physics ATAR

Explain the role of an iron core in a transformer.

The iron core increases the efficiency of the transformer. Being a ferromagnetic material, it increases the magnetic flux linkage between the primary and secondary coil.

If students have a motor, and they apply a force to the handle in such a way that the coil rotates with a constant circular speed. Explain why this force cannot have a constant magnitude. Describe how its magnitude changes as the coil rotates.

As the coil rotates an EMF is generated that opposes the direction of its rotation. When the coil is perpendicular to the field, no EMF is generated in the coil. When the coil is parallel to the field, the maximum EMF is generated in the coil. Hence, the magnitude of the force varies from a maximum when the coil is parallel to the field to zero when the coil is perpendicular to the field.

A stunt man on a bicycle is attempting to navigate a vertically circular loop-the-loop. When the cyclist enters the bottom of the loop-the-loop after a full circuit, they observe that they 'feel heavier' than they usually do. Explain.

Apparent weight is due to the normal force exerted on the cyclist from the track. When entering the vertical loop, a net upward force is required to produce a centripetal force in order to move in a circle. This additional force must be provided by the normal force from the track. Since the normal force from the track increases, so will the apparent weight.

Chandra's images and extra data gathered from other telescopes indicated that these flares were cooling down over time. In terms of black body radiation, explain how the data illustrates evidence of cooling.

Over time, the evidence shows that the emissions from the flare moved from the x-ray region to the infrared and then to the radiowaves region. This indicates that the peak wavelength of the flares emissions had increased: an increase in the peak wavelength of black body radiation indicates a decrease in the temperature of the black body.

What is the ultraviolet catastrophe?

Classical physics predicted that for black body radiation, the energy of radiation was high at shorter wavelengths and became infinite around the ultraviolet spectrum. However, the observed spectrum showed a peak of energy in the visible spectrum then a sharp fall at lower wavelengths. Classical physics' inability to explain this was called the ultraviolet catastrophe.

Explain what is meant by the term "polarised" and describe whether this is evidence of the wave or particle nature of light.

Polarised means that the electromagnetic wave is oscillating in a single orientation. Polarisation confirms that light behaves as transverse oscillations of the electromagnetic fields providing evidence for the wave nature of light.

What is AU?

Astronomical Units- a unit of length equal to the average distance between Earth and the Sun.

Electric current

The direction of flow of positive charge

Faraday's Law

A changing magnetic flux through a loop will induce an EMF

Lenz's Law

A changing magnetic flux induces an EMF (Faraday's Law)

The induced EMF drives a current

The induced current creates a magnetic field

The newly formed magnetic field has a direction that opposes the changing magnetic flux.

How to find the expected mass (kg) of a substance given the mass on the standard model table (mass is given in MeV/c^2).

The expected mass in kilograms is equal to the value on the standard model table multiplied by the charge of an electron, divided by the speed of light squared

Electromagnetic radiation of a particular wavelength is shone onto some sodium metal and current reading is measured by the ammeter. The intensity of the incident electromagnetic radiation is slowly increased whilst the wavelength remains constant. Describe and explain what happens to the current measured by the ammeter.

Increasing the intensity of the incident electromagnetic radiation without changing the wavelength will increase the number of photons incident on the metal surface. This will increase the rate of production of photoelectrons and, hence, increase the reading on the ammeter.

Electromagnetic radiation of a particular wavelength is shone onto some sodium metal and current reading is measured by the ammeter. The intensity of the incident electromagnetic radiation is slowly increased whilst the wavelength remains constant. The intensity of the incident electromagnetic radiation is returned to its original value and its wavelength is continually increased. Describe and explain what would be observed in the ammeter over time.

Increasing the wavelength of the incident electromagnetic radiation will decrease its photon energy. This will decrease the kinetic energy of the photoelectrons and, hence, decrease the reading on the ammeter. Eventually, the frequency of the incident radiation will become less than the threshold frequency and zero photoelectric current will be detected in the ammeter.

Explain how an airplane can create a 'weightless' environment at the top of a vertical circle.

If the weight is equal to the centripetal force, the normal force will be equal to zero.

If all other factors were kept equal, would an increase in AC frequency from 50Hz to 60Hz increase or decrease the power loss in a transformer due to eddy currents?

Increasing the AC power frequency will also increase the rate of change of flux due to the alternating currents in the transformer coils. Increasing the rate of change of flux will increase the size of the eddy currents generated in the iron cores. Hence, power losses due to eddy currents will also increase.

The James Webb Telescope is tuned to infrared radiation that is emitted by extremely distant luminous objects. To be able to detect this faint IR radiation, the telescope needs to be cooled to a very low operating temperature of 50K. Suggest a reason for this.

The infrared radiation from different galaxies will be very low intensity. At greater than 50K, the James Webb Telescope, will emit blackbody radiation in the infrared region. This will overwhelm the faint incoming infrared signals from distant objects. By cooling the James Webb Telescope to 50K or less, the James Webb Telescope will emit body radiation in the radio waves region and not interfere with the faint incoming infrared signals.

Explain why conducting astronomy in the infrared region of the electromagnetic spectrum will allow the James Webb Telescope to view the universe as it was 100 million to 250 million years after the Big Bang.

The Universe is expanding and eventually stars and galaxies formed. The light emitted by these first stars and galaxies is still travelling to us and has been redshifted into the infrared region.

Explain how the Cosmic Microwave Background Radiation is evidence supporting the Big Bang theory.

According to the Big Bang Theory, the universe was filled with plasma which absorbed and scattered any radiation travelling through it. Eventually after the Big Bang, atoms began to form, and radiation was no longer absorbed or scattered- the cosmic background radiation is a remnant of this radiation.

The original temperature of the universe would need to be very high, thus the universe would have given off blackbody radiation in all directions.

Explain what is meant by 'mass-energy equivalence'.

E=mc^2. An object at rest has energy associated with it. This means that mass can be converted into energy and vice versa.

Split ring commutator (motors)

Reverse the direction of the current through the coils

Carbon brushes (motors)

Electrical contact between commutator and emf source with limited frictional resistance.

Armature coils/rotor coils (motors)

Carry the current through the magnetic field to cause a rotation

Slip rings (AC generator)

Rotates with the armature coils to provide a frictionless electrical contact with the external circuit. Allows the current entering the external circuit to remain AC.

How to calculate average EMF of a generator.

The average EMF is calculated by considering a quarter turn of the coil where the flux goes from maximum to zero. Find the time to do a quarter turn. Then use in equation

ε=-NΔφ/t

Eddy current

A circular current induced in a conductor by a changing magnetic field.

Since this induced magnetic field will oppose the changing magnetic flux, it creates a force that opposes the motion that created the changing magnetic flux.

Eddy currents cause energy losses in the form of resistive heating in the core.

Limiting transmission power loss

Use a step up transformer before transmitting (this steps down the current)

Use a step down transformer close to the destination (this steps up the current)

Peak output voltage of a transformer

Find the output voltage using the Vp/Vs=Np/Ns. This is the rms value. So to find the peak voltage substitute this into the E(rms) value.

De Broglie wavelength

Indicates the length scale at which wave-like properties are important for that particle.

Students in a physics laboratory launch plastic discs across an aluminium air table. Air is blown vertically through small holes in the surface of the table, allowing the discs to float above the surface as they move. This is a nearly frictionless environment and the discs barely slow down as they cross the table. The students then attach a small but strong magnet on top of a disc and repeat the experiment. The disc slows down quite quickly, even though there is still no contact between it and the table.

Explain why the disc with the magnet slows down quickly.

aluminium is a conductor. The changing magnetic field due to the magnet moving induces eddy currents in the aluminium. Lenz's Law states that an induced current will flow in a way to oppose the change producing it. The disc experiences an opposing force so it slows down.

In Hubble's early data, he noticed that one particular spiral galaxy close to the Earth, seen edge on, had two values of v at its extremes. One was positive and one was negative. Assuming this was not an instrumental or human error, explain how this could occur.

Positive value of v means galaxy is moving away from observer. Negative value of v means light is blue shifted and galaxy is moving towards the Earth. This means galaxy is swirling where one side is coming towards the Earth. Rotational speed of side of galaxy must be greater than recessional velocity of galaxy.

Explain why increasing the strength of the magnetic field would increase the velocity of the particles leaving the cyclotron.

r = mv/Bq 1 as B increases, r decreases. If r decreases, the number of revolutions before exiting increases. The particle accelerates every time in passes between the magnets so its velocity will be greater when it exits.

Explain why the voltage across the magnets in a particle accelerator must alternate.

In order to accelerate across a potential difference, the plate opposite must have the opposite charge to the particle to attract it. 1 When the particle approaches the electric field from the opposite direction, the polarity of the field must change for the acceleration to remain positive.

Explain why a step-up transformer is used to increase the voltage before transporting the electricity.

It is more efficient to step-up the voltage. P = VI so if we increase V we have less current for the same power. P lost = I^2 R power is lost as heat. The lower the current, the lower the power lost due to heat.

State two main differences between hadrons and leptons.

Hadrons are made of quarks (not fermions), leptons are fundamental particles (fermions).

Leptons are generally less massive than hadrons.

Hadrons are subject to the strong nuclear force that holds the nuclei together, and leptons are subject only to the weak nuclear force

A bobo doll can never be tipped over. Even if its head is held on the ground, it will stand back up when released. Explain how this works. You must include in your answer the relevant conditions required for static equilibrium.

Person holding doll down provides a balancing torque to keep the doll in static equilibrium 1 When opposing moment holding head on the ground is removed, the restoring moment of the weight force rights the doll

What conditions are required to be in static equilibrium?

For an object to be in static equilibrium, it must be in both translational and rotational equilibrium.

How does the study of Cosmic Background Radiation provide evidence that the universe is expanding?

Originally the radiation given off would have had much shorter wavelengths and higher energy. As the universe expanded, so did the wavelengths of this background radiation and it became less energetic microwaves.

Dispersion

The separation of light into its component wavelengths (supports the wave behaviour of light)

Explain why aeroplanes need to increase their speed to maintain altitude when banking.

Upthrust counters weight force.

Upthrust is directly proportional to forward speed. Upthrust acts perpendicular to wings.

Only vertical component of upthrust counters weight therefore needs to increase speed to maintain altitude

List two properties of X-rays that make them suitable for x-ray spectrometry.

High energy

High penetrative ability

Small wavelength

Explain how we know that a painting had been edited.changed by the artist (through x-ray spectrometry)

Depending on its energy, the X-ray beam penetrates into different depths of the painting's surface. This allowed us to examine different layers of paint

Give two advantages of using X-ray spectrometry to analyse old paintings.

Expensive paintings can harmlessly be investigated before restoration. It also allows the paintings to be restored securely in the buildings where the public can still enjoy them.

Explain why the fluorescent radiation is element specific.

Energy level values are dependent on Z, the charge on the nucleus. Z is directly proportional to the number of protons in the nucleus, which defines the element. Fluorescent wavelengths correspond to energy transitions; therefore are element-specific.

What does increasing the power of an x-ray beam do?

It means more photons were second are hitting the target, thus increasing the intensity of photons emitted.

A simple AC generator is shown in the diagram below. A coil is manually rotated in a fixed magnetic field, producing an alternating current in the external circuit. Explain how the alternating current is produced.

As the coil rotates, each side travels up or down, cutting across the magnetic flux present.

The current induced in each side of the coil changes direction every half turn of the coil.

Each side of the coil is attached to the external circuit by a slip ring.

The slip ring maintains sliding contact with the carbon brushes so the current in each ring changes direction every half turn, which produces an AC current.

Two people have identical 1m rulers. When one of them sets off into a rocket, her mate on earth measures the length of her ruler from earth and finds that its only 0.6m long.

How are they able to explain these seemingly contradictory results?

To successfully measure the length of the ruler moving relative to them, they determine the position of the ends of the ruler at the same time and measure the distance between these two positions.

Each thought the other's measurements were not made simultaneously.

Therefore they both measure the other's ruler as a different length to 1.00 m.

One of the features that increases efficiency in a step up transformer is the laminated soft iron core. Explain why laminating the core increases the transformer's efficiency.

Changing magnetic fields induce eddy currents in the metal. This heats up the metal and power is lost. A laminated core reduces the magnitude of the eddy currents and therefore reduces power lost as heat.

Explain why transformers require AC current to function in electricity transmission.

A current-carrying conductor has a magnetic field around it. An alternating current produces an alternating magnetic field. This changing magnetic field induces an EMF in the secondary coil. 1 The EMF allows current to flow in the transmission lines.

Swinging a ball on a string in a horizontal circle. Explain why θ can never equal 90.0°, regardless of how great the frequency of the pendulum becomes.

Tension consists of two components, vertical and horizontal.

The vertical component counters mg.

When angle = 90°, the vertical component = 0.

This means there is nothing holding the weight of the 'bob', which is impossible.

Photoelectron

The ejected electron resulting from ionization during the photoelectric effect

Strong Nuclear Force

Holds nucleus together

Very strong

Small range

Gluon

Electromagnetic Force

Electric and magnetic field

Relatively strong

Infinite range

Photon

Weak Nuclear force

Causes radioactive decay

Relatively weak strength

Mid range

All W and Z bosons

Gravitational Force

Attraction between 2 masses

Very weak

Infinite range

Graviton

Meson

Consists of a quark and an antiquark. Eg a pion

Baryon

Consists of 3 quarks typically. Eg a proton

Lepton

A fundamental particle that is not affected by the strong nuclear force.

Torque of a motor

Multiply end result by two because there are two sides of the motor rotating.

Describe how, and under what circumstances, electrons are liberated from the target by incoming photons.

The incoming photons are completely absorbed by electrons in the target metal. If the photon energy is larger than the work function of the metal, an electron is released from the metal.

Discuss how the maximum kinetic energy of the liberated electrons is experimentally determined in the photoelectric effect.

a reverse potential is applied which turns the anode into a negatively charged cathode. As the reverse potential increases, fewer electrons have the necessary kinetic energy (KE) to reach it. When KE = Vq, 0 current is recorded. This is the maximum KE of a liberated electron.

Lenz's Law

The direction of the induced current by a changing magnetic field is such that the magnetic field created opposes the changes in the initial magnetic field.

How to describe a magnetic field around a magnet, specifically the north pole.

The magnetic field around the magnet is not uniform.

If an object is not accelerating...

If an object is not accelerating it means that all of its forces are balanced.

Explain why the magnitude of the velocity of B as measured by A would be the same as that of A measured by B, only in the opposite direction.

There is no preferred frame of reference.

Each spaceship sees the other receding from them.

Due to symmetry |v| will be the same.

Explain how one proton and one pion can be converted into one proton and five pions.

E=mc^2

Energy and mass are interchangeable. Some of the energy of the incoming pion is converted to the mass of the extra pions.

Pion

A meson that consists of an up or down quark and an up or down antiquark

Explain how inserting a third filter for polarised panels allowed light to hit the screen when no light was hitting it before.

The polarised light hitting the third filter strikes the axis of polarization at 45.0°.

A component of the light is transmitted to the last screen at 45.0° to the axis of polarisation.

Therefore a component of light emerges from the last filter and hits the screen.

A photon's energy is given by E = hf. When light passes through a polarising filter, the total energy transmitted is reduced but the frequency of each photon remains the same. Using the particle model of light, account for the reduction in transmitted energy.

The total amount of energy is proportional to the number of photons emerging per second.

The intensity is reduced by reducing the number of photons per second rather than reducing the energy of each photon.

Therefore the frequency and hence wavelength of each photon is unaffected by polarisation.