Current-voltage graphs
Different devices have different The opposition in an electrical component to the movement of electrical charge through it. Resistance is measured in ohms. characteristics. These can be investigated using graphs that show:
- Moving electric charges, eg electrons moving through a metal wire. on the vertical axis
- The potential difference (or voltage) of a supply is a measure of the energy given to the charge carriers in a circuit. Units = volts (V). This is the voltage between two points that makes an electric current flow between them. on the horizontal axis
These are known as current-voltage graphs or A graph of current vs potential difference for a circuit component..
A device in an electric circuit, such as a switch or lamp. which show a straight line I–V graph are linear circuit elements. They are also sometimes called 'ohmic circuit elements'. This means that current is Relationship between two variables where one variable is equal to a constant multiplying the other variable. Variables are plotted as a straight line though the origin. to potential difference for that component. This is a mathematical statement of The rule that states that the current (I) flowing through a resistor (R) is directly proportional to the voltage (V) across the resistor, provided the temperature remains constant. which states that the current in a conductor is proportional to the potential difference between its ends, provided temperature remains constant. It is true for a Electrical component with a resistance that is fixed and cannot be changed. or resistance wire at a constant temperature.
Non-linear circuit elements
Components which show I–V graphs that are not straight lines are non-linear circuit elements. The current through them is not directly proportional to the potential difference across them.
Resistors and filament bulbs
Non-linear circuit elements include fixed resistors that become hot, and filament bulbs which become hot and glow.
The shape of the I-V graph changes as temperature of the component increases.
When the component becomes hotter, the The smallest part of an element that can exist. in the metal A regular grid-like arrangement of atoms in a material. vibrate more. More collisions occur between the free Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons. and atoms, increasing the resistance.
An increase in the potential difference will allow the current to increase, but only up to a certain point.
Diodes
A An electrical device that allows current to flow in one direction only. has a very high resistance in one direction and a low resistance in the other. This means that current can only pass in one direction.
LDRs and thermistors
Light dependent resistors or A Light Dependent Resistor is a type of resistor which is affected by changes in light levels. A cadmium sulphide layer causes a decrease in resistance in the light and increase in the dark. are made of Insulating material doped with impurities to affect electron energy bands and therefore affect conduction properties. material. Their resistance decreases as the light intensity increases. LDRs can be used to switch lights on or off automatically, such as stadium lights which come on when it gets dark.
An electrical device whose resistance decreases as its temperature increases. are also made of semiconductor material. Their resistance decreases as the temperature increases. Thermistors are used to control temperatures in many devices, such as ovens and central heating systems.
More guides on this topic
- Electric charge - OCR 21st Century
- Series and parallel circuits - OCR 21st Century
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- Electric motors - Higher - OCR 21st Century
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- Sample exam questions - electric circuits - OCR 21st Century
