Physics Recipes

Demonstrating Hooke’s Law

Required: stand, clamp, spring, masses, ruler

Measure length/position of bottom of spring at start

Add masses/weights one at a time

For each mass/weight, measure the new length/position

Calculate extension by subtracting each length from original length

Plot force vs extension: straight line section at start shows Hooke’s Law

Producing current-voltage graphs "(filament, resistor, diode)

Connect a power supply in series with a variable resistor (or use a variable power supply) with an ammeter and the test component.

Add a voltmeter in parallel with the test component.

Take readings of current and voltage.

Vary the resistor/ power supply.

Record 6 different values of voltage and current.

Plot a graph of voltage (y-axis) against current (x-axis)

Finding a frequency from an oscilloscope screen

Count the number of squares that represents one complete wave.

Multiply this by the value for one square. (eg 0.002s)

This is the Time period.

Frequency = 1/Time Period

Measuring the speed of sound

Measure distance with trundle wheel/tape measure/metre rulers

Person at one end makes a loud sound with clappers/wood blocks/balloon

Person at other end starts timer when they see the sound being made and stops timer when they hear the sound (light travels so fast it is virtually no time in travelling – sound is slower)

Use *s = d/t* to calculate speed of sound

Repeat and average more results

Change ends to allow for wind/make sure each experiment is at same outside temperature.

Determining the refractive index of a medium

Place a block (Perspex/glass) onto a piece of paper and draw around it.

Direct a ray of light from a ray box onto the side of the block at an angle.

Mark the ray of light into and out of the block.

Remove the block and complete the ray lines through the block.

Add a normal line where the ray hits the surface of the block.

Measure the angle between the normal and the ray of light for both the incident and refracted rays.

Repeat for different angles of incidence and plot a graph of sin i/sin r.

The gradient is the refractive index.

OR

Use multiple values of i and r to calculate the refractive index and then find the mean value.

Determining the specific hear capacity of a substance

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Measure the mass with a balance

Measure the initial temperature with thermometer or temperature probe

Record the heating power and energy, perhaps using *P = I x V* or *energy = IVt*

Measure the final temperature

Calculate the temperature rise by subtraction

Calculate the specific heat capacity using Q = mcT

Producing magnetic field patterns (shape only)

Place a bar magnet on a piece of paper.

Sprinkle iron filings on top of the paper.

Tap the paper to align the filings.

Producing magnetic field patterns (direction and shape)

Place a bar magnet on a piece of paper.

Put a compass next to the pole of the magnet.

Mark on the paper where the compass points.

Move the compass to the other side of the mark and repeat the process.

Join the dots up and add an arrow in the direction that the compass points.

Terminal velocity

At start, the only force acting is weight downwards.

There is a resultant force downwards

which causes the object to accelerate downwards

As the object speeds up, air resistance/drag upwards increases

Resultant force is still downwards, but is decreasing in size, so object accelerates at a lower rate (but is still getting faster).

Eventually air resistance/drag builds up with speed until it is equal and opposite to the weight.

The forces are balanced. There is no resultant force.

The object does not accelerate.

This is terminal velocity.

Earthing a mians device

If a fault develops inside the appliance, then *Live* wire might touch the metal case/metal case may become live

*Earth* wire is connected to metal case

Since *Earth* wire has low resistance it allows a large current to flow to the ground

This large current blows/melts the fuse which breaks the circuit

Appliance now isolated from *Live* supply, so no further risk of electrocution.

Convection

A fluid (liquid or a gas) is heated up

It expands

It becomes less dense

The warmer fluid rises or floats upwards

It is replaced by cooler, denser fluid.

As it rises, it may cool down,

then contract, and sink making a convection current.

The expansion is because faster particles push against each other more,

so the distance between them increases.

Gas pressure and temperature: particle motion

Particles hit the walls of the container

Each collision exerts a force on the wall

This makes a pressure on the wall since *P = F/A*

At higher temperatures, the particles move faster,

so the particles hit the sides with more force

and the particles hit the sides more frequently.

Pressure increases

Gas pressure and volume: particle motion

When gas has its volume reduced but at constant temperature,

the average speed of particles is unchanged.

But each particle will collide with the walls more frequently

so the pressure will increase

Electric motor

An electric current flows through the coil

A magnetic field is created around the wires

This interacts with the permanent magnetic field

A force is experienced by the coil

Electric generator or dynamo

The wire moves and cuts the magnetic field lines (flux)

This induces a voltage across the ends of the coil

This voltage pushes a current to flow

Electric generator or dynamo

The wire moves and cuts the magnetic field lines (flux)

This induces a voltage across the ends of the coil

This voltage pushes a current to flow

Transformers

An alternating current is passed through the primary coil.

This causes an alternating magnetic field to be produced in the core.

The secondary coil cuts the magnetic field (flux).

A voltage is induced in the secondary coil.

This voltage pushes a current to flow

Nuclear fission chain reaction

A neutron is absorbed by a uranium nucleus.

the nucleus undergoes fission (splits)

making 2 daughter nuclei, a few neutrons and energy.

These neutrons go on to hit other uranium nuclei,

causing these nuclei to undergo fission and release more neutrons.

Life cycles of small stars

Hydrogen nebulae form in space.

The gas collapses under the attraction due to gravity.

This causes the pressure and temperature to rise.

If the increase in pressure and temperature is enough, fusion will start.

This forms a protostar.

The protostar continues to collect hydrogen.

Eventually the gravitational forces inwards are equal to the radiation pressure outwards.

The star is now in the main sequence.

Once all the hydrogen has fused to form Helium, the radiation pressure reduces and the star collapses under the gravitational forces.

This causes fusion to restart and the star also starts expanding, this forms a red giant

Once the extra stages of fusion have finished the outer layers of the star drift off to form planetary nebula and the core of the star remains as a white dwarf. (small, hot, but not undergoing fusion.)

Eventually the energy will dissipate outwards and the star will become a black dwarf.

Life cycles of large stars

Hydrogen nebulae form in space.

The gas collapses under the attraction due to gravity.

This causes the pressure and temperature to rise.

If the increase in pressure and temperature is enough, fusion will start.

This forms a protostar.

The protostar continues to collect hydrogen.

Eventually the gravitational forces inwards are equal to the radiation pressure outwards.

The star is now in the main sequence.

Once all the hydrogen has fused to form Helium, the radiation pressure reduces and the star collapses under the gravitational forces.

This causes the fusion process to begin again and this creates a layer around the core of the star. This process carries on for a lot longer than in a smaller star. This forms a red supergiant.

Eventually, once iron has been formed in the core, the star will undergo a catastrophic collapse and explosion which will create heavier elements and eject them into space. This is known as a supernova.

The star will then form a neutron star or a black hole depending on its size.