When the Chips Are Down

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This physics extension module from the Salters' Science course focuses on the physics and electronics behind each part of a television set. Production of electron beams and their deflection by electrical or magnetic fields are shown and students see how colours can be built up from the three primary colours of light. The cathode ray oscilloscope is used to demonstrate the use of ‘ramp’ voltages to control scanning, and the use of time-base and Y-gain controls. The use of diodes to produce half-wave and full wave rectification is shown to explain the production of a dc voltage to accelerate the electron beam. Other uses of diodes and LEDs are considered. Text exercises consider the properties of semiconductors and the transfer of charge by movement of electrons and positive holes. The use of doping impurities and the operation of thermistors and p-n junctions are described. The use of capacitors in smoothing voltages and storing charge is explored. The use of time delay CR circuits to control frequency and adjust tuning circuits is considered.

Lesson 1: Watching TV
The module is introduced by considering the main features of a TV set.

Lesson 2: The CRO
The operation of the time base and gain controls is demonstrated and these are related to the control of the electron beam in the tube of a TV.

Lesson 3: Diodes
Students use diodes to rectify an ac signal with both half and full wave rectification. They compare LEDs with light bulbs. The function of Zener diodes is also considered as protective devices.

Lesson 4: Charge carriers
Students compare conduction in various materials, including semiconductors. They use the idea of ‘holes’ and n- and p- type semiconductors to explain the operastion of thermistors, light dependent resistors (LDRs) and the diode.

Lesson 5: Capacitors
The use of capacitors to ‘smooth’ the output from a rectifier is demonstrated. Students carry out experiments with capacitors.

Lesson 6: Why the delay?
Students experiment with RC circuits and the use of the time constant to link capacity, resistance and rate of discharge. They see how time delays can be varied and consider applications of this.

URL: http://stem.org.uk/rx5qm

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