MAXIMIZING YOUR PHYSICS EXAMINATION MARKS

Transcription

1 MAXIMIZING YOUR PHYSICS EXAMINATION MARKS Ian Cooper Honorary Lecturer School of Physics The University of Sydney DO PHYSICS ONLINE HOME PAGE N.S.W. HIGH SCHOOL PHYSICS HSC COURSE When studying Physics say to yourself puts you in the right frame of mind SIMPLE??? Physicists use models to explain the physical world around us make simplifying assumptions eliminate what is not important concentrate on only the essential ideas what is most important Students focus on the real world lump too many ideas together than are related but different focus on real world not appreciate the role of simple models don t differentiate between the everyday use of language and the its use in a scientific context All of chemistry is in equations like this you can t do physics without mathematics 1 r 1 1 sin 0 r r r r sin r r r sin E U PRACTICE DOES NOT MAKE PERFECT need quality practice Maximizing Your Physics Examination Marks Ian Cooper 1

2 STUDYING PHYSICS THINK ON PAPER WITH A PEN IN HAND AND DO NOT THINK TOO MUCH IN YOUR HEAD Don t just read use pen and paper to process information Summarize what you read and study MINDMAPS (concepts maps) THINK, VISUALIZE and PROCESS while you are studying Memorize your summaries: short term and long term memories are different. Memory is the most important process in learning. Strive for understanding it only comes slowly after lots of memory work and exposure. Often comes in a flash. Doing questions that have answers: Read question / process it / think about what it asking / think about what you know / consult your summaries or textbook / review and process the given answer / after a short time interval do the question like in an exam, then check (mark) your answer against the published answer. Study sessions 40 to 60 min on physics eg doing problems, creating mindmaps Review sessions only about 10 minutes. These are short reflection sessions: short term memory long term memory. Reflection: review / reflect upon a study period a short time after the end of that study period very beneficial in transferring knowledge to long term memory you can process ~5 chunks of information at any one time MINDMAPS (special type of concept map) Summaries with minimum padding words Key words Symbols / equation / units Colour Graphs Images annotated diagrams (difficult to remember words, easy to remember dramatic images Make mindmaps for each equation summary of what the equation is telling you Maximizing Your Physics Examination Marks Ian Cooper

3 Motion of charged particle in a magnetic field F qv B q v sin B F B B qv Bsin Right Hand Palm Rule Fingers point along the direction of the B-field Thumb points in the direction in which positive charges would move right) (negative is moving to the left, then the thumb points to the Palm gives the direction of the force on the charged particle + q v + I F out of page B F palm face thumb v v B q fingers motion of a positive charge in a magnetic field S N e v (+q) F v v q q thumb B fingers palm face motion of a negative charge in a magnetic field B thumb palm facing down right hand palm rule fingers v q F v q B Maximizing Your Physics Examination Marks Ian Cooper 3

4 13/0 PHOTOELECTRIC EFFECT incident photon E = h f max KE of ejected electron E Kmax bound electron incident photon E blue = h f blue all photon energy acquired by electron: min energy required to remove electron from material W min (work function) photoelectron ejected from material Conservation of energy: h f W m v min 1 e Electron ejected from surface without delay ( kicked-out ) max bound electron energy absorbed by electron f blue > f red hf blue > W min hf red < W min incident photon E red = h f red bound electron energy absorbed by material insufficient energy acquired to eject electron Maximizing Your Physics Examination Marks Ian Cooper 4

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7 Equation Mindmap Equation of uniform accelerated motion one dimension v v0 at ( x x 1 0) v0t at v v 0 a ( x x0 ) u v s v t t = 0 v = v o x = x 0 an object in free fall (ignoring dissipative forces) has a constant acceleration + Need to define a reference frame (origin, coordinate system) y can replace x when the motion is in the vertical direction For projectile motion can use subscripts for the independent quantities in the horizontal (x) and vertical (y) directions x, y x 0, y 0 v x, v y v 0x, v 0y a x = 0, a y = g = 9.80 m.s - Symbol Physical quantity SI unit (other) t time interval measured from t = 0 s no h x y displacement at time t from the m + - initial (t = 0) position at x 0 km x 0 y 0 initial (t = 0) position: select frame of m + - reference, often can choose x 0 = 0 km v velocity of object at time t m.s km.h -1 Sign (+ / -) v 0 initial velocity velocity at time t = 0 s m.s km.h -1 a acceleration a = constant m.s Maximizing Your Physics Examination Marks Ian Cooper 7

8 Graphs v = v 0 + a t straight line: variables t and v, constants v 0 (intercept) a (slope) v slope a > 0 v v 0 > 0 v 0 > 0 (0,0) area = change in displacement t (0,0) slope a < 0 t v v 01 > 0 slope a 1 v v 0 > 0 a = 0 (0,0) slope a t (0,0) t v 0 < 0 a 1 < a v(t) = v 0 Maximizing Your Physics Examination Marks Ian Cooper 8

9 Problem A 5.0 kg ball is thrown vertically into the air at a speed of 0 m.s -1. What can you easily calculate? Maximizing Your Physics Examination Marks Ian Cooper 9

10 Problem A ball is thrown vertically into the air. Draw three time graphs: displacement, velocity, acceleration Maximizing Your Physics Examination Marks Ian Cooper 10

11 PROBLEM SOLVING GUIDELINES Think with pen and paper, not in your head Ask yourself How do I approach the problem ISEE I see Identify and Setup Read interpret the problem or question what is the marking expecting Identify the system (object of interest) Visualize the problem Identify the type of problem Data: known and unknown (?) physical quantities things that you measure name, symbol, value, unit (SI unit), significant figures eg speed of car A v A = 30.5 km.h -1 v A =? m.s -1 Use subscripts to identify different objects or different times mass: car A m A car B m B Initial velocity v i Final velocity v f Often best to use SI units: conversion of units km.h -1 m.s -1 ev J s y Create annotated scientific diagrams (need to be useful one of the most important approaches to answering problems always used by the best students); show directions for vector quantities What do I know: physical principles, laws, concepts, key words, equations Execute Answer the question Linear setting out one line after the other (you can ask for more paper to answer questions if space on exam paper insufficient) Qualitative answers: point form; only answer the question just don t write what you know, avoid making incorrect physics statements; use words in a scientific context; do not re-write question Quantitative answers: should be mathematically correct, proper use of = sign, correct use of symbols and equations; correct units; correct significant figures Do algebra before substituting numbers Evaluate Have I answered the question Check for incorrect physics statements Numerical answers: reasonable (sensible); units; significant figures; never use fractions in an answers always use decimals Maximizing Your Physics Examination Marks Ian Cooper 11

12 MATHEMATICS AN INDISPENSABLE TOOL Be able to interpret symbols an equations Equations tell a story Be able to change units Be careful with significant figures Be able to use scientific notation Be able to use your calculator using BIG and small numbers exponential (scientific) notation use x10 x and ENG buttons Be able to rearrange an equation Draw a scientific graph; interpret a graph; know about straight lines y = mx + b; find the slope m and intercept b of a straight line; know about the tangent and area under a curve Problems Find an expression for P A T T o 4 4 from the equation A car is travelling at a speed of 165 km.h -1. What is the speed of the car in m.s -1? In the Bohr model of the hydrogen atom, the energy levels of the atom are given by m e = 9.11x10-31 kg 0 = 8.854x10-1 C.N -1.m - h = 6.66x10-34 J.s c = 3.00x10 8 m.s -1 4 me e 1 En 80 h n Calculate the wavelength of the photon emitted in the transition from n = to n = 1 in nanometres. Maximizing Your Physics Examination Marks Ian Cooper 1

13 Problems Find an expression for P A T T o 4 4 from the equation T =? on left hand side of equation / Rearrange equation step by step A T T P T T T 4 4 o 4 4 o 4 4 o P A P T A T T 4 o P A 1 4 A car is travelling at a speed of 165 km.h -1. What is the speed of the car in m.s -1? Basic conversions 1 km = 10 3 m 1 h = (60)(60) s = s 1 km. h -1 = (10 3 ) / ( ) m.s km.h -1 = (165) / (3.6) m.s -1 = 45.8 m.s -1 In the Bohr model of the hydrogen atom, the energy levels of the atom are given by m e = 9.11x10-31 kg 0 = 8.854x10-1 C.N -1.m - h = 6.66x10-34 J.s c = 3.00x108 m.s -1 E n m e e 0 h n Calculate E 1 and E in (J and ev) and the wavelength of the photon emitted in the transition from n = to n = 1 (n and nm) E 1 = -.17x10-18 J = ev 1 ev = 1.60x10-19 J E = -5.4x10-19 J = ev h f = E 1 E / h =.45x10 15 Hz = 1.x10-7 m = 1 nm c = f c = 3.00x10 8 m.s -1 1 nm = 10-9 m Maximizing Your Physics Examination Marks Ian Cooper 13

14 Problem What is this equation all about? t0 tv v 1 c t 0 =.56 y and v = c t v =? y t 0 =.56 y and t v = 9.84 y v =? Maximizing Your Physics Examination Marks Ian Cooper 14

15 What is this equation all about? t0 tv v 1 c t 0 =.56 y and v = c t v =? y t 0 =.56 y and t v = 9.84 y v =? Time Dilation Effect moving clocks are observed to run slow t v dilated time interval / t 0 proper time interval / v speed of frame of reference / c speed of light t 0 =.56 y v = c t v =? y t v =.98 y >.56 y = t 0 Different observers observe different time intervals Rearranging the equation v/ c 1 t 0 tv t v = 9.84 y t 0 =.56 y v =? c v = c Maximizing Your Physics Examination Marks Ian Cooper 15

16 GRAPHS Mathematical conclusions can only be made from straight lines Linear relationship straight line Proportional relationship straight line passing through origin (0,0) Equation of a straight line y = m x + b x and y are variables m (slope) and b (intercept) are constant x = 0 y = b slope y m x rise run Problem sketch straight line graphs for b = 0 m > 0, b > 0 m > 0, b > 0 m = 0, b < 0 m < 0 v = u + at u > 0, a < 0 h f ev W f (x-axis) and V s (y-axis) are the variable h, e and W min are constants s min From the graph how do you find h, e, W min and f c Maximizing Your Physics Examination Marks Ian Cooper 16

17 Problem Photoelectric Effect h f ev W e = 1.60x10-19 C s min An experiment was performed using a photocell. The surface was illuminated by light of different frequencies f and the stopping voltages V s were measured. frequency f (x10 14 Hz) stopping voltage V S (V) Plot the stopping voltage V s versus the frequency f of the light. (y-axis V s from - V to + V x-axis f from 0 to 10x10 14 Hz) Determine: the cut-off frequency f c, the work function W min in ev and J, and the value of Planck s constant h. Another surface was used in the experiment. Its work function was 0.5 ev. Draw a line on the graph for this surface. Maximizing Your Physics Examination Marks Ian Cooper 17

18 Stopping Voltage Vs (V) Stopping voltage Cut-off frequency Voltage required to reduce photocurrent to zero ev s is the measurement of the maximum kinetic energy a photoelectron Minimum frequency for the release of electrons from the surface of the material frequency f (Hz) x h f mv W ev W V V V stopping 1 h f e max min stopping min stopping s W e min slope slope = h/e = (V.Hz -1 ) intercept b = V Work function W = 1.7 ev = J 1 ev = 1.6x10-19 J Planck s constant h = J.s note answer low than accepted value Cut-off frequency f c = Hz The two lines have the same slope = h / e. Maximizing Your Physics Examination Marks Ian Cooper 18

19 Problem A 5.0 kg ball is thrown vertically into the air at a speed of 0 m.s -1. What can you easily calculate? Apply: How to approach the problem: Identify Setup Execute Evaluate a = - 10 m.s - max height v 1 = 0 m.s -1 s 1 =? m t 1 =? s m = 5.0 kg + ground: v =? m.s -1 s = 0 m t =? s F G = - mg u = +0 m.s -1 Problem Type motion with constant acceleration v u at s ut at v u a s 1 At maximum height v1 u 0 0 v1 u a s1 s1 m 0 m a 10 v u a t t v u a Just before impact 0 0 s.0 s 10 v = - u = - 0 m.s -1 t = ( t 1 ) = 4.0 s or v u a s u v u 0 m. s 1 v u a t v a t t v a 0 s s Force on ball during flight F G = m g = m a = (5)(-10) N = N Maximizing Your Physics Examination Marks Ian Cooper 19

20 a y (m/s ) a x (m/s ) v y (m/s) v x (m) y (m) x (m) Problem A ball is thrown vertically into the air. Draw three time graphs: displacement, velocity, acceleration Apply: How to approach the problem: Identify Setup Execute Evaluate time t (s) time t (s) time t (s) time t (s) time t (s) time t (s) Displacement curve parabola s ut at 1 slope of tangent ds v dt Velocity curve straight line v u at slope is constant and negative dv a dt Acceleration curve constant independent of time Maximizing Your Physics Examination Marks Ian Cooper 0

21 Problem Two balls with the same mass are dropped from rest and from the same height onto the ground. One ball bounces off the floor to nearly its original height, while the other ball does not bounce at all. Which ball exerts the greatest force on the ground during the impact? Maximizing Your Physics Examination Marks Ian Cooper 1

22 Problem Two balls with the same mass are dropped from rest and from the same height onto the ground. One ball bounces off the floor to nearly its original height, while the other ball does not bounce at all. Which ball exerts the greatest force on the ground during the impact? Apply: How to approach the problem: Identify Setup Execute Evaluate Ball A m A = m B = m Ball B + velocities of ball just before impact u A = -u u B = -u velocities of balls just after impact v A = 0 v B = +u Impulse = change in momentum F t mv mu F t mv m u mu mu Ball A 0 A A A Ball B F t mv m u mu mu m u B B B Impulse exerted on ball B by the ground is twice the impact exerted on ball A Ball A sticks to the ground ta tb FB FA The force on the bouncing ball is greater because this ball has the greater change in its momentum Explain how a person can walk on a corn flour mixture? Maximizing Your Physics Examination Marks Ian Cooper

23 Problem Predict Observe Explain (POE) Compare the time it takes for a magnet to fall through a glass tube and copper tube of the same length. Maximizing Your Physics Examination Marks Ian Cooper 3

24 Problem Predict Observe Explain POE Compare the time it takes for a magnet to fall through a glass tube and copper tube of the same length. Apply: How to approach the problem: Identify Setup Execute Evaluate Faraday s law of electromagnetic induction: an emf is induced in a conductor when the conductor is in relative motion with a magnet a changing magnetic flux produces an emf in a conductor emf d dt B magnetic flux B BAcos If there is a complete circuit, the induced emf will produce an induced current through the conductor. Lenz s law the direction of the induced current will produce a magnetic field to oppose the change in magnetic flux due to the relative motion. glass tube magnet falls freely F G = mg S N S N copper tube magnet falls slowly F B B induced right hand screw rule S N Newton s nd law a glass > a copper F a m F G = mg B magnet induced current (Farady s / Lenz s laws) Induced current produces a magnetic field to oppose the motion of the falling magnet (same magnetic poles repel) Maximizing Your Physics Examination Marks Ian Cooper 4

25 Problem Two light globes when connected separately to a 10 V battery results in one globe being very bright whereas the other globe is very dim. One globe is rated 100 W, 10 V and the other 0 W, 10 V which globe is bright and which is dim? Comment on the brightness of each globe when both are connected to the battery in (1) series and () parallel. Maximizing Your Physics Examination Marks Ian Cooper 5