# Physics Core Practicals

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## Core Practical 1 - Acceleration

Method 1 - Trapdoor Method:

Ball-bearing held at height (h) above a trapdoor switch. When the switch is turned, the circuit is broken and the electromagnet releases the ball-bearing. The switch causes the stopwatch to start and this circuit is broken once the ball hits the trapdoor.

{h = (1/2)gt^2} from {s = ut + (1/2)at^2}

Graph of t^2 against h is plotted. Gradient is 2/g.

Method 2 - Light Gate Method

Object is dropped in between two sets of light gates, one at the top of the drop and one near the bottom. The distance between light gates is height (h), and they measure the time taken for the object to fall.

{v^2 = 2gh} from {v^2 = u^2 + 2as}

Using v = x/t work out velocity. Graph of v^2 against h is plotted. Gradient is 2g

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## Core Practical 2 - Resistivity

Measure the diameter of the wire using a micrometer screw gauge at 3 places along the wire and calculate an average. Attach the circuit to the wire, having placed the wire on a metre rule. Current (I) and voltage (V) is measured for a range of lengths (L).

Calculate the cross-sectional area using the diameter.

Using R = V/I, calculate the resistance for the range of lengths.

Graph of R against L. Gradient is R/L, multiplied by area gives resistivity.

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## Core Practical 3 - E.M.F and Internal Resistance

Set up a circuit with a cell, ammeter, variable resistor and a voltmeter across the cell.

Measure the current (I) and potential difference (V) for varying levels on the variable resistor, starting at the highest value.

Rearrange {V = ε - Ir} to {V = -Ir + ε}

Graph of V against I. Gradient is -r, and y-intercept is ε.

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## Core Practical 4 - Viscosity

A large measuring cylinder filled with the liquid. Lines are drawn on the cylinder at intervals of 5cm. Measure diameter of the ball-bearing using micrometer screw gauge, then radius of the ball bearing found. Density of the liquid must be known, or measure by using mass/volume.

A small ball bearing is dropped into the cylinder. Measure time taken for the ball to fall between each level of 5cm. Timings repeated 3 times.

Terminal velocity is calculated by dividing distance between markers by average time taken for the ball to fall between them.

Variables sustituted into this equation to find viscosity:

η = [2({solid} ρ - {fluid} ρ)gr^2] / 9v

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## Core Practical 5 - Young's Modulus

Set up a clamp with two wooden blocks on a desk, and a pulley at the other end of the desk. Attach the wire between the blocks and over the pulley, having a marker on the wire. Have a metre rule next to the wire with 0 at the marker.

Measure the diameter of the wire using a micrometer screw gauge, and thus find the radius.

Measure the distance (x) between the wooden blocks and the marker.

Add masses to the end of the pulley and measure extension (Δx) for a range of masses.

Find force using F = mg for the range of masses.

Stress = Force/Area

Strain = Extension/Original Length

Graph of stress against strain. Gradient is Young's Modulus.

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## Core Practical 6 - Speed of Sound

Signal generator connected to a speaker and one of the inputs to the oscilloscope. A microphone is also connected to the other input of the oscilloscope. When the signal generator is turned on, the oscilloscope shows two waveforms - one represents the sound from the speaker and the other from the microphone.

The microphone is placed on a metre rule where the waveforms are first in antiphase. This is shown by the oscilloscope. This distance is recorded. The microphone is moved again along the ruler until the waveforms are in antiphase again. This distance is 1 wavelength.

This is repeated for a range of frequencies, creating a table of frequency and associated wavelength.

Graph of λ against (1/f). Gradient is speed of sound in air.

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## Core Practical 7 - Standing Waves

Signal generator connected to a vibration generator. A string is tied onto a retort stand, and connected to the vibration generator. String is hung over a moveable bridge and it hangs over the edge of the desk over a pulley.

Method 1 - Frequency and length

Record the frequency displayed on the signal generator for a range of lengths. Mass on the pulley remains constant.

Graph of f against 1/L. Gradient is v/2. Straight line through origin shows frequency is inversely proportional to length.

Method 2 - Frequency and tension

Record frequency displayed on the signal generator for a range of masses on the pulley. Force or tension is calculated using F = mg.

Graph of f^2 against T. Gradient is (λ^2)/μ. Straight line through origin shows frequency is proportional to the square root of tension.

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## Core Practical 8 - Diffraction

Set up a laser to shine on a screen, with a diffraction grating in front of the laser. Measure distance (D) between the grating and the wall.

Mark the central maximum, where the light goes straight through the grating. Mark the 1st order, 2nd order and 3rd order maximum on the screen.

Measure the distance (x) between the central and each other maximum. Find angle θ using tan θ = x/D.

nλ = dSinθ

Substitute values in to find the wavelength.

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## Core Practical 9 - Impulse

Set up a trolley, with card of length L, attached to a pulley over a desk with a hanger with a slight tilt. Set up one light gate which the card will pass through.

Start with all of the masses on the trolley, and record the time the card takes to pass through the light gate (t) and measure time taken from release to the light gate (T). Repeat three times for this mass and find average times for t and T.

Move one mass to the hanger, and repeat the recordings until all the masses are on the hanger.

The force for each recording = (mass on hanger)(g)

Velocity for each recording = (L) / (average t)

FT = Δp

(mass on hanger)(g)(T) = (mass of trolley + mass ontop of trolley)(velocity)

Graph of mT against v, gradient is M/g.

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## Core Practical 11 - Capacitance

Set up a circuit with a power supply connected to a capacitor and resistor by a switch, so that when the switch is turned, the capacitor is connected to the resistor only. Attach a voltmeter across the resistor.

Charge the capacitor and record the voltage (V0). Turn the switch and discharge the capacitor through the resisitor, and measure the time taken for the voltage to decrease every 0.5V until it stops decreasing.

Graph of V against t should show exponential decrease.

Graph of lnV against t, y intercept of lnV0, gradient of -1/RC.

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## Core Practical 12 - Calibrating a Thermistor

Set up an ohmmeter in series with a thermistor. Pour boiling water into the beaker and put the thermistor and thermometer into the beaker. Record the initial temperature and resistance on the ohmmeter. Add ice cubes to the water, until the temperature decreases by 10oC. Record the resistance and repeat until the temperature is 0oC.

R = R0eb/T so InR = b/T + InR0

Plot lnR = b/T + lnR0. This is the calibration graph.

Use the calibration graph to find the resistance at a given temperature, and use to set up a potential divider circuit using:

Vout = Vin + R1/(R1 + R2)

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## Core Practical 13 - Specific Latent Heat of Water

Wchange temperature of water = Wice to water + Wchange temperature of ice

mwcΔTw = miL + micΔTi

mwc(ϴ1 - ϴ2) = mi+ mic(ϴ2 - 0)

Measure mass of water (mw), measure initial temperature of the water ϴ1, measure the mass of the ice (mi). c = 4182J/kgoC

Crush the ice (to make melt faster) and funnel the crushed ice to make sure no water is present in the ice.

Drop the ice into the water and stir. Measure the lowest temperature the water reaches (ϴ2).

Put values into equation and rearrange to find L.

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## Core Practical 14 - Boyle's Law

Fixed mass of gas trapped by oil in a sealed measuring cylinder with fixed dimensions.

Increase pressure of gas using pump. Measure the pressure on gauge and volume of air from cylinder. Level of oil will rise and air will compress.

Plot pressure against 1/volume. Straight line through origin, gradient nRT. (p = 1/volume x nRT)

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## Core Practical 16 - Oscillations

Attach known masses to a mass-spring system and measure the time (t) it takes for each mass to oscillate 20 times. Find T = t/20.

As T = 2π√m/k, T2 = (4π2m) / k.

Graph of T2 against m, straight line throught the origin, gradient 4π2/k.

Time an unknown mass to oscillate 20 times and find its T. Use the graph to find its mass.

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