High school physics FAQ

High school physics poses questions from the profound to the peculiar. This page collects frequently asked questions from the high school physics forum created for students studying "HSC Physics" in the state of New South Wales, Australia. If you wish to add questions or to extend answers, please do so via that forum. We also maintain a web site of high school physics resources for the NSW syllabus. Other questions may be addressed to J.Wolfe@unsw.edu.au


Relativity

For background, see our web sites
Einstein Light: relativity in 10 minutes... or 10 hours
is our contribution to the 100th anniversary of relativity. It has a set of multimedia presentations of some of the key points, and a large set of web pages going into more detail.

Inertial and non-inertial frames of reference.

"We are asked to perform an investigation between non-inertial and inertial frames of reference. (As in to do an experiment in the real world to see whether we are in inertial frame or non-inertial frame.)"

Centripital forces and inertial frames

"My physics teacher tells me that when I go around a sharp curve in my car, there is no force causing me to move away from the center of cuvature. So what is happening to make me feel as if I am sliding towards the outside?"

The two airplane version of the twin paradox: is General Relativity involved?

(See the twin paradox for an introduction.) "We were discussing proof of special relativity/time dilation in class and used, as an example, the idea of a clock being taken on a fast plane having time run slower than an identical clock left on the ground. The suggestion was made, however, that this plane would really be accelerating (circling the earth) and would therefore be in a non-inertial frame of reference and we would need to use general relativity!"
    Yes, you're right. The fast plane (the one going East) is subject to General Relativity (GR) corrections (a) because of its variable altitude in the Earth's gravitational field and (b) because of the centripital acceleration. For the plane going West, the centripital acceleration is much smaller: roughly speaking, it stays between the Earth and the sun while the Earth rotates below it. (at temperate latitudes this is possible). The gravitational field term would be rougly the same for both, but not exactly, because East and West bound planes often fly at different altitudes to take advantage of or to avoid the jet stream.

    So for both reasons (gravity and accelerating frame, which are locally indistinguishable according to the Principle of GR), there is a GR correction. The gravitational term is of the same order for both planes. In fact, the gravitational and SR terms turn out to be of comparable size: both are hundreds of nanoseconds. The acceleration term is smaller than the gravitational term. (The acceleration and gravitational terms would be comparable for a satellite, but planes travel much more slowly than near-earth satellites. The SR and gravitational terms are comparable for an object on the Earth's surface.)

    So yes, an explanation of the time difference in the two clocks requires either an explicit calculation of the two terms. The original report is: J.C. Hafele and R. E. Keating, Science 177, 166 (1972). In fact, the GR terms, while of the same order as the SR terms, are fairly similar for the two planes. So the main effect is the SR effect, and it is in agreement with SR calculations.

sketch of planes and the Earth

Seen from a non-rotating frame of reference, above the South pole, the Eastward flying plane has its speed plus the speed of the ground (the atmosphere travels with the Earth, to a good approximation). The Earth turns below the Westward flying plane. At sufficiently high latitude, it can stay in the same time zone for the entire flight.

Analyse and interpret some of Einstein's thought experiments involving mirrors and trains and discuss the relationship between thought and reality.

Mass defect

The syllabus says: "Explain the concept of a mass defect using Einstein's equivalence between mass and energy" How?

Mass dilation

"What is mass dilation?"

Relativity and space travel

"What are the implications of mass increase, time dilation and length contraction for space travel?"
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Space travel

See also the site
Space on the UNSW HSC site for background.

Please explain why the forces acting on an astronaut increase to approximately |3W| during the intial periods of launch.

"Weightlessness"

What is the advantage of setting PE = 0 at r = infinity instead of having, lets say, the centre of the earth to be zero?

The syllabus says 'Define gravitational potenial energy as the work done to move an object from a very large distance away to a point in a gravitational field.' Can anyone explain this? Why do we define gravitational potential energy?

We are supposed to discuss the relative energy costs associated with Space Travel.

"The main energy cost associated with space travel is currently fuel to reach Low Earth Orbit. Energy is also needed to leave this orbit, change direction/ accelerate and for communication. What else is there to say?"

What are the advantages and disadvantages of microwave and radiowave as effective communcation in space travel? Does a bullet that is shot straight up return to the ground at the same speed and if so, why? Return to top of page and menu


Astrophysics

Question: Recall the components of the electromagnetic spectrum and describe the properties of each component. Explain why some of these wavebands can only be detected from space-based observations. Question: Define the terms "resolution" and "sensitivity". New generation telescopes. What is the impact of astrophysics on society?. Parallax, resolution, Airy disc, parsecs, distances Return to top of page and menu

Cepheid variables---measuring longer distances

The most distant objects are reported to be about 13 billion light years away, and the universe is said to be 14 billion light years away. What stops us seeing further?

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The atom, photoelectric effect, energy levels, quanta, black body radiation,.

How does the quantisation of emitted radiation explain the black body radiation curve? Why does it have a peak?What is Wien's law?

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The photoelectric effect

"How is the photoelectric effect used in the following: breathalysers, solar cells and photocells?" Could you please tell me about the relationship in solar cells among the photoelectric effect, semiconductors, electric fields and current? Return to top of page and menu

By thinking that electrons behave like waves, how does it help to explain that the accelerating particle does not give out energy?

Electron microscope

What is magnetic diffraction and focussing of electron beams? What are the differences in resolving power between optical and electron microscopes?

Heisenberg's Uncertainty Principle

Pauli's exclusion prinicple

The Zeeman effect, and why the Rutherford atom doesn't account for it

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How did Chadwick and Fermi's work change our understanding of the atom?

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The Bohr-de Broglie-Sommerfeld atom and the Hydrogen spectrum

The spin quantum number

What does "spin" refer to in particle physics? And why is this a necessary concept?

Accelerators as probes of nuclear structure

Can you please explain why accelerators are used to probe into the structure of matter?
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Semiconductors, transistors, solar cells etc

What are n-type and p-type semiconductors?

How do diodes and transistors work?

Transistors as amplifiers and logic gates

History of the invention of the transistor.

What was the impact of the invention of transistors, microchips and microprocessors on society?

More about transistors, computers

Can you provide more resources for teaching the Age of Silicon (NSW syllabus topic)?

Solar cells and the photovoltaic effect

Could you please tell me about the relationship in solar cells between the photoelectric effect, semiconductors, electric fields and current?
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States of matter

The syllabus asks us to recall the states of matter and their properties and debate whether superconductivity is a new "state"

Bose-Einstein condensates

Why is large wavelength so imporant? How does this tie in with quantum mechanics?
Are Bose-Einstein condensates a new state of matter. Return to top of page and menu

Superconductors

How do superconductors work?

What is a phonon?

What causes the lattice distortions in a superconductor?

Levitation of a magnet by a superconductor

The Meissner effect

Superconductivity and computers

I've read several books now all describing superconductors, if used in computers, will allow them to operate at higher speeds. None of them describe how this actually happens.

Applications to magnetic fields, motors, power distributions, MRI

I'm finding it really difficult to find any info of "the effects of those applications [of superconductivity] on computers, generators, motors and transmission of electricity through power grids".
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Nuclear physics, radioisotopes, neutrinos etc

What are some of the industrial and medical applications of radioactivity and nuclear physics?

and how is this property utilised in medicine and industries?

How are isotopes used in engineering or agriculture?

How are isotopes used in agriculture?

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What are the links between high energy particle physics and cosmology?

What is a neutrino, what is an anti-neutrino?

How is beta decay described in terms of quarks?

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Medical Physics

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Magnetic focusing


Motors and Generators

See
Motors and generators for a background to this topic. There are descriptions and diagrams of the main classes of motors.

Analyse the environmental impact of the development of AC generators.

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Force between two wires

The HSC physics syllabus, in the Motors and Generators topic, asks us to "solve problems and analyse information about simple motors using F/L = k.I1I2/d". I have never seen a problem relating this equation to motors. Can you suggest any? What relevance does this equation have to the operation of a simple motor? Does the force between current carrying wires on either the same or opposite side of the coil effect the torque? What is the implication of the force between current carrying wires for power distribution networks? Return to top of page and menu


Transformers

This section includes transformers, power lines, induction cooktops, eddy current switching and regenerative braking. See
Transformers for background.

Explain why voltage transformations are related to conservation of energy

Transformers in electricity supply and the home

How is heating caused by eddy currents in transformers overcome?

How does the principle of induction apply to cooktops in electric ranges?

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How have eddy currents been utilised in electromagnetic braking?

How are eddy currents used in switching devices?

Eddy current braking

Electromagnetic current braking is smoother, but why is it an advantage over conventional braking? Does it brake smoother in less time and less distance then conventional braking?
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Oscilloscope

Could someone please explain to me the timebase properties in CRO. Does it relate to horizontal or vertical movements?

Simple Harmonic Motion under gravity


Drift velocity

There is a page about
drift velocity and Ohm's law that introduces this topic. The text below is a summary of several questions about drift velocity of charge carriers in a conductor. It arises from a peculiar and confusing statement about drift velocity in the physics syllabus in New South Wales, Australia, and two multiple choice quesions in specimen papers in which insufficient information was given to allow one to answer. Briefly, if you are not studying in a NSW high school, you don't need to read this section.

Advice to students trying to answer a question to which there is no correct answer Return to top of page and menu


Miscelleaneous questions in history and social studies

The Einstein-Planck debate The Nobel prizes in physics What has been the impact of advances in understanding of matter on work of physicists ? Return to top of page and menu


2001 HSC Paper on Physics

Question 4 caused a lot of feedback. Was it B or C? Other problems with the 2001 paper

2003 HSC Paper on Physics

Question 5
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Short answers suitable for the high school syllabus in NSW

Glossary and skill list

Hints on doing tests

Other useful links

Thanks to all those who asked the original questions, and to John Storey for comments on the original version of this page. However, the opinions expressed here are those of
Joe Wolfe / J.Wolfe@unsw.edu.au  phone 61-2-9385 4954 (UT + 10, +11 Oct-Mar).


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Happy birthday, theory of relativity!

As of June 2005, relativity is 100 years old. Our contribution is Einstein Light: relativity in 10 minutes... or 10 hours. It explains the key ideas in a short multimedia presentation, which is supported by links to broader and deeper explanations.