS of skin and other tissues

power is the rate in change of...

energy.


If energy is moving from one place to another, the power tells us how fast the energy is moving.

units for amount of energy in a certain time

Joules per second

Kcal per hour

Or we can use watts. One watt is equal to 1 joule per second.

power

power = work/time

work = force x distance

metabolic rate

Amount of energy an animal uses in a unit of time; the sum of all the energy-requiring biochemical reactions.


metabolic rate = power (energy consumption rate) divided by SA of the body.
common units: Watts per metres squared or kcal per metres squared

how to estimate the surface area of a person

surface area = 0.202 x mass in kg to the power 0.425 x height in metres to the power 0.725

rate of energy consumption for a certain person doing a certain activity equation

rate of energy consumption = metabolic rate for the activity x surface area of the body

Total energy consumption for a certain person doing a certain activity equation

total energy consumption = metabolic rate for the activity x surface area of the body x time

resting metabolic rate

MET is a ratio of your working metabolic rate relative to your resting metabolic rate. Metabolic rate is the rate of energy expended per unit of time. It's one way to describe the intensity of an exercise or activity.

One MET is the energy you spend sitting at rest — your resting or basal metabolic rate. So, an activity with a MET value of 4 means you're exerting four times the energy than you would if you were sitting still.

MET = metabolic rate of activity/basal metabolic rate

Equation for the food we eat and energy we gain

We digest glucose and oxygen to get co2, water and energy. The amount of energy produced can be worked out from looking at the amount of oxygen we used as the reactant.

1 litre of oxygen produces 4.83kcal of energy.

We can measure how much energy we are using by monitory respiratory intake of oxygen. This process is called indirect calorimetry.

Metabolic ratees for activity are based on oxygen consumption.

80% of the energy we use is converted to...

heat

keeping our body warm

conduction isn't enough to account for all the heat transfer around the blood.

Heat is transferred into the blood where it is then passed all around the body.

how to control our body temp

cooling down:
1. wear less clothes, increase contact with air, more convection, more heat loss
2. fan - increases air flow, more convection, removes thermal energy from skin
3. wearing lighter colours - limits absorption of thermal radiation so we are cooler
4. limit chance of absorbing radiation - sit in shade
5. time of day affects amount of light per unit of area - noon is warmer

heating up:
1. wear more clothes, less contact with air, less convection, less heat loss
2. wearing darker colours - increase absorption of thermal radiation, we heat up

perspiration

the process of sweating

how we loose heat

1. sweating
2. breathing/panting

how we prevent loosing heat

in a cold environment we need to generate more heat to make up for the heat lost - therefore basal metabolic rate increases.



why we feel cold:
1. cold external temperature - loose heat quickly
2. wind speed - large amount of thermal exchange with environment through convection
3. humidity - low water density in surrounding air means we loose more heat through loosing water


how to keep warm:
1. multiple clothing layer - each layer traps air, air is a poor thermal conductor so it can't conduct much thermal energy so but humans can so we can conduct the thermal energy from air which heats us up
2. fur - works same way as clothing layer. traps a layer of air between animals and environment. but also does opposite and keeps animal cool - traps layer of cool air to reduce amount of cool air lost to the environment.
3. shivering - our bodies way of generating our own thermal energy



frostbite
at below 0 degrees our skin and tissues become damaged leading to frost bite.

absorption of EM radiation

EM radiation absorbed and emitted by pretty much everything.

x rays

x ray electromagnetic waves are waves with wavelengths shorter than ultraviolet rays, but longer than gamma rays.

X rays travel well through tissue. Allow us to see deep in the body. X rays are strongly absorbed by bone and metals therefore x rays are good for looking deep into the body at bone structure and metal implants. X rays are not good for looking at the tissues themselves e.g cancer growth. Also when x rays are absorbed by tissue then can damage the tissue and cause mutation of a cell in the tissue and cause cancer - this is why people wear led protection.

Ultraviolet radiation

broken down into 3 regions:

1. UV-A,
highest penetration so longest wave length so lowest frequency so has the lowest energy. 400-315nm. Results in skin tanning. Also used to treat certain skin disorders.

2. UV-B
mid penetration, mid wave length so has mid energy. 315-280nm. Results in skin burning, skin disease, cataracts.

3. UV-C 280-100nm
almost no penetration to skin. Shortest wave length means largest frequency so has the highest energy. Used for killing micro-organisms on surfaces.



summary:
short wave length = high frequency = high penetration
long wave length = lower frequency = less penetration

Ozone layer

Protective layer in atmosphere that shields earth from nearly all the UV radiation. Blocks UV-C and UV-B but not UV-A

Loss of ozone layer

there is widespread concern that the ozone layer is deteriorating due to the release of pollution containing the CFC molecules. Such deterioration allows large amounts of ultraviolet B rays to reach Earth, which can cause skin cancer and cataracts in humans and harm animals as well.

CFC molecules catalyse chain reactions which break up the ozone layers. CFCs are highly prominent in aerosols and fridges. CFCs are compounds made up of chlorine, fluorine and carbon bound together. Because they are extremely stable molecules, CFCs do not react easily with other chemicals in the lower atmosphere. One of the few forces that can break up CFC molecules is ultraviolet radiation.


Antarctica
In polar winter no sunlight reaches the poles. Cold temps allows PCS to form. Increased risk of skin cancer due to increase UV radiation. We absorb more energy into the environment so average temp of earth increases - this contributes to global warming.

How loss of ozone occurs

1. UV radiation causes a chlorine atom to break away from the CFC molecule

2. Free chlorine atom hits an ozone molecule. Ozone molecule is made up of three oxygen atoms.

3. Chlorine pulls an oxygen away from the ozone molecule which forms a stable o2 molecule and chlorine monoxide

4. Another free oxygen from the surroundings will come along and hit the chlorine monoxide molecule and take the oxygen molecule which leaves chlorine.

5. This leads to another free floating chlorine atom

6. Free chlorine will continue to deplete ozone in the stratosphere - because we are back to step 1 where we have a free chlorine, this process will repeat

Energy balance on earth

Life on earth depends on the biology of the surface of the soil. Plants, fungi, algae, insects are all responsible for maintaining life on earth by keeping the soil fertile.



The soil temp is critical and needs to be maintained:
1. soil heating - thermal energy gained by solar radiation - radiation coming from the sun. Gain small amount of heat via conduction from the molten core - centre of earth is very hot and this heat slowly makes its way out to us
2. soil cooling - looses thermal energy through convection with air. Moisture evaporation. Loose heat through radiation.

Atmospheric temp also affects soil temp - hotter climates have warmer soil.

how does a green house work?

1. We receive solar radiation from the sun - this can easily get through the glass.

2. This radiation is absorbed by plants and other objects in the greenhouse

3. This radiation is then re emitted but now at infra red due to lower temp of the objects inside the greenhouse - wave length is longer with lower energy

4. Thermal infra red energy can't easily transmit through the glass - gets blocked inside which increases temp.

greenhouse effect

Natural situation in which heat is retained in Earth's atmosphere by carbon dioxide, methane, water vapor, and other gases.


How this effect has happened:
1. We receive solar radiation from the sun - this can easily get through the atmosphere to reach us.

2. This radiation is absorbed by plants and other objects on earth.

3. This radiation is then re emitted but now at infra red due to lower temp of the objects - wave length is longer with lower energy

4. Thermal infra red energy is blocked by water vapour and co2 in the atmosphere and can't easily transmit through - gets blocked inside and is absorbed by water vapour and co2.

Effects of green house effect

effects:
1. drying of many ecosystems
2. erratic changes in weather patterns
3. changes in ocean circulation
4. melting ice capes


reason for this effects:
1. more fossil fuels being burnt
2. deforestation
3. CFCs
4. oil and petrol engines