Chp 11&12: COE & DC Flashcards
Explain why the current in a filament lamp is greatest when the current is first switched on?
Before the lamp is switched on, the lamp is cold.
When the switch is on for some time, due to joule heating, the filament lamp becomes hotter, causing more vigorous vibrations and the amplitude of the vibrations of the metal lattice increases.
causes greater obstruction to the flow of electrons, decrease drift velocity of e, increase R.
No. charge carriers does not increase significantly; hence the overall effect is that the resistance of the filament lamp increases with temperature.
As the resistance increases the current decreases, hence current is the greatest when the lamp is first switched on
Despite the small drift velocity of electrons, explain why when a switch is closed, the lamp is lit almost instantaneously
- when switch is closed, E-field set up instantly.
- wires and filament are fully filled w closely packed e
- e- closest to filament lamp will move
how the resistance of the lamp can be obtained from an I-V graph
ratio of V and I, not the gradient
Why for a thermistor, as pd increase, resistance decreases?
As the temperature of the NTC thermistor rises, more charge carriers in the thermistor are liberated and become mobile (free to move).
effect of increased mobile charge carriers > effect of increased amplitude of lattice vibrations of lattice ions
hence the electrical conduction of the thermistor improves, and the resistance of the thermistor decreases.
Maximum power theorem:
For the power transferred from the EMF source to the external load to be maximum, the resistance of the load must be the same as the internal resistance.
Explain why a potentiometer can give a more accurate reading of the EMF of a cell compared to using a voltmeter.
At the balance length of a potentiometer, there is no current flowing through the secondary cell, as such even with an internal resistor, we can accurately measure the EMF of the cell.
