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Thermal Physics

States of matter Solids - Solids have a definite shape and a definite volume - Solids cannot flow and are not compre­ssible Liquids - Liquids have no definite shape but do have a definite volume - Liquids are able to flow to take the shape of a container but they are not compre­ssible Gases - Gases have no definite shape and no fixed volume - Gases can flow to take the shape of their container and are highly compre­ssible
Thermal Equili­brium - As an object absorbs thermal radiation it will become hotter - As it gets hotter it will also emit more thermal radiation - The temper­ature of a body increases when the body absorbs radiation faster than it emits radiation - Eventu­ally, an object will reach a point of constant temper­ature where it is absorbing radiation at the same rate as it is emitting radiation - At this point, the object will be in thermal equili­brium An object will remain at a constant temper­ature if it absorbs heat at the same rate as it loses heat
Changes of State When a substance changes state, the number of molecules in that substance doesn't change and so neither does its mass - The only thing that changes is its energy - Changes of state are physical changes and so they are reversible Melting & Freezing - Melting occurs when a solid turns into a liquid (e.g. ice to water) - Freezing occurs when a liquid turns into a solid Boiling & Condensing - Boiling occurs when a liquid turns into a gas - This is also called evapor­ating - Condensing occurs when a gas turns into a liquid
The greenhouse effect. - If the Earth had no atmosp­here, the temper­ature on the surface would drop to about −180 °C at night, the same as the Moon’s surface at night - This would happen because the surface would be emitting all the radiation from the Sun into space - The temper­ature of the Earth is affected by factors contro­lling the balance between incoming radiation and radiation emitted - The Earth receives the majority of its heat in the form of thermal radiation from the Sun - At the same time, the Earth emits its own thermal radiation, with a slightly longer wavelength than the thermal radiation it receives (the surface temper­ature of the Earth is signif­icantly smaller than the surface temper­ature of the Sun) - Some gases in the atmosp­here, such as water vapour, methane, and carbon dioxide (green­house gases) absorb and reflect back longer­-wa­vel­ength infrared radiation from the Earth and prevent it from escaping into space - These gases absorb the radiation and then emit it back to the surface - This process makes the Earth warmer than it would be if these gases were not in its atmosphere
Thermal Conduction in Solids - Conduction is the main method of thermal energy transfer in solids - Conduction occurs when: Two solids of different temper­atures come in contact with one another, thermal energy is transf­erred from the hotter object to the cooler object - Metals are the best thermal conductors - This is because they have a high number of free electrons. Conduc­tion: the atoms in a solid vibrate and bump into each other - Conduction can occur through two mechan­isms: - Atomic vibrations - Free electron collisions - When a substance is heated, the atoms, or ions, start to move around (vibrate) more - The atoms at the hotter end of the solid will vibrate more than the atoms at the cooler end - As they do so they bump into each other, transf­erring energy from atom to atom - These collisions transfer internal energy until thermal equili­brium is achieved throughout the substance - This occurs in all solids, metals and non-metals alike
Thermal Expansion - When materials are heated, they expand - This expansion happens because the molecules start to move around (or vibrate) faster, which causes them to knock into each other and push each other apart - Thermal expansion occurs in solids, liquids and gases - When temper­ature is increased (at constant pressure); - Solids will tend to expand the least - Gases expand the most - Liquids fall in between the two - Molecules do not expand, but the space in between them does - When solids, liquids and gases are heated:
Thermal Conduction in Liquids & Gases - For thermal conduction to occur the particles need to be close together so that when they vibrate the vibrations are passed along - This does not happen easily in fluids - In liquids particles are close, but slide past each other - In gases particles are widely spread apart and will not 'nudge' each other - Both types of fluid, liquids and gases, are poor conductors of heat
Relative Thermal Conduc­tivity - Conductors tend to be metals - Better thermal conductors are those with deloca­lised electrons which can easily transfer energy - This means that there is a wide range of thermal conduc­tivity
Convection - Convection is the main way that heat travels through liquids and gases - Convec­tion only occurs in fluids - Convec­tion cannot happen in solids
Density & Convection Descri­ptions of convection currents always need to refer to changes in temper­ature causing changes in density - The temper­ature may fall or rise, both can create a convection current - When a liquid (or gas) is heated (for example by a radiator near the floor): - The molecules push each other apart, making the liquid/gas expand - This makes the hot liquid­/gas less dense than the surrou­ndings - The hot liquid/gas rises, and the cooler (surro­unding) liquid/gas moves in to take its place - Eventually the hot liquid/gas cools, contracts and sinks back down again - The resulting motion is called a convection current. When a liquid or gas is heated, it becomes less dense and rises - When a liquid (or gas) is cooled (for example by an A.C. unit high up on a wall): - The molecules move together, making the liquid/gas contract - This makes the hot liquid­/gas more dense than the surrou­ndings - The cold liquid/gas falls, so that warmer liquid or gas can move into the space created - The warmer liquid or gas gets cooled and also contracts and falls down - The resulting motion is called a convection current
Thermal Radiation - All objects give off thermal radiation - The hotter an object is, the more thermal radiation it emits - Thermal radiation is the part of the electr­oma­gnetic spectrum called infrared - Thermal radiation is the only way in which heat can travel through a vacuum - It is the way in which heat reaches us from the Sun through the vacuum of space - The colour of an object affects how good it is at emitting and absorbing thermal radiation:

Electr­oma­gnetism

Electr­omotive force and potential difference - The Electr­omotive Force (e.m.f.) is the name given to the potential difference of the power source in a circuit - It is defined as The electrical work done by a source in moving a unit charge around a complete circuit - The Electr­omotive Force (EMF) is measured in volts (V)
Altern­ating Current - An altern­ating current (a.c.) is defined as A current that contin­uously changes its direction, going back and forth around a circuit - An a.c. power supply has two identical terminals that switches between positive and negative - The current is therefore defined as positive or negative, depending on which direction it is flowing at that time - The frequency of an altern­ating current is the number of times the current changes direction back and forth each second - In the UK, mains electr­icity is an altern­ating current with a frequency of 50 Hz and a potential difference of around 230 V
The EMF is the voltage supplied by a power supply: 12 V in the above case - The definition of e.m.f. can also be expressed using an equation - Where - E = electr­omotive force (e.m.f.) (V) - W = energy supplied to the charges from the power source (J) - Q = charge on each charge carrier (C)
Mains Electr­icity - Mains electr­icity is the electr­icity generated by power stations and transp­orted around the country through the National Grid - Everyone connects to the mains when plugging in an appliance such as a phone charger or kettle - Mains electr­icity is an altern­ating current (a.c.) supply - In the UK, the domestic electr­icity supply has a frequency of 50 Hz and a potential difference of about 230 V - A frequency of 50 Hz means the direction of the current changes back and forth 50 times every second - Mains electr­icity, being an altern­ating current, does not have positive and negative sides to the power source - The equivalent to positive and negative are called live and neutral and these form either end of the electrical circuit
Electric Power - Power is defined as The rate of energy transfer or the amount of energy transf­erred per second - The power of a device depends on: - The voltage (pote­ntial differ­ence) of the device - The current of the device - The power of an electrical component (or appliance) is given by the equation: P = IV
Induced EMF - An EMF will be induced in a conductor if there is relative movement between the conductor and the magnetic field - It will also be induced if the conductor is stationary in a changing magnetic field - For an electrical conductor moving in a fixed magnetic field - The conductor (e.g wire) cuts through the fields lines - This induces an EMF in the wire
- Work is done when charge flows through a circuit - Work done is equal to the energy transf­erred - The amount of energy transf­erred by electrical work in a component (or appliance) depends upon: - The current, I - The potential difference, V - The amount of time the component is used for, t - When charge flows through a resistor, for example, the energy transf­erred is what makes the resistor hot - The energy transf­erred can be calculated using the equation: E = P × t - Where: - E = energy transf­erred in joules (J) - P = power in watts (W) - t = time in seconds (s) - Since P = IV, this equation can also be written as: E = I × V × t
Lenz’s Law The direction of an induced potential difference always opposes the change that produces it. - This means that any magnetic field created by the potential difference will act so that it tries to stop the wire or magnet from moving
Direct Current - A direct current (d.c.) is defined as A current that is steady, constantly flowing in the same direction in a circuit, from positive to negative - The potential difference across a cell in a d.c. circuit travels in one direction only - This means the current is only positive or only negative - A d.c. power supply has a fixed positive terminal and a fixed negative terminal - Electric cells, or batteries, produce direct current (d.c.)
Demons­trating Lenz's Law - If a magnet is pushed north end first into a coil of wire then the end of the coil closest to the magnet will become a north pole - Explan­ation - Due to the generator effect, a potential difference will be induced in the coil - The induced potential difference always opposes the change that produces it - The coil will apply a force to oppose the magnet being pushed into the coil - Therefore, the end of the coil closest to the magnet will become a north pole - This means it will repel the north pole of the magnet

Nuclear and Atomic Physics

- Atoms are the smallest particles in the world and they cannot be broken down into smaller parts since they are the smallest. - in Nuclear Physics there are 3 particles: - Alpha - Beta - Gamma
Gamma These are the most penetr­ating and are stopped only by many centim­etres of lead. They ionise a gas even less than beta particles and are not deflected by electric and magnetic fields.
Ionizing effect of radiation - A charged electr­oscope discharges when a lighted match or a radium source (held in forceps) is brought near the cap and this causes radiation
Half Life The rate of Decay is unaffected by temper­ature but every radioa­ctive element has its own definitive decay rate, expressed by it’s half life. This is the average time for half the atoms in a given sample to decay.
Alpha These are stopped by a thick sheet of paper and have a range in air of only few centim­etres since they cause ionisation in a gas due to frequent collisions with gas molecules.
Background Radiation A type of radiation that is being recieved from the surrou­ndings like e.g. Underg­round, In the sky etc.
Beta These are stopped by a few millim­etres of aluminium and some have a range in air of several metres and their ionizing power is much less than that of alpha particles. As well as being easily deflected by electric fields, they are more easily deflected by magnetic fields.

General Physics

 
Momentum - An object with that is in motion has momentum which is defined by the equation (Momentum = mass x velocity).
Kinetic energy - The kinetic energy, E, of an object (also known as its kinetic store) is defined as: K The energy an object has as a result of its mass and speed - This means that any object in motion has energy in its kinetic energy store - Kinetic energy can be calculated using the equation: EK = ½ × m × v2
Collisions The total momentum before a collision = The total momentum after a collision - Before the collision: - The momentum is only of mass m which is moving - If the right is taken as the positive direction, the total momentum of the system is m × u - After the collision: - Mass M also now has momentum - The velocity of m is now -v (since it is now travelling to the left) and the velocity of M is V - The total momentum is now the momentum of M + momentum of m - This is (M × V) + (m × -v) or (M × V) – (m × v)
Gravit­ational potential energy - The gravit­ational potential energy, E, of an object (also known as its gravit­ational store) is defined as: P The energy an object has due to its height in a gravit­ational field - This means: - If an object is lifted up, energy will be transf­erred to its gravit­ational store - If an object falls,­ energy will be transf­erred away from its gravit­ational store - The GPE of an object can be calculated using the equation: ΔEP = mgΔh
Impulse - When a resultant (unbal­anced) force acts on a mass, the momentum of that mass will change - The impulse of a force is equal to that force multiplied by the time for which it acts: impulse  =  force × change in time impulse = FΔt - The change in momentum of a mass is equal to the impulse provided by the force: impulse = change in momentum impulse = FΔt  = Δp - Change in momentum can also be described as: Δp = Δ(mv) Δp = mv − mu - Where: - m = mass in kg - v = final velocity in m/s - u = initial velocity in m/s - Therefore: impulse = FΔt  = Δp = mv − mu
Work done - Work is done when an object is moved over a distance by a force applied in the direction of its displa­cement - It is said that the force does work on the object - If a force is applied to an object but doesn’t result in any movement, no work is done Work is done when a force is used to move an object - The formula for work done is: Work done = force × distance W = fd
Energy - Energy is a property that must be transf­erred to an object in order to perform work on or heat up that object - It is measured in units of Joules (J) - Energy will often be described as part of an energy system - In physics, a system is defined as: An object or group of objects - Therefore, when describing the changes within a system, only the objects or group of objects and the surrou­ndings need to be considered - Energy can be stored in different ways, and there are changes in the way it is stored when a system changes - The principle of conser­vation of energy states that: Energy cannot be created or destroyed, it can only be transf­erred from one store to another - This means that for a closed system, the total amount of energy is constant
Efficiency of energy transfer - The efficiency of a system is a measure of how well energy is transf­erred in a system - Efficiency is defined as: The ratio of the useful power or energy transfer output from a system to its total power or energy transfer input - If a system has high effic­iency, this means most of the energy transf­erred is useful - If a system has low effic­iency, this means most of the energy transf­erred is wasted
Pressure - Pressure is defined as The concen­tra­tion of a force or the force per unit area - For example, when a drawing pin is pushed downwards: - It is pushed into the surface, rather than up towards the finger - This is because the sharp point is more concen­trated (a small area) creating a larger pressure
Liquid Pressure - A fluid is either a liquid or a gas When an object is immersed in a fluid, the fluid will exert pressure, squeezing the object - This pressure is exerted evenly across the whole surface of the fluid and in all directions - The pressure exerted on objects in fluids creates forces against surfaces - These forces act at 90 degrees (at right angles) to the surface The pressure of a fluid on an object creates a force normal (at right angles) to the surface - The pressure of a fluid on an object will increase with: - Depth within the fluid - Increased density of the fluid ### Calcul­ating Pressure in Liquids - The pressure is more accurately the difference in pressure at different depths h in a liquid, since the pressure changes with the depth - The pressure due to a column of liquid can be calculated using the equation Δp = ρgΔh or in simple words —> $Rho Gravity Height$
revision notes

Properties of Waves

- Waves transfer energy and inform­ation - Waves are described as oscill­ations or vibrations about a fixed point - For example, ripples cause particles of water to oscillate up and down - Sound waves cause particles of air to vibrate back and forth - In all cases, waves transfer energy without trans­fer­ring matter - For water waves, this means it is the wave and not the water (the matter) itself that travels - For sound waves, this means it is the wave and not the air molecules (the matter) itself that travels - Objects floating on water provide evidence that waves only transfer energy and not matter
Frequency - Frequency is defined as: The number of waves passing a point in a second - Frequency is given the sy­mbol f and is measured in Hertz (Hz)
Types of Waves - Transverse - e.g. vibrations of guitar string. - Longit­udinal - e.g. Tsunami waves.
Wave Speed - Wave speed is the speed at which energy is transf­erred through a medium - Wave speed is defined as: The distance travelled by a wave each second - Wave speed is given the symbol, ν, and is measured in metres per second (m/s), it can be calculated using: $wave speed = frequency × wavele­ngth$
Features of a wave - When describing wave motion, there are several terms which are important to know, including: - Crest (Peak) - Trough - Amplitude - Wavelength - Frequency - Wave speed - Wavefront
Wavefront - Wavefronts are a useful way of picturing waves from above: each wavefront is used to represent a single wave - The image below illust­rates how wavefronts are visual­ised: - The arrow shows the direction the wave is moving and is sometimes called a ray - The space between each wavefront represents the wavelength - When the wavefronts are close together, this represents a wave with a short wavel­ength - When the wavefronts are far apart, this represents a wave with a long wavel­ength - Wave speed is defined as: The distance travelled by a wave each second - Wave speed is given the symbol ν and is measured in metres per second (m/s) - Wave speed is the speed at which energy is transf­erred through a medium - Transverse and longit­udinal waves both obey the wave equation: V = f x lambda
Crests & Troughs - A crest, or a peak, is defined as: The highest point on a wave above the equili­brium, or rest, position - A trough is defined as The lowest point on a wave below the equili­brium, or rest, position**
- Where: - v = wave speed in metres per second (m/s) - f = frequency in Hertz (Hz) - λ = wavelength in metres (m)
Amplitude - Amplitude is defined as: The distance from the undist­urbed position to the peak or trough of a wave - It is given the symbol A and is measured in metres (m) - Amplitude is the maximum or minimum displa­cement from the undist­urbed position
Transverse Waves - Transverse waves are defined as: Waves where the points along its length vibrate at 90 degrees to the direction of energy transfer
Wavelength - Wavelength is defined as: The distance from one point on the wave to the same point on the next wave - In a transverse wave: - The wavelength can be measured from one peak to the next peak - In a longit­udinal wave - The wavelength can be measured from the centre of one compre­ssion to the centre of the next - The wavelength is given the symbol λ (lambda) and is measured in metres (m) - The distance along a wave is typically put on the x-axis of a wave diagram
Longit­udinal Waves - Longit­udinal waves are defined as: Waves where the points along its length vibrate parallel to the direction of energy transfer
   
 

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  • Published: 9th March, 2023

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