Chapter 12 - Electric Current Flashcards
Current
The rate of flow of charge
Potential difference
Energy transferred (work) from each coulomb of charge
emf
Energy transferred (work) to each coulomb of charge
Resistance of an ideal voltmeter
Infinite
Resistance of an ideal ammeter
Zero
Resistance
Opposition to the how of charge, ohms
Ohmic conductor
Pd is proportional to current if temperature is constant
Diodes
Only allow current to flow in one direction
Filament lamp
Resistance increases as temperature increases
Thermistors
Resistance decreases as temperature increases
Why does a conductors resistance increase as temp increases?
The positive ions in the conductor vibrate more when its temp is increased. The conduction electrons therefore cannot pass through the metal as easily. This is a positive temperature coefficient because its resistance increases with increase of temperature
Why does a semiconductors resistance decrease as temperature increases?
The number of conduction electrons increases when the temperature is increased. A thermistor made from an intrinsic semiconductor therefore has a negative temperature coefficient
Superconductors
Have zero resistiviry below a certain temperature
Critical temperature
Temperature below which the resistivity of a superconductive material is zero
Uses of superconductors
Produces large magnetic fields
Transmission of electricity
Charge carriers in metals and non-metals
Metals - charge carriers are conduction electrons. They repeatedly collide with each other and the fixed positive ions in the metal
Salt solution - charge carried by ions
Insulator
Each electron is attached to an atom and cannot move away from the atom. When a voltage is applied across an insulator, no current passes through the insulator, because no electrons can move through the insulator
Conductor
Most electrons are attached to atoms but some are delocalised - the delocalised electrons are the charge carriers in the metal. When a voltage is applied across the metal, these conduction electrons are attracted towards the positive terminal of the metal
Semiconductor
The number of charge carriers increases with an increase of temperature. The resistance of a semiconductor therefore decreases as its temperature is raised
Equation for power
Power = potential difference x current
Power = energy / time
Ohm’s law
That the pd across a metallic conductor is proportional to the current through it, provided the physical conditions do not change
Unit of resistivity
Ohm metre
W
Work done, joules
Work done
- energy transferred
- energy lost from a store
- energy changed between stores
What effects the resistance of a component?
- length
- cross sectional area
- material
- temperature
Resistivity equation with length as the subject
Length = (resistance x area) / resistivity
Resistivity equation with resistance the subject
Resistance = (resistivity x length) / area
Cross-sectional area
Pi x r^2
How does the resistance of a lamp vary as the voltage is increased from zero to its working voltage
The resistance of he lamp increases with the voltage. Resistance and voltage are directly proportional to each other. The resistance increases as the filament lamp gets hotter because the positive ions are vibrating more which makes it more difficult for the charge carrying electrons to get through as there are more collisions of the electrons with the lattice ions.
What’s the significance of the critical temperature of a material?
Below the critical temperature the material has no resistance and thus no potential difference
The resistance of a metallic conductor increases with temperature because, at higher temperatures
The amplitude of vibration of lattice ions increases
