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Resistors for OCR GCSE Physics

Resistors

This page covers the following topics:

1. Ohmic conductors
2. Filament lamps
3. Diodes
4. Thermistors
5. LDRs

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.

Ohmic conductors

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.

Filament lamps

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.

Diodes

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.

Thermistors

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.

LDRs

1

Sketch a graph to display the relationship between current and voltage of an ohmic conductor.

image

Sketch a graph to display the relationship between current and voltage of an ohmic conductor.

2

What is a thermistor?

A thermistor is an electrical component used in circuits to detect temperature changes.

electrical component that detects temperature changes

What is a thermistor?

3

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.

Will current flow through a thermistor more easily at high or low temperatures?

4

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.

Describe the relationship between voltage and current for a filament lamp.

5

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.

Explain how street lamps can be turned on at night automatically using an LDR.

End of page

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