HSC Physics Binding Energy

One of the most misunderstood concepts in the quanta to quarks section of the current NSW HSC Physics syllabus is binding energy. Listed below are some tutorial points on this concept. 

1. The binding energy of a nucleus is the work that must be done on the separate nucleons to assemble them from an initial state where they do not influence each other into the final nucleus.
2. Binding energy has a negative sign.
3. A lone proton or neutron has no binding energy.
4. The total energy of a nucleus is the sum of the rest energy of each component particle and the binding energy of the nucleus.
5. Energy is released in a nuclear reaction when the product nuclei are more strongly bound (more stable) than the initial nuclei (less stable).

HSC Physics Band Theory

One of the most difficult and misunderstood concepts in the ideas to implementation section of the current NSW HSC Physics syllabus is band theory. Listed below are some tutorial points on this concept. 

1. Bands are not the same as the orbits of electrons around a nucleus (these are called shells).
2. Bands refer to an entire material not a particular atom.
3. The valence band is the set of possible energy states an electron can have if it is bonded to an atom. In a full valence band conduction is not possible as there are no states available for an electron to move to (no net flow of electrons).
4. The conduction band is the set of possible energy states that an electron can have if it is free of an atom. In a full conduction band conduction is not possible.
5. In a conductor at room temperature there is a partial overlap of the valence and conduction bands. The valence band is full and the conduction band is partially filled. Electrons can be moved easily by an applied electric field and enter nearby unoccupied states in the conduction band. In a semiconductor at room temperature there is a small energy gap between the valence and conduction bands. The valence band has some unoccupied states and some electrons occupy states in the conduction band. An electron in the valence band can gain thermal energy and enter the conduction band creating a hole in the valence band that behaves like a positive charge carrier. In an insulator there is a large energy difference between the valence and conduction bands. The valence band is full and the conduction band is empty.

HSC Physics Simultaneity

One of the most difficult and misunderstood concepts in the space section of the current NSW HSC Physics syllabus is simultaneity. Listed below are some tutorial points on this concept. Two sentences from Einstein's own popular exposition on relativity, Relativity, the Special and General Theory, Methuen, 1920, are given italics.

1. A frame of reference is a measurement system (coordinate system) where time and position values of an event can be determined. A railway platform is a frame of reference. A moving train is a frame of reference. A person on the platform makes measurements in the platform reference frame. A passenger on the train makes measurements in the reference frame of the train.
2. Two events are simultaneous in a reference frame if each has the same time coordinate in this reference frame.
3. Every reference body (coordinate system) has its own particular time; unless we are told the reference body to which the statement of time refers, there is no meaning in a statement of the time of an event.
4. Events which are simultaneous with reference to the embankment are not simultaneous with respect to the train, and vice versa (relativity of simultaneity).
5. Suppose that two simultaneous events (such as lightning strikes) occur at the different points x1 and x2 in the reference frame of the platform. The times at which these events occur in the reference frame of the train are not the same as those in the reference frame of the platform. The time interval between these events in the platform reference frame is zero. The time interval between these events in the reference frame of the train is -v/c²(1-v²/c²)^-1/2(x2-x1). This expression shows that the time interval between two events depends on the location of the events and the relative speed of the reference frames. The factor c is the speed of light in a vacuum, this being the same in all inertial (non accelerating) frames of reference.

HSC Physics Back EMF

During the next 4 weeks I will list the most misunderstood concept in each of the four sections of the current NSW HSC Physics syllabus. Back emf is the most misunderstood concept in the motors and generators section. Here are some tutorial points on back emf.

1. Look at a demonstration motor and a generator. Draw diagrams of these labelling the parts.
2. When the magnetic flux through a coil changes an emf is induced in the coil.
3. Imagine a coil of wire (solenoid) carrying a constant current. As the current flows through the windings it makes a magnetic field that passes through the coil. If the current is turned off it does not drop to zero immediately. For a short time the current keeps flowing at a value that exponentially falls towards zero. During this time interval an emf is induced in the coil due to the changing magnetic flux through it causing the current to "keep going" for a short time, opposing the change that is happening. Sketch graphs showing (A) the current flowing in the coil versus time, (B) the potential difference across the coil versus time.
4. As the coil of a motor rotates in a magnetic field the changing magnetic flux through it induces an emf in the coil in accordance with Lenz's law. This emf tends to make an induced current in the opposite direction to the current supplied to the coil and is called a back emf.
5. When a motor is first turned on the current in the coils is large as there is no back emf. As the coils gain speed the back emf increases and the current in the coil decreases.