Resistors for Edexcel A-level Physics
This page covers the following topics:
1. Ohmic conductors
2. Filament lamps
Ohmic conductors are conductors that obey Ohm's Law, therefore there is a linear relationship between current and potential difference. Examples of ohmic conductors are electrical wires and resistors.
A filament lamp heats up as current flows through it in a circuit and lights up. As the temperature of the filament lamp increases, its resistance also increases and thus the current is reduced. Filament lamps do not obey Ohm's Law and thus the voltage across them are not directly proportional to the current through them.
A diode is an electrical component used in circuits to allow current to only flow in one direction. This allows for the voltage in a circuit to be controlled and for equipment to be protected. The very high resistance of the diode in one direction means that current can only flow through it in the opposite direction, and thus the diode does not obey Ohm's Law. A small voltage is needed across a diode for it to be able to allow current to flow through it.
A light dependent resistor (LDR) is an electrical component whose resistance changes depending on the light intensity falling on it. The resistance of an LDR decreases as the light intensity increases. In low lighting, the resistance will be high and the amount of current flowing through it will be low, whereas in brighter places, the resistance will decrease, allownig for more current to flow through it. LDRs can be used in circuits to detect light changes.
Sketch a graph to display the relationship between current and voltage of an ohmic conductor.
Describe the relationship between voltage and current for a filament lamp.
Filament lamps do not obey Ohm's Law and thus the voltage across them are not directly proportional to the current through them.
Explain how street lamps can be turned on at night automatically using an LDR.
The resistance of an LDR will be high in the dark. This will allow for current to flow through the street lamp instead of the LDR, thus turning it on.
Is the resistance of a diode constant regardless of the way it is connected? Explain your answer.
The resistance of a diode in one diretcion is very high so as not to allow current to flow in that direction, and doesn't in the opposite direction.
No, the resistance depends on the direction it is connected in.
Are diodes ohmic conductors?
Since current only flows through a diode in one direction, the relationship between current and voltage is not linear, therefore they are not ohmic conductors.
End of page