Define electric current:
A flow of negatively charged particles (electrons) or “charge carriers”. Electric current flows from negative to positive.
What direction does conventional current flow in?
+ to -
What is the charge of one electron?
1.6 x10-19 C
What is the difference between DC and AC?
Direct current only flows in one direction
Alternating current changes direction 50 times every second (50Hz)
What is the equation for charge?
Charge= current x time
A measure of the amount of energy a component transfers per unit of charge passing through it. V=E/Q
The force that pushes electrons around a circuit (requiring a p.d)
What is the equation for voltage (in terms of energy):
Voltage= Energy/ Charge
Define Electromotive force (EMF):
The total work done by a cel per coulomb of charge. V= W/Q
This is the correct term for supply voltage.
Define potential difference:
The voltage across components in a circuit (the work done to them by the current). V=W/Q
The correct term for voltage.
What is an electron volt?
A unit of energy that is used with sub-atomic particles. It is a derivation from the V=E/Q equation and states that:
“if an electron is accelerated by a p.d of 1V then the energy it will gain is E=V x e
The opposition to the flow of current within a conductor.
What is the equation for resistance?
Resistance= p.d / current
What is Ohm’s Law?
The current through a component is directly proportional to the voltage.
Describe the relationship between current and voltage for a filament lamp (and the IV graph):
A filament lamp is an ohmic conductor until the current becomes too large, then it heats up and increases resistance (curving the graph)
Describe the relationship between current and voltage for a fixed resistor (and the IV graph):
Resistors are ohmic conductors so I is directly proportional to V.
Describe the relationship between current and voltage for a diode (and the IV graph):
A diode only conducts in the forward direction.
Describe the relationship between current and voltage for a thermistor (and the IV graph):
A thermistor alters resistance with temperature (in the reverse manner to a filament bulb). Resistance reduces with temperature.
The general property of a material to resist the flow of electric current.
Give the equation for resistivity:
Resistance= (Resistivity x Sample length) / Cross sectional area
R= pl / A
Define carrier density:
Assuming each atom has one free electron, the carrier density is the number of free charges per unit volume (n).
Define drift speed:
The speed with which electrons will move down a wire.
What is a semiconductor?
A solid material (generally) with small numbers of delocalised electrons that are free to conduct.
Give the equation for current in terms of drift velocity:
Current= number of electrons x cross sectional area x average drift velocity x electron charge
How would increasing the temperature of a wire impact the resistance, current and drift velocity in a conductor?
The vibrations would increase so the resistance would increase, decreasing the current. This would decrease the drift velocity as I=nAve.
How would increasing the temperature impact the resistance, current and carrier density of a semiconductor?
Increasing the temperature of a semiconductor increases the already small carrier density, so the resistance decreases and the current increases (as I=nAve). This is known as a negative temperature coefficient.
What are 3 properties of free atoms which are not found in solid materials?
- Series of discrete energy levels
- Number of energy levels depends on the electrons recieved energy
- Electrons can leave their atoms if they have enough energy
Describe the layout of energy levels in solid materials:
As there are many atoms close together, energy levels become wider and form energy bands. These bands are the valence band and the conduction band. Electrons of many different energy values are in the valence band but if they get enough energy they can move to the conduction band and become delocalised.
Explain the difference between the energy bands for insulators, semiconductors and metals:
Insulator: Empty conduction band and full valence band.
Semiconductor: Almost empty conduction band and almost full valence band.
Metal: Almost full conduction band.
What is a conduction ‘hole’?
An electron hole is the lack of an electron in a certain position where one could exist in an atom or atomic lattice. As an electron is missing, a net positive charge remains.
These holes are not particles but quasiparticles.
How do semiconductor diodes work?
They are made from joining together two different types of semiconductor (P and N types). This creates an energy barrier at the junction which blocks charge carriers like holes or electrons. The barrier can be overcome in the forwards but not the reverse direction
What is superconductivity?
A quality of a device with zero resistance below a critical temperature. This means that it also has infinite current. They can be used to make high power electromagnets.