Physics Final Exam UNIT 5 PDF

UNIT 5 - Solids, Liquids and Gases Density Density is defined as the mass per unit volume of a material. Objects made from low density materials have a lower mass while objects made from high density materials have a higher mass. Equation for density Gases are less dense than...

UNIT 5 - Solids, Liquids and Gases Density Density is defined as the mass per unit volume of a material. Objects made from low density materials have a lower mass while objects made from high density materials have a higher mass. Equation for density Gases are less dense than solids because the particles are more spread out.  The volume of a shape may not always be given but it can be calculated using the following equations. CORE PRACTICAL:DETERMINING DENSITY Types of objects include: Regular shapes like a sphere ball Irregular shapes like a rock Liquids like water steps for finding the density of regular shaped objects: Find the mass of the object using a digital balance. Take multiple readings for the mass and any dimensions obtained for volume measured. Find the average for all readings obtained as this improves the accuracy and reliability of the results Use the density equation to calculate the density of the object. Measure the diameters of the ball bearings with a vernier calliper to find the dimensions of the shape. Calculate the volume of the object then use the density formula to find the density of the object. Steps for finding the density of irregular shaped objects. Find the mass of the object by placing it on a digital balance. Fill up the eureka can with water to a point below the spout (tube projecting from container) Place an empty measuring cylinder below the spout. Carefully lower the object into the eureka can. Measure the volume of displaced water. Repeat the measurements and calculate the average for accurate results. Calculate the density. Apparatus needed: Independent variable = Volume of water added Dependent variable = Mass of cylinder Possible errors: systematic errors - Ensure that the digital balance is set to zero before using it. Random errors - incorrect measurements of length. Take repeated readings to calculate Place the object into the eureka can slowly as placing it from a height can displace more water by splashing and will give incorrect results. Pressure Pressure is defined as the concentration of force or the force per unit area. For example when a drawing pin is pushed downwards, it is pushed into the surface and not the finger because the sharp point is more concentrated and has a smaller surface area. Pressure can be calculated using the equation:  If a force is applied to a large surface area, it will result in a smaller amount of pressure. If a force is applied to a small surface area, it will result in a larger amount of pressure. Pressure in liquids A fluid is a gas or liquid. When an object is immersed in a fluid, the fluid will create pressure, squeezing the object. This pressure is exerted evenly across the whole surface and in all directions. The pressure exerted on objects in fluids creates forces against surfaces. These forces act at 90 degrees to the surface. Pressure in liquids can be calculated using the equation: p= h x p x g p= pressure (Pa) h= height of the column in metres ρ= density of the liquid in (kg/m^3) g= gravitational field strength on earth in (N/kg) The pressure is more accurately the difference at different depths h in a liquid since the pressure changes with depth.  kPa to Pa (multiply by 1000) Solids, LIquids and Gases Matter can exist in solids, liquids and gases. In solids, the particles are closely packed together and they vibrate in fixed positions. They have a definite shape (they are rigid) and a definite volume. In liquids, the particles are closely packed with spaces in between. They can flow over one another and they have no definite shape (take up the shape of the container) and a definite volume. In gases, the particles are far apart and they have a random movement. Gases have no definite shape and no definite volume. Gases are highly compressible because there are large gaps between the particles. It is easier to push the particles together than in liquids and solids. Changes of state When a substance changes its state, the number of particles or mass does not change. The only thing that changes is its energy. Unlike chemical changes, physical changes are reversible. There are 6 changes of state. Melting - when a solid turns into a liquid. Freezing - when a liquid turns into a solid. evaporating - when a liquid turns into a gas. Condensing - when a gas turns into a liquid. Subliming - when a solid turns into a gas. Deposition - when a gas turns into a solid. Heating a system will change the energy stored in the system as it increases its kinetic energy. The temperature of the material is related to the average kinetic energy of the molecules.  This increase in kinetic energy can cause the temperature of the system to increase or cause a change in the state of the system. The higher the temperature, the higher the kinetic energy of the particles. Core practical - investigating the changes of state The aim of this experiment is to investigate the temperature of an ice cube as it changes states. Equipment needed: Thermometer Ice cubes Beaker Tripod and gauze Bunsen burner Stopwatch Method: Place the ice cubes in the beaker (it should be about half full) Place the thermometer in the beaker Place the beaker on the tripod and gauze and slowly start to heat it using the bunsen burner As the beaker is heated, take regular temperature measurements (e.g. at one minute intervals) Continue this whilst the substance changes state (from solid to liquid) Plot a graph of the temperature (y-axis) against time (x-axis) The graph will show regions where the temperature increases and where there is no temperature change. Systematic errors: measurements from the thermometer should be kept at eye level for accurate measurements. Random errors: ensure there are enough ice cubes to surround the thermometer and only begin when the temperature is below 0 degrees. Safety considerations: wear goggles, place the bunsen burner on a heat proof mat to avoid damaging the surface. Specific heat capacity How much the energy of a substance increases depends on the mass of the substance, the material and the amount of energy put into the system. The specific heat capacity of a substance is defined as the amount of energy required to raise the temperature of 1kg of a substance by 1°C. If a substance has a low heat capacity, it heats up and cools down quickly. If a substance has a high specific heat capacity, it heats up and cools down slowly. The amount of energy needed to raise the temperature of a given mass can be calculated using the equation: ΔQ = mcΔT  Change in thermal energy = Mass × Specific heat capacity × Change in temperature ΔQ = change in thermal energy, in joules (J) m = mass, in kilograms (kg) c = specific heat capacity, in joules per kilogram per degree Celsius (J/kg °C) ΔT = change in temperature, in degrees Celsius (°C) Core practical:investigating specific heat capacity The aim of this experiment is to determine the heat capacity of a liquid and a solid by measuring energy required to increase the temperature by one degree. Method: Place the beaker on a digital balance and press ‘tare’ Add approximately 250ml of water in the beaker and measure the mass.  Place the immersion heater and thermometer in the water beaker. Connect a circuit to the power supply, immersion heater and an ammeter and voltmeter. Record the initial temperature at time 0s. Turn on the power supply to 10V and start the stopwatch. Record the voltage from the voltmeter and current from the ammeter. Record the temperature, voltage and current every 60 secs for 10 mins. Repeat the steps replacing the beaker of water with your aluminium steel block. Analysis of results Calculate the energy supplied every 60 secs using the formula: Electrical energy = voltage x current x time Calculate the temperature change by subtracting the temperature at 0s from the temperature recorded each minute. Plot a graph of the energy supplied and the temperature change. Calculate the gradient of the graph. Systematic errors: Ensure the digital balance is set to 0 before taking measurements of the mass. Calculate an average mass of the water as some of it may evaporate during the experiment. Random errors: Stir the water frequently to ensure that the temperature is the same in the fluid. Kinetic theory of gases Molecules in gases are constantly moving at high speed in random directions. Random motions means that the particles are travelling in no specific path and change direction when they collide either with the walls of the container that they are in or with other particles. The random motion of particles in a fluid is known as brownian motion. Brownian motion is evidence for the existence of small particles as large particles of pollen and smoke can be observed in the air moving at random directions. This is because the small particles that are invisible to the naked eye are colliding with the larger particles causing them to change their direction. Pressure = Force/Area Since the particles collide with the walls of their container, the collisions produce a net force at right angles to the wall of the gas container. A gas at higher pressure will produce more frequent collisions. Hence the higher force, higher pressure per unit area will be exerted. Absolute zero The amount of pressure that a gas exerts on its container depends on the temperature of the gas because the particles move with more energy as their temperature increases. As the temperature of the gas decreases, the pressure on the container also decreases. In 1848 a physicist, Lord Kelvin recognised that there must be a temperature in which the particles in a gas exert no pressure. At this temperature they must no longer be moving and hence not colliding in their container. This temperature is called absolute zero and is equal to -273°C. It is not possible to have a temperature lower than absolute zero because the particles will have no movement. Absolute zero is defined as the temperature at which molecules in a substance have zero kinetic energy. This means that it is not possible to remove any more kinetic energy from it. A temperature in kelvin can never be a negative value. Temperature The temperature of a gas is related to the average speed of the molecules. The higher the temperature, the faster the molecules move and vice versa. Hence the molecules collide with the surface of the container more frequently and this is because their kinetic energy increases. Heating a system will increase the energy store by increasing the kinetic energy of its particles. The increase in kinetic energy can: Cause the temperature of the system to increase. Produce a change of state (melting, freezing, condensing) The internal energy of a gas is the sum of kinetic energy of all the molecules. The higher the temperature, the higher the average kinetic energy which means they will move around faster. The temperature in kelvin is proportional to the average kinetic energy of the molecules. The gas laws If the temperature of a gas remains constant, the pressure of a gas changes when it is: compressed - decreases the volume and increases the pressure. Expanded - increases the volume and decreases the pressure. A vacuum pump can be used to remove air from a sealed container.  The motion of molecules in a gas changes according to the temperature. As the temperature of a gas increases, the speed of the molecules also increases. The hotter the gas, the higher the kinetic energy and the higher the speed. The cooler the gas, the lower the kinetic energy and the lower the speed of the molecules If the gas is heated up, the molecules will travel at higher speeds and collide with each other more often. This will cause more pressure to be exerted. The pressure law If the volume of a gas is constant, the pressure law is given by: P∝T This means the pressure is proportional to the temperature. The higher the temperature, the higher the pressure and the lower the temperature, the lower the pressure. The relationship between pressure and temperature in kelvin for a fixed mass of gas at a constant volume can also be written as: P1 = initial pressure (Pa) P2 = final pressure (Pa) T1 = initial temperature (K) T2 = final temperature (K) The Boyle’s law For a fixed mass of a gas held at a constant temperature: pV = constant This means that the pressure and volume are inversely proportional to each other. When the volume decreases, the pressure increases. When the volume increases, the pressure decreases. This is because when the volume decreases, the particles will collide more often since there is less space for them to move in. This equation can also be rewritten for comparing the pressure and volume before and after a change in a gas: P1V1 = P2V2 (Boyle’s Law) P1 = initial pressure in pascals (Pa) V1 = initial volume in metres cubed (m3) P2 = final pressure in pascals (Pa) V2 = final volume in metres cubed (m3)