Physics 6 PDF - Temperature and Heat

# 12. Temperature and heat - Temperature scales. - Heat expansion of bodies. - Volume expansion. - Thermal properties of water. - Specific heat, calorimetry. - Thermostats, thermoregulators, sterilizers. ## Temperature and temperature scale - There are three temperature scales in use today, Fahrenh...

# 12. Temperature and heat - Temperature scales. - Heat expansion of bodies. - Volume expansion. - Thermal properties of water. - Specific heat, calorimetry. - Thermostats, thermoregulators, sterilizers. ## Temperature and temperature scale - There are three temperature scales in use today, Fahrenheit, Celsius and Kelvin. - Fahrenheit temperature scale is a scale based on 32 for the freezing point of water and 212 for the boiling point of water. - Celsius temperature scale also called centigrade temperature scale, is the scale based on 0 for the freezing point of water and 100 for the boiling point of water. - Kelvin temperature scale is the base unit of thermodynamic temperature measurement in the International System (SI) of measurement. It is defined as 1/273.16 of the triple point of pure water. ## Temperature scales - **Celsius**: The point at which ice melts at 0°C and water boils at 100°C (assuming standard pressure). - **Fahrenheit**: - Ice melts at 32°F, water boils at 212°F, 180°F apart. - 9/5 F= 1 C. The intersection between F and C is 20°. - **Kelvin**: 0 Kelvin = -273°C and is absolute zero, the coldest possible. ## Heat expansion of bodies - Thermal expansion is the tendency of matter to change in shape, area, and volume in response to a change in temperature. - **Temperature → molecular kinetic energy of a substance** - When a substance is heated, the kinetic energy of its molecules increases → the molecules begin vibrating/moving more and usually maintain a greater average separation. ## Specific heat, calorimeter - Calorimetry is the science determining the changes in energy of a system by measuring the heat exchanged with the surroundings. - Calorimeters are designed to be well-insulated, so no heat is gained from or lost to the surroundings. - No heating element is used to introduce heat in the system = the total heat transferred (q) for the entire calorimeter system equals zero. - 0 = q = qcu + qcol + qw (between copper, water) # Water, properties and importance for living organism - Water (volume expansion coefficient in the liquid state is 2.1, and 0.5 in the solid state) - boiling point -100°C (212°F) to 4°C (39.2°F) → steadily contract. - Normally a substance continues to become denser as it turns from liquid to solid, but this does not occur with water. - That's the reason why ice floats, making it possible for fish and other animals to survive the winter and thus life is possible. ## Thermostat, thermoregulators, sterilizer - **Core temperature** is regulated by the hypothalamus (in the brain). - Temperature figures are only true if they come from a sterilized room. # 13. Thermal exchange, laws. - Thermal exchange by thermal conductivity, convection, radiation and adsorption. - Kirchoff's law of thermal radiation. Stefan-Boltzmann law, Wien's law. ## Thermal exchange by thermal conductivity, Convection, radiation and absorption - **Thermal conductivity** (k, λ, or κ) is the property of a material to conduct heat. Higher thermal conductivity means higher heat transfer. Expressed in kelvin-meters per watt. - **Convection**: Transfer of heat from one place to another by the movement of fluids usually the dominant form of heat transfer in liquids and gases. - Can be natural or be forced eg the draft of a chimney or the water pump of a car. - Energy transfer due to specific molecular motion (diffusion) - Also energy is transferred by motion of the fluid (motion because a large number of fluid are moving) - Such motion (+ temperature gradient) adds to heat transfer - **Heat transfer through radiation** takes place in form of electromagnetic waves (called black body) - Radiation emitted by a body is a consequence of thermal agitation of its composing molecules. - The radiation energy per unit time is proportional to the fourth power of the absolute temperature and can be expressed with **Stefan-Boltzmann Law** as: - q = σ² A - (q = heat transfer per unit time (W); σ = 5.6703 10-8 (W/m²K4) - The Stefan-Boltzmann Constant; - T = absolute temperature in Kelvin; (K)A = area of the emitting body (m²) - The total energy I, total, irradiated from unit surface area of a black body per unit time (also known as emitting power) is proportional to the fourth power of the black body temperature T: 1 = σΤ^4 ## Kirchhoff's law of thermal radiation - Kirchhoff's law of thermal radiation refers to **wavelength-specific radiative emission + absorption by a material body in thermodynamic equilibrium**. - Kirchhoff's law basically states that a body at temperature T radiates electromagnetic energy. - A perfect black body in thermodynamic equilibrium absorbs all light that strikes it + radiates energy according to a unique law of radiative emissive power for temperature T (universal for all perfect black bodies). ## Wien's Law - **Wien's Law** describes a spectrum of thermal radiation (frequently called the blackbody function). - The equation does accurately describe the short wavelength spectrum of thermal emission from objects (not long wavelength though!). - Radiative energy dW per wavelength interval da has a maximum at a certain wavelength m → the maximum shifts to shorter wavelengths as the temperature T is increased - The product A㎞T is an absolute constant: AmT = 0.2898 centimetre-degree Kelvin. - **Heat absorption** is an endothermic process described by the second law of thermodynamics. - Thermal energy transitions from a hotter mass to a cooler mass → reach thermodynamic equilibrium. - Both objects reach thermodynamic equilibrium/same temperatures. - The heat transfer between the two objects will equal zero. ## Thermal exchange between human body and surrounding medium - The rates of convection and evaporation at the interface between the human body and the surrounding air are expressed by the parameters convective heat transfer coefficient hc, in W 2degrees C-1 and evaporative heat transfer coefficient h(e), W-2 hPa-1. - Basically we are warm blooded and need to keep our body temperature up. These are the parameters for the heats we lose. Our body will replace this heat though. ## Hypothermy - Can happen because of environment or disease. - More warmth is discharged than produced. - Can lead to frostbite or death. ## Hyperthermy - Overheating of the body. - In opposite of fever not because of pyrogenes (fever reducing medicament don't work). ## Cyrosurgery - The use of extrem cold to in surgery to destroy abnormal or diseased tissue (eg warts) # 14. Electric field - Intensity and potential of point charge and electric dipole in vacuum. - Electric field in material medium. - Orientational and induced polarization. - Electric capacity of a conductor and a flat capacitor. - **A capacitor** is a device that stores electric charge and consists of two conducting objects placed near each other but not touching. Charge Q = CV (C=constant of proportionality, capacitance, V=potential difference between them). The unit of capacitance is coulombs per volt and is called Farad (f). - **Every electrically charged particle creates its own energy field, E that determines the interaction F, between them.** F = q. E where q is the magnitude of the charge. - **Two equal point charges Q, of opposite sign by a distance I, are called electric dipole.** ## Coulomb's law states - **Electric force between two charges is directly proportional to the product of their charges and inversely proportional to the distance between their centers.** - When applied to our two charges - the source charge (also called point charge) (Q) and the test charge (q) (which is used to find the intensity at one point) - the formula for electric force can be written as: - F = k•q.Q / d² - Where: - k = 9.0•10º N•m²/C2 - d = separation distance between charges (meters) - If the expression for electric force as given by Coulomb's law is substituted for force with E =F/q equation (E= electrical field strength, F=electric force, q=quantity of charge of the source charge), a new equation can be derived as shown below. - E = F / q = k•q.Q / d² / q = k.Q / d² - E= kQ / d² - The electric field strength is dependent upon the quantity of charge on the source charge (Q) and the distance of separation (d) from the source charge. ## Electric field in material medium - **The electric displacement field is a vector field that appears in Maxwell's equations.** - It accounts for the effects of free and bound charge within materials. - Units of coulomb per metre squared (C-m-2). - **Light can be polarized.** In the case of shaking a rope in a horizontal plane the movement is said to be linearly polarized or plane-polarized. Polarization exist only for transverse waves and not for longitudinal waves such as sounds waves. - **Light can also be un polarized which means the source has oscillations in many planes at once.** A light bulb emits un polarized light and so does the Sun. - **Plane-polarized light can be obtained from un polarized light using certain crystals such as tourmaline.** More commonly used today are Polaroid sheets. It can be used to obtain plane-polarized light from un polarized light. - **In a good conductor the electrons are bound very loosely and can move very freely within the material and are often referred to as free electrons or conduction electrons.** - **An example of the flow of electric charge is the human nervous system that gives us mean for being aware of the world, of communication within the body and for controlling the body's muscles.** - The direction of polarization itself rotates. - 1011 Hz is the frequency a microwave works on. ## Dielectric constant - **Dielectric material** (dielectric for short) **is an electrical insulator that can be polarized by an applied electric field.** - **When a dielectric is placed in an electric field, electric charges do not flow through the material as they do in a conductor.** - **Only slightly shift from their average equilibrium positions causing dielectric polarization because of that positive charges are displaced toward the field and negative charges shift in the opposite direction.** ## Electric capacity - **Ability of a body to store an electric charge.** - The SI unit of capacitance is the farad (symbol: F). - For many dielectric materials the capacitance is independent of the potential difference between the conductors and the total charge on them. # 15. Electrical conductivity - Electrical conductivity of solids and liquids. - Electrolytic dissociation. - Solvation (hydration) and recombination of ions. - Degree of electrolytic dissociation. - Conductivity of electrolytes. - Faraday's law. - Direct and alternating electric current. - **When two terminals are immersed in liquid there will be an electric current between them.** Depending on the concentration of ions there will be stronger or weaker conductivity. - **Solids that can conduct charges are called conductors for example metals, and those who can't are called nonconductors or insulators.** There is also an intermediate type known as semiconductors. - **In insulators the electrons are bound very tightly to the nuclei.** In a good conductor some of the electrons are bound very loosely and can move freely within the material. They are referred to as free electrons or conducting electrons. ## Electrical conductivity of solids and liquids - **Electrical conductivity measures a material's ability to conduct an electric current.** - Commonly represented by the Greek letter σ (sigma) - SI unit is siemens per metre (S/m) and CGSE unit is reciprocal second (s¯¹) - **Solid materials can be classified into two main categories:** - 1. Electrical conductors that can be crossed by an electric current - 2. Insulators that don't conduct electric current - Classified in terms of their resistivity or conductivity as conductors, insulators, or semiconductors. - The electron energy levels in a solid are often expressed in relation to the Fermi energy. - Conducting electron density in a metal can be calculated from the Fermi energy. - **Electrical conduction is solids is the drift of free electrons in solids.** - **In liquid electricity is conducted by the migration of positive and negative ions through liquid.** - When two terminals (or electrodes) are put in liquid and a source of electricity is connected to them, there will be a current through the liquid. - An electrical field is established between the positives terminal (anode) and the negative terminal (cathode) and ions drift through the liquid. ## Electrolytic association and Conductivity of electrolytes - **Conductivity of an electrolyte solution is a measure of its ability to conduct electricity.** - SI unit of conductivity is siemens per meter (S/m) - Ion association is a chemical reaction. - Ions of opposite electrical charge come together in solution to form a distinct chemical entity. - **Associated as ion pairs, ion triplets, etc.** - Ion pairs are classified according to the nature of the interaction as contact, solvent-shared or solvent-separated. - Most important factor to determine the extent of ion association is the dielectric constant of the solvent. ## Solvation (hydration) and recombination of ions - **Solvation is the process in which molecules of a solvent attract the particles of a solute.** - Electron-ion recombination proceeds in several different modes. - 1. three-body recombination (TBR) - 2. one-step radiative recombination (RR), all to the ground-and singly-excited states of the target ions. - 3. the indirect resonant mode is a two-step dielectronic recombination (DR) ## Degree of electrolytic dissociation - **Dissociation the breaking up of a compound into simpler constituents that are usually capable of recombining under other conditions.** - **In electrolytic (or ionic) dissociation the addition of a solvent or of energy in the form of heat causes molecules or crystals of the substance to break up into ions (electrically charged particles).** - The idea of ionic dissociation is used to explain electrical conductivity and many other properties of electrolytic solutions. # Faraday's law ε = - Ν Δφα/Δt - The more rapidly the magnetic field changes, the greater the induced emf in a loop of wire. - The emf depends also on the loop's area and angle. - **Emf is proportional to the rate of change of the magnetic flux through the loop.** - **Magnetic flux for uniform magnetic field through a loop area A is defined as Q = B1A = BAcos=** - B1 = component of magnetic field B perpendicular to the face of the loop. - **The induced emf of this instant is e = ∆Q/At**. - **Faraday's law of induction is a basic law of electromagnetism predicting how a magnetic field will interact with an electric circuit to produce an electromotive force (EMF)→ called electromagnetic induction.** - **It is the fundamental operating principle of transformers, inductors, and many types of electrical motors, generators and solenoids.** ## Faraday's Law of incuction - **E = - N(A/B/At** ## Direct and alternating electric current - AC/DC the Australian rockband is named after these currents. - **In direct currents energetic flow is in one direction, in alternating current it flows in both directions.** - **An alternator can be used to generate an electric current; a wire is spun in a magnetic field to produce the alternations.** - **Direct current has a constant voltage.** #16. Magnetic field - Magnetic field of a permanent magnet. - Magnetic field of direct current. - Ampere's law-magnetic induction. - Magnetic field of electric current in circular conductor, solenoid. - Charge motion in magnetic and electric fields - Lorentz force. - Mass spectrometry. ## Magnetic field of a permanent magnet - Permanent magnet should produce a high magnetic field with a low mass. - Should be stable against the influences which would demagnetize it. - Desirable properties of such magnets are typically stated in terms of the remanence and coercivity of the magnet materials. - **The magnetic field B (measured in tesla) of an ideal dipole is measured along the axis** - B-axis = uo/4π x 2u/d^3 - Uo = permeability constant, u = magnetic moment, - d = distance from the center of bipole in meters. ## Magnetic field of direct current - DC magnetic field emanates from the earth naturally (that is what allows a compass to find magnetic North) ## Ampere's law- magnetic induction - **The magnetic field in space around an electric current is proportional to the electric current which serves as its source**. - **Ampere's Law states that for any closed loop path, the sum of the length elements times the magnetic field in the direction of the length element is equal to the permeability times the electric current enclosed in the loop.** ## Magnetic field of electric current in circular conductor, solenoid - **The magnetic field lines around a long wire which carries an electric current form concentric circles around the wire**. - **The direction of the magnetic field is perpendicular to the wire**. - **A long straight coil of wire can be used to generate a nearly uniform magnetic field similar to that of a bar magnet**. - Coils, called solenoids, have an enormous number of practical applications. - **The field can be greatly strengthened by the addition of an iron core (typical in electromagnets)**.