Edexcel A levels Physics Unit 3 Notes

Phys ics – Unit 3 Note s Graph: How to find ___ value from the graph?

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Gradient or area under graph

How does it improve results?

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Enables average to be taken Shows anomalies Will show if ___ value is changing

Why graph is straight?

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Mention constant an mention the gradient Either A ∝ B or compare with y = mx + c

Advantages of graphs?

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Anomalies can be identified Allows interpolation/extrapolation Systematic errors can be detected Relationship/trend can be indentified Equation can be derived Line of best fit averages results Intercept/gradient/area can be determined

P.D v/s Current Graph

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Linear relationship Remember “E = V + Ir” Internal Resistance = negative of gradient E.M.F = intercept with the potential difference axis/yaxis

Stress v/s Strain Graph Gradient = Young’s Modulus

Area = energy stored per unit volume

Tables/Readings: Points to remember – Fill in column headings with units Criticize the readings/set of measurements.

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Inconsistent precision / inconsistent significant figures in __ column Less readings Small range Accuracy (e.g. only recorded to nearest 10 cm) No mention of repeats/no repeats

Average taken should be of all normal readings and have same number of significant figures as the readings

V/S: Advantages and Disadvantages for [Analogue] device v/s [Digital Device]: ∑

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Advantages of Analogue: - No power supply required - Simple to operate or to set up - Readily available, cheaper, easily transportable Disadvantage of Analogue: - More errors - Effect of reaction time - Insufficient precision for short values (e.g. short times) - Graph would have to be drawn manually Advantage of Digital: - Fewer errors - No reaction time

- Precise readings - Graph drawn automatically Disadvantage of Analogue: - Power supply needed - Needs training, setting up, alignment issues, time to set up - Not easily available, expensive, not easily transportable

Advantages and Disadvantages for manual v/s data logger + graph method: ∑

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Advantage of manual: - No power supply required - Easily transportable - cheaper Disadvantage of manual - Small no. of readings/ large time interval b/w readings - Reaction time - Random/systematic/parallax errors - Easily broken (if apparatus like glass beaker is used) Advantage of data logger + graph - Large no. of readings/ small time intervals - Graph drawn automatically - Simultaneous reading of two values (if applicable) Disadvantage of graph

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Power supply needed Zero/systematic errors

Advantages and disadvantages of Digital Multimeter v/s Ammeter + Voltmeter (analogue) ∑

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Advantage of Digital Multimeter - Only one meter needed/may be cheaper option - No parallax error - Variable scale/scale can be changed - Two decimal places - Only requires series connection - Unlikely to be much heating effect - Smaller uncertainty as only one reading - No calculation required Disadvantage of Digital Multimeter - Zero error, contact resistance - Internal battery required - Less simple to graph for a fixed wire Advantage of A+V - Do not require individual batteries - Graphical method possible Disadvantage of A+V - Two meters needed/may be more expensive4 - Parallax error - Scales are fixed so requires interpolation - Limited by size of scale divisions - Requires both series and parallel connection - Heating effect - Greater uncertainty as two readings - Need calculation from two readings

Experiments: Percentage Uncertainty Mean – lower value × 100

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Higher value – Mean × 100

Mean

Mean

Percentage difference (Given value – your value) × 100 Given Value Precautions:

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Rule vertical/horizontal Release object from rest Practice

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Repeat Avoid parallax errors

Sources of uncertainty

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(if wire) small diameter/diameter only measured once/kinks in the wire (if wire and circuit involved) Contact resistance/resistance of connecting wires (if circuit) accuracy of ohmmeter or voltmeter or ammeter (any device) Zero error Wires snapping Weights falling Didn’t reach terminal velocity/tube too narrow/distance between markers is too small/ misreading of stopwatch or micrometer Reaction time Parallax error Inaccuracy in measuring _______.

Why repeat?

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Can identify anomalies, average can be taken, reduces random error/uncertainty

Why the voltmeter should have a very high resistance:

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Voltmeter is in parallel with wire Current through voltmeter should be negligible (accept very small or zero)

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The voltmeter needs a high resistance reduce Ammeter should measure current in justtothe wire current through it. If the voltmeter takes any current, the current recorded for the wire will not be correct

Assumptions for calculating stress:

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Cross section has remained the same

Refraction Experiment o

Also take readings at second face

Wire-extension ∑ ∑

Vertical length of wire with masses on end OR horizontal wire with masses over pulley Measure extension Δ

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use of mark on wire OR length use of OR F = comparison kx extension = final – initial of wires Variables to be kept constant – Initial length of the wires Precautions - Use of goggles OR means of catching/avoiding falling masses, Protection (of eyes) from snapping wire OR protection of feet/floor from falling masses

Viscosity Experiment ß

Apparatus: Oil. Ball Bearing, Measuring cylinder, light gates, timing device, rule, micrometer

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Quantities to be measured: Diameter, distance, time (alt Diameter, velocity) Instruments: Micrometer, metre rule, stopwatch Independent: diameter/radius Dependent: Terminal Velocit/time Radius determined from diameter, velocity from dist and time. Graph of v against2 rand find gradient Source of uncertainty/systematic error: terminal velocity not reached, reaction time, temperature not constant, zero error, measurement of diameter/distance fallen, parallax error Safety precautions: low risk experiment, mop up spills, use gloves, wear goggles, normal laboratory rules should be followed

Determining Young’s Modulus o

o o

o o o o o

Apparatus: Wire, support, weight, micrometer screw gauge/vernier callipers/ digital vernier Quantities to be measured: Length, extension, diameter, mass/weight/force Instruments: Metre rule, travelling microscope, Micrometer for diameter, balance for size of mass Independent – Weight/force Dependent – Extension Use of ? r2 and youngs modulus equation The main source of uncertainty/systematic error: Extension/diameter Risk of weights falling (hence toe protection) and snapping wires (hence safety glasses)

% Loss of Kinetic Energy ∑ ∑ ∑ ∑

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Drops ball of known mass vertically from known height Determines height of bounce using a metre rule mgΔh = 1/2mv2 States that mass cancels OR mass stays the same OR mass measured OR mass known % loss in KE = loss in PE x 100% initial PE Precaution: make measurement of height of bounce at eye level ∑

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Thermistor resistance ß

Apparatus: Circuit, Thermistor in water, means of heating/cooling (Bunsen or water bath), thermometer, stirrer

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Quantities to be measured: Resistance, temperature (alt Current, pd, temp) Graph of resistance v/s temperature Sources of uncertainty/systematic error: simultaneous reading of two variables, systematic error on thermometer, parallax error, meter zero error, uneven temperature of liquid Safety: hot water and heated wires

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Spring o Apparatus: Spring, mass/weight, support, rule, set square, pin, balance Created by: Niharika Shinde www.ighelp.blogspot.com Email: nihakalp@gm ail.com

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Independent: Force/Weight/Mass Dependent: Extension Plot force v/s extension, gradient = k Uncertainty sources: parallax, zero error, elastic limit not exceeded

Semi-circular block refractive index – accuracy ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑

Use of paper/pins to trace path of ray Ensure there is a thin ray (from the ray box). Comment on using centre of block Normal drawn (at A) / measure from normal Mark the ray, then measure the angle Use large angles of incidence/wide range Repeat and take average Work in a dark room.

Circuit-related ß ß ß ß ß

Instruments: Ammeter, voltmeter, power supply, variable resistor/variable power supply Determine R from V and I Draw graph of R v/s V ‡ Positive intercept on R axis (either curve or straight line) Uncertainty source: zero error on meter or difficulty of taking simultaneous measurements Safety Precautions: If low voltage supply ‡ low risk If bulb involved: Hot bulb, so do not touch

Efficiency of motor o o o o o

Quantities to be measured: P.d, current, height and time Repeat?: Yes = for average. No = motor heats up, battery p.d reduces Power input = VI Power Output = mgh/t Efficiency = poweroutput/powerinput Uncertainty/Systematic error source: Zero error in meter, reaction time, parallax Safety Precautions: Risk of mass falling (wear shoes), low risk if p.d. is low, risk to eye if string breaks (wear goggles)

Instruments: Metre rule: Longer length so measurement to nearest mm appropriate Micrometer: Measures to 1/100 mm Stopwatch: reading to 0.1s or 0.01s Ammeter/Voltmeter: 0.1A or 0.1V interval and state indentified range if bulb involved and value given >> Note: Resistance of variable resistor should not be reduced to zero so that circuit is not short circuited (otherwise will get hot) OR to prevent high current through circuit

Reading: 5.5mm + 0.35mm = 5.85mm

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