Effects of an Electric Current and Domestic Circuits. Chapter 24. Heating Effect of Electric Circuits. It was James Watt who experimentally investigated the effects of the heat, W, from a current carrying wire. It was found that the following equation may be used to find W:
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I = current through the wire
R = resistance of the conductor
t = time that the current flows for
thin plating of another.
A metallic conductor
A filament bulb conductors
As the voltage across the filament increases, so does the current and in turn the temperature. Since resistance α current, resistance increases. So as the bulb gets hotter a given increase in V doesn’t produce an increase of I when it was colder.
Charge carriers in a filament bulb are negative electrons.
A semi conductor, e.g. a thermistor conductors
As the p.d. across the semiconductor is increased, the current increases, and in so doing the semiconductor gets hotter. This produces more holes and electrons for conduction and the resistance drops. Thus a further increase in V produces a larger increase of I when it was cold.
Charge carriers are negative electrons and positive holes.
Active electrodes conductors
As p.d. increases so will current. Active electrode takes part in the chemical reaction-also obey Ohm’s law and the graph is linear through the origin. If the electrodes are inactive, the voltameter behaves like a cell and has an emf that must be overcome before current will flow.
Charge carriers are positive and negative ions.
A Gas conductors
An example would be a discharge tube. In region OA the positive ions in the tube are attracted to the negative electrode and the electrons move towards the positive electrode once a p.d. is applied, as number of ions crossing the tube increases so does the current. In region AB all the ions in the tube cross without recombination so no increases in current. Voltage increases to a stage where collision between fast moving ions and electrons produces more ions, corresponding to region BC.
Charge carriers are positive ions, negative electrons and a few negative ions.
E.g. neon lamps.
A Vacuum conductors
No charge carriers in a vacuum, however if the cathode is heated sufficiently electrons will be produced by thermionic emission. A certain voltage is reached where all the electrons from the cathode are carried across the tube and the curve flattens out.
1000 W=1000 joules per second
energy=power x time.