Resistors for SQA National 5 Physics
This page covers the following topics:
1. Ohmic conductors
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 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.
Thermistors are electrical components used in circuits to detect temperature changes. The resistance of a thermistor changes with temperature. As the temperature increases, the resistance of a thermistor decreases. Low temperatures will cause a big resistance in the thermistor and thus little current will flow through it, whereas at higher temperatures, the reduced resistance will allow for more 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.
What is a thermistor?
A thermistor is an electrical component used in circuits to detect temperature changes.
electrical component that detects temperature changes
Will current flow through a thermistor more easily at high or low temperatures?
The resistance of a thermistor will decrease as the temperature increases, therefore current will flow through it more easily at high temperatures.
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.
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