Generators and motors

Generators and motors are applications of electromagnetic induction. Figure 2illustrates a simple electric generator. 

Figure 2 A simple electric generator.

The crank represents a mechanical method of turning the loop of wire in a magnetic field. The change in magnetic flux through the loop generates an induced current; thus, the generator converts mechanical energy into electrical energy. The operation of a motor is similar to that of a generator but in reverse. The motor has similar physical components except that the electric current supplied to the loop exerts a torque, which turns the loop. The motor, therefore, converts electrical energy into mechanical energy.

Mutual inductance and self-inductance

Mutual inductance occurs when two circuits are arranged so that the change in current in one causes an emf to be induced in the other.

Imagine a simple circuit of a switch, a coil, and a battery. When the switch is closed, the current through the coil sets up a magnetic field. As the current is increasing, the magnetic flux through the coil is also changing. This changing magnetic flux generates an emf opposing that of the battery. This effect occurs only while the current is either increasing to its steady state value immediately after the switch is closed or decreasing to zero when the switch is opened. This effect is called self-inductance. The proportional constant between the self-induced emf and the time rate of change of the current is called inductance (L) and is given by 

The SI unit for inductance is the henry, and 1 henry = 1(Vs/A).

Using Faraday's law, inductance can be expressed in terms of the change of flux and current: 

Electromagnetic Induction problems

Faraday’s Law

1. (a) A coil of radius 20 cm consisting of 20 turns is held 2 meters above the south

end of a magnet. If the coil is dropped, determine the average induced EMF

(voltage) once the loop hits the magnet if the field strength at the 2 m height

is 0.005 T, and is 0.01 T at the surface of the magnet.

(b) What is the direction of the induced current, as seen from above the coil?

2..

A square loop of copper coil 10 cm on each side is in static magnetic field of

0.005 T perpendicular to the loop. The coil is deformed into a circle having the

same circumference as the square loop. If this shape-change occurs in 5 seconds,

and the coil has a resistance of 1 , determine the induced current in the coil.

3. A 100 turn conducting circular coil of radius 1 cm is placed in a magnetic field

of variable.

3. A 100 turn conducting circular coil of radius 1 cm is placed in a magnetic field

of variable strength B(t) = 0.01t + 0.01 Tesla, which is perpendicular to the

plane of the loop. Determine the induced EMF.

Lenz’s Law

When an emf is generated by a change in magnetic flux according to Faraday's Law, the polarity or direction of the induced emf is such that it produces a current whose magnetic field opposes the change which produces it.

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Einstein's Special Relativity

Inertial Frame of Reference

An inertial frame of reference is one in which

 have no acceleration when no net forces act on it.

The Speed of Light

It is quite natural to ask whether the principle of Galilean relativity also applies to

electricity, magnetism, and optics. Experiments indicate that the answer is no. Recall

that Maxwell showed that the speed of light in free space is 3x10^8 m/s. 

Physicists of the late 1800s thought that light waves moved 

through a medium called the ether and that the speed of light was c only in a special, 

absolute frame at rest with respect to the ether.

ARTIFICIAL GRAVITY - Simulation

(Ref: NASA)

Artificial Gravity - NASA Simulator

Prof Walter Lewis Lecture - MIT


Work & Energy Lecture By Walter Lewis

Unit 3 Motion Exercise
(Call 0405 610 255 or wanigara@yahoo.com for VCE Physics Tutoring)

Resistor Colour Codes

Diodes

Unit 3 VCE PHYSICS AREA OF STUDY 2
ELECTRONICS & PHOTONICS (Study design)

(Ref: http://www.vcaa.vic.edu.au/vce/studies/physics/physicsd.pdf)

Photonics is the science of using light to manipulate information and energy and involves all facets of visible, ultraviolet and infrared radiation; this includes its detection, transport, storage and manipulation. Photonics is the basis of much of modern communication technology. Photonic devices are used with electronic components in smoke detectors, burglar alarms, safety interlocks, televisions, cathode ray oscilloscopes (CRO), relative position sensors, communication devices including fibre optic cables, modulators and demodulators, CD readers and writers, and computer networks. Some phenomena which characterise the interface between electronics and photonics are introduced.

Students will use electronic and photonic devices and systems in domestic and industrial contexts.

Outcome 
On completion of this unit the student should be able to investigate, describe, compare and explain
the operation of electronic and photonic devices, and analyse their use in domestic and industrial
systems.
To achieve this outcome the student will draw on the following key knowledge and apply the key
skills listed.
Key knowledge
To achieve this outcome the student should be able to:
• apply the concepts of current, resistance, potential difference (voltage drop) and power to the
operation of electronic circuits comprising diodes, resistors, thermistors and photonic transducers
including light dependent resistors (LDR), photodiodes and light emitting diodes (LED), (V=IR,
P=VI);
• calculate the effective resistance of circuits comprising parallel and series resistance and unloaded
voltage dividers;
• describe energy transfers and transformations in opto-electronic devices;
• describe the transfer of information in analogue form (not including the technical aspects of
modulation and demodulation) using:
– light intensity modulation, i.e. changing the intensity of the carrier wave to replicate the amplitude
variation of the information signal so that the signal may propagate more efficiently
– demodulation, i.e. the separation of the information signal from the carrier wave;
• design, investigate and analyse circuits for particular purposes using technical specifications related
to potential difference (voltage drop), current, resistance, power, temperature and illumination
for electronic components such as diodes, resistors, thermistors, light dependent resistors (LDR),
photodiodes and light emitting diodes (LED);
* Analyse voltage characteristics of amplifiers including linear voltage gain (ΔVout/ΔVin) and
clipping;
• identify and apply safe and responsible practices when conducting investigations involving
electrical, electronic and photonic equipment.

Work, Energy & Momentum problems

A rifle bullet of mass 0.01 kg strikes and embeds itself in a block of mass 0.99 kg, which rests on a horizontal friction less surface. The block is attached to a coil spring as shown in the figure. The impact compresses the spring by 0.10 m. 

(a) Calculate the Velocity of the block immediately after the impact

(b) Calculate the velocity of the bullet just prior to hitting the block. 

(c) At the instant that the block momentarily comes to rest, what has happened to the momentum?

            (A) It is now Stored in the spring

            (B) It has been transferred to the earth

            (C) It has been dissipated as sound and heat

            (D) It has been lost because the collision was not perfectly elastic