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PHYSICS UNIT ONE What is in Unit One? Nuclear Physics and Radioactivity Electricity One of … Astrophysics Astronomy Energy from the Nucleus… What is required? Pass the SACs in the three areas of study Turn up to <90% of classes Complete all weekly homeworks Optional … 40+ club homework What do I need? Yr 11 Physics Textbook Scientific Calculator (CAS not allowed in exam) Workbook NUCLEAR PHYSICS AND RADIATION What is an atom Three key historic figures in discovery of what an atom is: 1) Democritus – concept of atomos 2) J. J. Thompson – the plum pudding model 3) Lord Rutherford – the solar system model Rutherford’s Gold foil experiment A small amount of the radiation is deflected by very large angles Fired radiation at a thin sheet of gold (only 200 atoms thick) Most radiation particles pass through undeflected Rutherford’s gold foil experiment He concluded two things: 1) The atom is mainly empty space 2) It has a small, but dense and positive nucleus. Activity – work in groups Your teacher wants to make a scale model of an atom for the classroom. We want to use a tennis ball for the nucleus and marbles will be the orbiting electrons. If the size of a nucleus is r = 1x10-14m, and the electrons orbit at a distance of 1 x 10-10m; how far away do my electron marbles have to orbit the tennis ball? Activity: Extension How small will the scale model nucleus have to be such that the entire scale model atom will fit in the classroom? The atom Nucleus: Made of neutrons and protons Electrons orbit the nucleus The atom Name Charge Mass (atomic units: a.u.) Proton +1 1 Neutron 0 1 Electron -1 0.0005 Particles that live in the nucleus are called nucleons. Protons and neutrons are nucleons. Electromagnetic Force and Nuclear Force Protons have the same charge, and just like having two magnets with similar poles, protons repel each other How does the nucleus stick together? There is a force called the strong nuclear force. It only works at very small distances (when nucleons are almost touching). Adding neutrons to a nucleus increases the attractive strong nuclear force without increasing the repulsive electromagnetic force Electromagnetic Force and Nuclear Force This is similar to adding a piece of metal between “like” poles of two magnets. It helps them stick! Isotopes Isotopes are atoms of the same element that differ in the number of NUETRONS they have. Isotopes have similar chemical properties but different physical properties. Example: Carbon has 3 isotopes: Carbon -12, 13, and 14. All three isotopes have 6 protons. C-12 has 6 neutrons. C-13 has 7 neutrons. C-14 has 8 neutrons. Describing an atom Mass Number: Protons + Neutrons Z E A Symbol of element Atomic Number: Protons This is the isotopic symbol for an atom Isotopic Symbol Why are there mass numbers with decimal places on the periodic table? eg 35.5 17𝐶𝑙 How can you half a neutron? On the periodic table, the mass numbers are called the relative atomic mass numbers. They are an average of all isotopes of that element found on earth. Questions: Complete the table Element 12 6𝐶 No. of protons No. of neutrons 143 85 37𝑅𝑏 53 3 1𝐻 72 Name Uranium Radioisotopes Sometimes the nucleus of a particular isotope is unstable, and it may undergo a radioactive decay or transmutation This is known as a radioisotope Radiation is emitted Every element heavier than Bismuth (A = 83) is radioactive. Every element heavier than Uranium (A = 92) has to be produced artificially. Nuclear Radiation Three types of radiation can be emitted in nuclear decay Alpha (α) Beta (β) Gamma (γ) Radiation Symbol α β γ Name alpha beta Gamma Made of Two protons, two neutrons (helium nucleus) electron energy +2 -1 Charge Mass Typical Energy Heavy (4 a.u.) 5MeV 0 Light (0.0005 a.u.) None 1MeV 0.1MeV Radiation α β γ Range in air Few cm 1 or 2 metres Many metres Shielded by Sheet of paper Ionizing ability High Speed 0.1c Isotopic Symbol 4 2𝛼 cm of Al Reasonable 0.9c 0 −1𝛽 c = the speed of light Many cm of Pb Poor c 0 0𝛾 What is an electron volt? (eV) We normally measure energy in the unit Joules (J) Atoms and electrons often contain very small amounts of energy if measured in Joules (~10-19J) 1eV = 1.6x10-19J It is the energy 1 electron gains after being accelerated by 1V Questions 1) Convert into Joules a) 5eV b) 1keV c) 3MeV d)0.2keV 2) Convert into eV a) 3.2x10-19J b) 6.4x10-14J c) 10-15J Prac: Detecting radiation Cloud chamber and glow-in-the-dark and geiger How radiation is detected 1) Geiger counter When it touches the anode, it becomes an electrical pulse. The counter counts these electrical pulses This collisions causes an electron to be ejected. It is attracted to the positive anode Incoming radiation strikes an atom in the Geiger counter How radiation is detected 2) Cloud Chamber Incoming radiation The radiation ionizes some of the alcohol atoms Super saturated alcohol vapour Clouds form around ionized alcohol atoms How radiation is detected 3) Fluorescent Materials – Some materials (like glow in the dark stickers, or phosphorus screens) will emit light when bombarded with nuclear radiation. Decay equations Polonium-210 decays via alpha emission. What product is formed? 210 84𝑃𝑜 → 42𝛼 + 𝑦𝑥𝑧 ??? The mass numbers must balance on both sides of the equation The atomic numbers must balance on both sides of the equation x= 206 y= 82 z= Pb Decay equations Carbon-14 decays via beta emission. 14 6𝐶 x= 14 y= 7 → 0 −1𝛽 + 𝑦𝑥𝑧 z= N → −10𝛽 + 147𝑁 + ῡ Note about beta decay: An antineutrino ( a particle of very small mass, but no charge) is also emitted. Symbol: ῡ 14 6𝐶 Decay equations The product nucleus is called the daughter nucleus Any further decay will lead to a grandaughter nucleus Questions: Write equations for the following: 1) Uranium-238 decays via alpha emmision 2) Francium-222 decays via beta emission 3) Polonium-214 decays via alpha emission 4) Carbon-12 emits gamma radiation 5) A new isotope is formed by adding a neutron to U-238 Decay equations prac Half-life Prac Half life It is random when a radio-isotope might decay. However, with a large number of atoms, they obey an exponential decay. Half the atoms have decayed in one half life. Half the remaining atoms have decayed in a second half life. Half Life Activity is a measure of atomic decays. Units are Becquerels 1 Bq = 1 decay per second Half Life Questions From the following graph, estimate the half life of the sample. Using this half-life, estimate at what time the sample will have an activity of 100 kBq Half Life Questions A hospital keeps a sample of Iodine-131 (I-131) for use in radiation therapy. It is noted that the activity of the sample has reduced by 75% in 17 days. What is the half life of I-131? Half-Life Question Carbon-14 is a radioisotope that is useful in obtaining the age of carbon containing materials (such as wood) up to about 50’000 years. Carbon14 is a naturally occurring radioisotope that plants will absorb from the atmosphere, along with the non-radioactive isotopes carbon-13 and carbon-12. After the plant dies, it stops absorbing carbon, and the carbon-14 within the plant decays away. Carbon-14 decays through beta decay with a half-life of 5730 years. Half-Life Questions A team of archaeologists have discovered the remains of what looks to be a fire-pit used by primitive homo sapiens. They measure the amount of carbon-14 in samples taken from the fire-pit and find the measured ratio of carbon-14 to be approximately 6.25% of the ratio of atmospheric carbon-14. Approximately how many half-lives have elapsed since this sample was alive? Approximately how old is the sample? Half-Life Questions Radiation Isn’t always a bad thing! Has many uses Medical Imaging and treatment Carbon Dating Energy production (nuclear fission) Smoke alarms But in large quantities it can be harmful… Which radiation is the most harmful? Alpha radiation has the highest ionizing ability. Ionizing: The radiation removes electrons from atoms in our cells/DNA. This causes the cells/DNA to mutate/die. However, alpha radiation is also the easiest to shield Our skin will effectively shield it. However, if we breath it in, or eat contaminated food = GAME OVER! Which radiation is the most harmful? What radiation are these Fukishima workers being protected from? How do you know? Absorbed Dose The absorbed dose is the amount of radiation energy that has been absorbed per kilogram. 𝐴𝑏𝑠𝑜𝑟𝑏𝑒𝑑 𝐷𝑜𝑠𝑒 = 𝐸𝑛𝑒𝑟𝑔𝑦 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 𝑏𝑦 𝑡𝑖𝑠𝑠𝑢𝑒 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑡𝑖𝑠𝑠𝑢𝑒 Units: Gray (Gy). 1 Jkg-1 = 1 Gy Dose Equivalent Different forms of radiation have different ionising abilities, and so cause varying amounts of damage to humans. As each type of radiation can affect tissue differently, the Dose Equivalent is used compare accurately radiation effects. Dose Equivalent 𝐷𝑜𝑠𝑒 𝐸𝑞𝑢𝑖𝑣𝑎𝑙𝑒𝑛𝑡 = 𝐴𝑏𝑠𝑜𝑟𝑏𝑒𝑑 𝐷𝑜𝑠𝑒 × 𝑄𝑢𝑎𝑙𝑖𝑡𝑦 𝐹𝑎𝑐𝑡𝑜𝑟 Units: Sieverts (Sv) Radiation Quality Factor Alpha 20 Beta 1 Gamma 1 Neutron 10 Effective Dose Different organs in the body have different sensitivities to radiation doses. eg. A person’s lungs would be more likely to develop cancer than the liver if they were both given the same amount of radiation. 𝐸𝑓𝑓𝑒𝑐𝑡𝑖𝑣𝑒 𝐷𝑜𝑠𝑒 = Σ 𝐷𝑜𝑠𝑒 𝐸𝑞𝑢𝑖𝑣𝑎𝑙𝑒𝑛𝑡 × 𝑊 W = Weighting Σ = “the sum of” Effective Dose Body Part Weighting (W) Ovaries/testes 0.2 Bone Marrow 0.12 Colon 0.12 Lung 0.12 Stomach 0.12 Bladder 0.05 Breast 0.05 Liver 0.05 Oesophagus 0.05 Thyroid 0.05 Rest of body 0.07 Total 1.00 Effects – Short Term Called Somatic effects Radiation ionizes molecules, causing cells to mutate or die Immediate Sickness/Death Effects – Long term Genetic Effects (mutates DNA in testes/ovaries) Radiation causes genetic defects that show up in offspring Questions 1. An 80kg tourist absorbs a gamma radiation dose of 200μGy during a return flight to England. Calculate the amount of radiation energy absorbed 2. Calculate the dose equivalent that has been received 3. During a medical procedure a patient receives a dose of gamma radiation. The organs which are affected include a bladder (5000Sv) and the ovaries (3000 Sv). Calculate the effective dose of radiation to which this woman has been exposed Area of Study Two: Electricity! Electrical Circuits A circuit must be closed Must have a voltage source (EMF) – otherwise no power! Must have a resistive load (bulb, resistor, bell etc) – otherwise you get a short circuit! Electrical Symbols Device Symbol Device Symbol Wires crossed (not joined) Cell Wires joined Battery of cells Resistor or other load AC supply CC VV Resistor Ammeter A Filament Lamp Voltmeter Diode DC Supply Earth or ground Switch V Circuit Diagrams Draw: A) Two 1.5V cells connected to a lightbulb, and a switch B) A heating element (a resistor) connected to an AC power supply with a switch Circuit Diagrams Are the following circuits different? No! They are the same. One bulb, and a DC power supply, connected to each other. Circuit Diagrams Just because there are corners in a circuit diagram, doesn’t mean there has to be corners in the real circuit. This brings some confusion. Circuits that may appear different as diagrams, may actually be identical. These are called equivalent circuits. Circuit Diagrams Which of the following are equivalent circuits? Why? Prac Simple Circuit. Part B. Using volt and ammeter Graph V/I Ohm’s Law Ohm’s Law relates the voltage across a load to the current it draws V = IR V = Voltage Units: Volts (V) I = Current Units: Amps (A) R = Resistance Units: Ohms (Ω) Ohm’s Law Ohmic Conductors obey Ohm’s Law They have linear relation between the voltage and current (straight line graph) Ohm’s Law Some devices don’t obey Ohm’s Law. Their resistance is not constant Called non-ohmic devices Ohm’s Law Examples of Ohmic Conductors: Resistors, most bulbs Example of non-Ohmic Conductors: Diodes Questions A resistor draws a current of 1mA and has a voltage across it of 8V. Find the resisitance What current does a 1000Ω light bulb draw from a 24V power supply You have an Ohmic resistor, which draws 75mA when connected to 12V power supply. You decrease the voltage to 5V, what current do you expect it to draw? Resistance Which has more resistance? a. Fuse wire or power lines? b. Copper wire or Iron wire? Resistance The more resistance something has, the less current can go through it Resistance in a wire depends on: 1) The length of the wire 2) The cross sectional area of the wire 3) What the wire is made of Resistance 𝑅= 𝜌𝐿 𝐴 ρ = resistivity (depends on the material) Unit: Ωm L = Length Unit: m A = Cross sectional area. Unit: m2 Resistance Example Normal household wire has a diameter of 1.8mm, and is made from copper (ρ =1.7x10-8) What is the resistance of a 10m long section? 𝜌 = 1.7 × 10−8 , 𝐷 = 1.8𝑚𝑚, 𝑙 = 10𝑚 𝐴 = 𝜋𝑟 2 = 𝜋 × 0.9 × 10−3 2 = 2.5 × 10−6 𝑚2 𝜌𝑙 1.7 × 108 × 10𝑚 𝑅= = = 0.068Ω −6 𝐴 2.5 × 10 Resistance 1) A 100m long section of telegraph wiring has a diameter of 3.6mm and a resistivity of 2.8x10-8. What is the resistance? 2) You have to design a household’s wiring system. Name three ways you could decrease the resistance as much as possible. 3) Why don’t all electricians try and use really thick wire all the time? Electrical Power Power is how much energy is used per second. Power = VI Measured in Watts (W) 1W = 1Js-1 Questions 1) A bulb is connected to a 12V supply and draws 2) 3) 4) 5) 0.875A, what power is it using? A toaster connected to the 240V mains supply uses 1kW of power. What current does it draw? Which of the three identical bulbs in the picture shown is using the most power? Which of these two bulbs (connected to different power supplies) is the brightest? Bulb 1 connected to 10V supply draws 0.8A of current. Bulb 2 connected to a 15V supply draws 0.4A. A 50Ω heating unit is connected to a 20V power supply. What power does it use? Questions Electrical Energy If power is “energy per second”, then energy is power multiplied by the time. 𝐸 = 𝑃𝑡 Measured in Joules (J) kWh is another unit of energy (Note it isn’t power!). 1kWh = 3.6MJ Activity The price for electricity supplied by the grid is about 20c per kWh. Find: The cost of heating something in the microwave for 5minutes How much a 1.5V battery should cost How much does it cost to boil a kettle (for 2 minutes) How much money are the solar panels on the science block roof saving us per day (with approx 8 hours sun) A iphone charger left plugged in to the wall will still use 1W. How much does this cost, if it is left plugged in all year? What is electricity? Electricity is moving charges carrying energy An example of a charge is an electron Charge is measured in Coulombs (C) 1e- = 1.6x10-19C How many electrons in 1 C? Current and Voltage What is current? Current is how much charge goes passed a given point in 1 second 𝐼= 𝑞 𝑡 What is voltage? Voltage is how much energy each Coulomb of charge carries 𝑉= 𝐸 𝑞 Questions 1. How many electrons make up a charge of 1μC? 2. A) A hairdryer draws a current of 1.6A. What charge flows through the hair dryer every second? B) How many electrons flow through the hairdryer in 1 minute? 3. Use the formulae 𝑉 = 𝐸 𝑞 𝑞 𝑡 and 𝐼 = , and substitute into P=VI, and simplify Electrostatics Charges don’t always have to move They may just accumulate somewhere Law of conservation of charges: Charges aren’t just created! To make some negative charge you have to start with neutral atoms and split into an equal negative charge (electrons) and positive charge (remaining ion) Electrostatics Like charges repel Unlike charges attract (like a magnet) An excess of electrons on something gives it a negative charge Deficit of electrons on something gives it a positive charge Prac: Charged Rods Demo: Van de Graff generator Van de Graaff Generator How does it work? Charge jumps! Positive charge left on dome Metal comb touches belt, causing friction and separating positive and negative charges Negative charge is taken to bottom of the machine. Conductors and Insulators Conductors: Allow the flow of electrons Insulators: Inhibit the flow of electrons Coulomb's Law What caused the Al can to roll? A force! Charges exert a force on one another. 𝐹= 𝑘𝑞1 𝑞2 𝑟2 k = 9x109 Nm2C-2 Positive force = repulsion ( a push ) Negative force = attraction ( a pull ) Coulomb’s Law Example: Two charges, with a charge equal to 1μC, are held 2m apart from each other. What is the force each charge exerts on each other? Coulombs law Examiners like to ask proportionality questions with Coulomb’ law Eg. Two charges, each with a charge of q, are positioned a set distance apart from each other. If the distance is doubled, how does the force change? Questions 1) A - 1μC and 6 μC are separated by 20cm. Calculate the force. 2) Two 1μC are separated by an unknown distance and experience a force of 3N. Calculate the distance separating the two charges. 3) A charge of q, and another of 2q are separated by a distance x. If the charge on the second is doubled to 4q, and the distance between them halves to x/2, how does the force change? Electric Fields The electrostatic force is invisible. However, as scientists, we have added lines around charges to help visualise this force Called “The Electric Field” Shows where a positive charge would fly off to if we dropped it into the field A negative charge would head in the opposite direction. Where lines are close together, the field is strong Where lines are far apart the field is weak Electric Fields If we had a positive charge that couldn’t move Electric Fields And we dropped another positive charge just north of it, which direction would the 2nd charge fly off to? Electric Fields So we put an arrow there Electric Fields If we were to drop another positive charge to the right of our main positive charge… Electric Fields If we were to drop another positive charge to the left of our main positive charge… Electric Fields Continue putting field lines around our charge… Electric Fields Question: Where is the electric field the strongest? Answer: Near the charge! Electric Fields Question: What would the field lines created by a negative charge look like? Discuss with your neighbour. Electric Fields Some more complicated fields… Electric Fields 𝐹 = 𝐸𝑞 E = Electric Field. Units: NC-1 Electric Fields - Questions Some small charged spheres are to be placed in an electric field which points downwards and has a strength of 5000NC-1. What force would be experienced by a charge of +5 μC and -0.6 μC ? Draw the electric field you would expect around the three charges below -q +q -q Electric Fields Questions Draw the electric field you would expect around the two charges below +q -2q Back to Electric Circuits Draw two different ways you could place two bulbs in a circuit with a battery One is called series One is called parallel Kirchhoff’s Laws Law 1: The current law Law: “In any circuit the sum of all currents flowing into a point equals the sum of all currents flowing out of that point Analogy: River: The amount of water flowing into a junction equals the sum of water flowing out. [Draw this] What this means. Current is same at all places on a series circuit. It splits up in a parallel circuit. Kirchhoff’s Laws Law 2: The voltage law Law: “The total potential drop around a closed circuit must be equal to the total EMF in the circuit Analogy: River. The pump (EMF) pumps water to the top. It drops down as it goes through devices [Draw this] What this means: Voltage is supplied by battery, and used in devices. Voltage is the same in parallel circuit. Activity Smarties run. Questions? Prac: Series and Parallel Circuits Resistors in Series 𝑅𝑇 = 𝑅1 + 𝑅2 + … Eg Total resistance in the circuit? 6Ω 2Ω 𝑅𝑇 = 6Ω + 2Ω + 3Ω = 11Ω 3Ω Resistors in Parallel 1 𝑅𝑇 = 1 𝑅1 + 1 𝑅2 + … 3Ω 6Ω 1 𝑅𝑇 1 1 3 = + = 3 6 6 6 𝑅𝑇 = = 2Ω 3 Resistors in Parallel 3Ω 6Ω The total resistance in a parallel circuit is always less than the smallest resistor. Sometimes examiners ask for the equivalent resistance (it’s the same thing as total resistance) Questions Find the equivalent resistance of the following circuits: 1) 2) 1Ω 2Ω 4Ω 3Ω 3Ω Total Resistance of Hard Circuits! Rules: Work out one section at a time : Work from the inside out 12Ω 6Ω 1Ω Questions 3) 3Ω 4Ω 6Ω 4) 5Ω 12Ω 5Ω 3Ω Questions 5) 3Ω 3Ω 6Ω 2Ω 6) 3Ω 3Ω 6Ω 10Ω Questions 7) Draw a circuit with three resistors that has a total resistance of 5Ω. 8) Draw the above circuit differently. 9) Using a 1Ω, 2Ω, 3Ω, 4Ω and 5Ω resistor draw a circuit with total resistance = 6Ω Final note about Voltages and Current in circuits The total voltage in a circuit is set by the battery/powerpack. However, the current drawn from this battery depends ON WHAT IS IN THE CIRCUIT Eg…POE Draw two circuit diagram with two bulbs (series and parallel). Guess which will draw the most current Measure total voltage in both, and total current. Which draws most power? Explain why one draws more current than the other CUPS 9 and 10. Prac: Which bulb is the brightest Report required for this prac. Demo: Internal Resistance Resistance within the powerpack/battery. Uses some of the voltage up! A V Internal Resistance Voltage without bulb = Voltage measured with bulb = Current = Voltage used in internal resistance Internal Resistance = Other sources of EMF EMF = electro-motive force (what drives the circuit) Solar 20% efficient 2kW per square meter of sunlight energy Don’t work at night! Less power in winter. Why? Household Electricity We have only been dealing with DC electricity (Direct Current) Household electricity is AC (Alternating Current) Why? We use AC because its relatively easily to change between voltages Household Electricity Power lines have a low resistance, but because electricity has to travel a large distance, this becomes an issue, and much power could be lost A high voltage has a lower current, and a lower current loses less power! Transformers change between high voltage (14000V) and low voltage (240V) Household Electricity Active: 240V Neutral: 0V Earth/Ground Current wants to flow from Active to Neutral just like it flows from +ve to -ve in a battery. Household Electricity Why do we have an earth wire? Imagine a fault in your toaster that touched the active wire to the casing. Then you touched the casing! The casing is safely connected to earth, so if a fault did occur, high current would flow to earth, and a fuse would blow. Electric Shocks! Our muscles move when they get small electrical shocks sent from our brain. Above 20mA of electric current, and we cant let go. A shock across the heart of 50mA for 5s = possible death Prac: Build a buzzer game with a bell