# HSC Physics Revision 2016

The HSC Physics examination is on October 31. From now until this date I will list exam tips to help students maximise their Physics mark. These pages will be updated regularly in the run up to the exam so check back to see the latest updates.

Students can increase their marks significantly during this period if they are positive, organised and have a plan. Remember that the HSC Physics examination is not "difficult Physics". What is required is a steady approach to study with adequate nutrition, sleep, exercise and recreation. Click on READ MORE below for exam tips. Grab the opportunity to increase your mark.

1. Gravitational Potential Energy. Remember that this increases as an object moves away from a planet. The work done by a force in lifting an object at a constant speed is equal to the gain in gravitational potential energy. The confusion arises due to the gravitational potential energy being negative. The gravitational potential energy of an object at infinity is zero. Watch the classic mistake, -2 is not less than -5! Learn the syllabus definition. Remember that the equation change in pe = mgh can only be applied if h is small compared to the radius of the planet. In exam questions use the syllabus equation to calculate gravitational potential energy.
2. Magnetic Force The magnetic force on a current or a moving charge is in a direction perpendicular to both the current direction and magnetic field direction.The right hand palm rule is used to determine the force direction. Remember that a constant magnetic field can do no work on a moving charge and so the speed of the charge is constant in the field. The path of the charge is a circle if the velocity is perpendicular to the field and a helix with a drift component along the field if it enters the field at an angle.In problems with moving electrons the current direction is the opposite direction to the electron movement.
3. Cathode Rays. These are streams of electrons moving from the cathode to the anode in a glass tube containing air at a very low pressure when a high voltage is applied between the electrodes in the tube.They can be deflected with E and B fields and this shows their particle nature. Learn the glass tube experiments that show that cathode rays have momentum and travel in straight lines. If cathode rays travel in a straight line through perpendicular E and B fields then v=E/B. Cathode rays are not green! The light given off when cathode rays strike glass has a wavelength in the green region due to the energy levels of the electrons in the glass atoms.
4. Rutherford Model of the Atom This has a small positively charged nucleus containing most of the mass of the atom with electrons outside of the nucleus. Remember that alpha particles (many students in exams write electrons!) were fired at a thin gold foil and 1 in 8000 of the particles were scattered through angles greater than 90 degrees. Good answers will have a diagram (look this up) showing the scattering but do not have the alpha particle hitting the nucleus! Why?
5. Gravitational Field Define what is meant by a gravitational field. Draw the gravitational field lines around a flat Earth and a round Earth. Plot acceleration due to gravity versus distance from the centre of the Earth. Do not call the weight of an object gravity! The weight of an object is the force acting on a mass in a gravitational field. What is the acceleration of a satellite as it orbits the Earth in a circular path?
6. Split-Ring Commutator Learn how a split-ring commutator in a DC motor works. This is an exam favorite. A common answer is "the split-ring commutator reverses the direction of the current flowing into the coil each half revolution of the coil". This is incorrect. It reverses the direction of the current in each side of the coil every half-turn so that the torque direction changes and so the coil spins continuously. Learn how the split-ring commutator enables the direction of the current leaving an AC generator to be in a constant direction.
7. Diffraction of x-rays A difficult concept is the diffraction of x-rays by crystals as in the Bragg's experiment. Make sure you use diffraction and interference in your answer and relate the atom spacing to the wavelength when constructive interference occurs. State Bragg's equation and draw a diagram showing the symbols. Even though the equation is derived using reflection from successive layers of atoms the effect is really one of interference!
8. Matter Waves are Not Electromagnetic Waves! de Broglie's hypothesis states that associated with a material particle there is a matter wave which dominates when the wavelength is comparable to characteristic distances in the experiment. These waves stay with the particle and do not travel at the speed of light. The resolution of an electron microscope is much greater than an optical microscope as the wavelength of electrons is much smaller than the wavelength of visible light.
9. Projectile Motion. Try this band 5/6 problem. A stone is thrown at an angle of 56 degrees to the horizontal at a certain speed on level ground and its horizontal range is R. Find the range when the stone is thrown at the same angle but at twice the first speed. Neglect air resistance.
10. Long Distance Electricity Transmission. To minimise the ohmic heating in power lines (Pav=I2R) electrical energy is transmitted as AC at the lowest possible current and the highest possible voltage. Students then ask "Pav also equals IV so doesn't this mean that the power losses are large?". No. When we use IV, V is the voltage drop along the power line (potential difference) not the high potential (relative to the ground) at the start of the transmission line.
11. Moving Charge An electron and a proton are released from rest and move through a potential difference V. Compare the final kinetic energy of each particle.
12. Beta Decay An unstable nucleus that decays spontaneously by emitting an electron/positron and a antineutrino/neutrino is said to undergo beta decay. These particles are not in the nucleus, they are created in the process. In the beta decay of a particular nucleus the same amount of total energy is shared between the electon and the antineutrino but in varying proportions. Wolfgang Pauli proposed the existence of the neutrino in 1932 to explain the range of energies of electrons in beta decay and also some "missing" angular momentum. Do not confuse the neutrino with the neutron!
13. Geostationary Satellite Compare the orbtial speed of a geostationary satellite with the rotational speed of a point on the Equator. Is it possible to have a geostationary satellite over Sydney?
14. Slip-Rings In what devices do we have slip-rings? Does an induction motor have slip-rings?
15. Maglev Train Many students write that the Maglev train uses the Meissner effect. This is incorrect. Superconducting electromagnets on the train produce very strong magnetic fields that are "repelled" by the magnetic field produced by electromagnets on the track and so the train experiences an upward force. The Meissner effect is the exclusion of magnetic field lines from the body of a superconductor when it is below its critical temperature.
16. Hyperfine Structure The Quanta to Quarks syllabus in the section on limitations of Bohr's model of the atom has a reference to "hyperfine structure". What does this mean? On close examination the ground state energy level of the hydrogen atom is actually split into two energy levels that are very close together. This is called hyperfine structure. The hyperfine structure is due to an interaction between the spinning proton and spinning electron in the hydrogen atom. When an electron makes a transition from the higher to the lower split energy level if emits a photon of frequency 1420 MHz. This is characteristic of hydrogen and is used to detect hydrogen in our galaxy.
17. Sling-Shot Effect This is often incorrectly applied in examination answers. The gravitational interaction where a small moving spacecraft passes a large moving planet from behind conserving momentum and kinetic energy is called the slingshot effect. The large planet gives some of its momentum to the spacecraft and the force of gravity between the objects deflects the path of the spacecraft so that the speed of the spacecraft relative to the Sun is incresaed by a large amount and the speed of the planet is reduced by a very tiny amount.
18. Lenz's Law Learn Lenz's law carefully. Many students express this poorly in exams. The induced currents in a loop create a magnetic field that opposes the change in external magnetic flux through the loop. See the Wikipedia article "Eddy current brake" for clear diagrams and explanations of magnetic braking. A band 5/6 skill is to determine the circulation direction of the induced eddy currents when a metal plate moves in a magnetic field.
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