Electricity Flashcards
Current
CURRENT = the rate at which charge passes a point (units: amperes, A) (Q= It)
DIRECT CURRENT (d.c) is when the current is always flowing in one direction. Appears as a straight line on an oscilloscope.
ALTERNATING CURRENT (a.c) is when the current changes direction multiple times a second.
- Appears as a wave-y line (with crests and troughs) on an oscilloscope.
- The maximum voltage is called the peak voltage: peak voltage(Vpeak)= Y-gain setting x number of divisions (on oscilloscope).
- Period(T) = timebase setting x number of divisions (on oscilloscope). (T= 1/f)
- The values for current and voltage are continuously varying, so the quoted values for appliances are an average value known as the root mean square (r.m.s) value
- Vpeak = √2 Vrms
- Ipeak = √2 Irms
Circuits
Series circuit:
- there is only one path for current so is equal at all points, Is = I1 = I2
- the sum of the voltages across components is equal to the supply voltage, Vs = V1 + V2
- the total resistance of resistors is equal to the sum of the individual resistances, Rt = R1 + R2
Parallel circuit:
- the sum of the currents in each branch is equal to the current in the supply, Is = I1 + I2
- the voltage across each branch is equal to one another, Vs = V1 = V2
- the total resistance of resistors is less than the smallest value of the individual resistors, 1/Rt = 1/R1 + 1/R2
Resistance
Resistance is the opposition of current. (increasing current, decreases resistance)
Measured in ohms
R = V/ I
Potential difference
Potential difference (voltage, V) is the energy given to or transferred by each unit of charge as it passes through an electrical circuit.
Units: volts (V)
V = Ew/ Q
Power
Power (P) is the amount of energy transferred per unit time
Units: Watts (W)
P = E/t, P = IV
Potential divider circuit
Potential divider circuit (/voltage divider circuit) is made up of two or more resistors connected in series
The supply voltage is shared between the resistors, and the ratio of the voltages across the resistors is the same as the the ratio their resistance, so: V1/V2 = R1/R2
V1 = (R1/ R1 + R2) x Vs
Wheatstone bridge
A wheatstone bridge circuit consists of two potential dividers connected in parallel, with a voltmeter forming a bridge between them.
A whetstone bridge circuit is said to be balanced when there is no potential difference across the bridge (i.e. when the voltmeter bridge reads zero volts). For this to happen the ratio of the voltages in the two potential dividers must be the same.
V1 = (R1/ R1+R2) x Vs V3 = (R3/ R3+R4) x Vs Vbridge = V1 - V3
E.m.f
ELECTRICAL SOURCE are derives that supply electrical energy
The ELECTROMOTIVE FORCE (e.m.f) (E) of a source is the electrical potential energy supplied per unit charge which passes through an electrical source.
Unit: volts (V)
The e.m.f is the voltage measured across the source when it is a open circuit (i.e. there is no current being drawn from the source)
Internal resistance
Internal resistance is the resistance present inside the electrical source itself
When current is drawn from an electrical source some energy is wasted due to the internal resistance (the greater the current the more energy that is wasted) (therefore an ideal source is one with no internal resistance)
A real electrical source can be considered as a source with an e.m.f, ‘E’, in series with a small resistance, ‘r’.
The ‘wasted energy’ inside the electrical source is known as ‘lost volts’, Vlost = Ir
The available energy per unit charge of the electrical source is known as the ‘terminal potential difference’ (Vtpd), E = Vtpd + Vlost (i.e. E = V + Ir)
When the electrical source is connected to a load resistance, R, then: Vtpd = I R
When a electrical source is short circuited (i.e. both the load resistance, R, and the terminal potential difference, Vtpd, are zero) the short circuit current is given by: Ishort = E/ r
When measuring e.m.f and internal resistance using an experiment then graphing the results, the:
e.m.f(E) = y-intercept, c [on Vtpd axis] (when I=0A)
internal resistance, r = negative gradient, -m
Conductors
Conductors have many free electrons which can easily move through the medium
In conductors the highest occupied band (conduction band) is not completely full allowing the electrons to move and therefore conduct. The band below this (valence band) is full so does not allow the movement of electrons. However, at room temperature the valence and conduction band overlap, so electrons can move from the valence to conduction band, assisting with conduction.
Insulators
Insulators are materials with very few to no free electrons.
In an insulator the highest occupied band (valence band) is full, meaning electrons cannot move. Since there is also a large gap between the valence and conduction band at room temperature the electrons in the valence band do not have enough energy to move up into the empty conduction band, preventing any conduction taking place.
Semiconductor
Semiconductors are materials that are insulators when pure, however when an impurity is added/ are exposed to heat or light are able to conduct.
Doping is the deliberate introduction of an impurity into a semiconductor. Adding an impurity which provides extra electrons forms an n-type semiconductor, meaning the majority of charge carriers present are negative. Adding an impurity which has less electrons than the semiconductor forms a p-type semiconductor, meaning the majority of charge carriers are positive.
In a semiconductor the gap between the valence band and conduction band is small so at room temperature the electrons have sufficient energy to move from the valence to conduction band allowing some conduction to place. When this happens positively charge ‘holes’ are left behind in the valence band, contributing to further conduction as it allows the movement of electrons in the valence band. The direction of movement of holes opposes to the direction of movement of electrons in the semiconductor. An increase in temperature increases the conductivity of a semiconductor.
[*The Fermi level (energy level) is the point where it is equally probable that an electron is or is not present.]
In an undoped (intrinsic) semiconductor there are equal numbers of electrons in the conduction band and holes in the valence band. This is represented by the Fermi level* being positioned halfway between the valence band and conduction band.
In an n-type semiconductor the extra electrons provided are able to occupy energy levels close to the conduction band (donor levels) and are therefore easily excited into the conduction band. This is represented by the movement of the Fermi level towards the conduction band. At room temperature there are therefore more electrons in the conduction band than holes in the valence band.
In a p-type semiconductor the missing electrons can be thought of as gaps at energy levels just above the valence band (acceptor levels) that electrons can move into. Electrons from the valence band are therefore easily excited into these levels, leaving behind additional holes in the valence band. This is represented by the movement of the Fermi level towards the valence band. At room temperature there are therefore more holes in the valence band than electrons in the conduction band.
Band theory
The electrical properties of materials can be explained using band theory. In isolated atoms electrons occupy discrete energy levels, but when atoms bond together their energy levels interact and form ‘bands’. These bands each represent a continuous range of energies, and certain groups of energies that are not allowed are known as ‘band gaps’.
p-n junctions
When a p-type and n-type semiconductor are placed in contact with each other a layer, known as a p-n junction, is formed.
In the absence of any external voltage (unbiased) the Fermi level is flat across the junction. So in order for electrons to cross the junction from n-type to p-type they must have sufficient energy to move against the potential difference that is set up by the difference in the energy level of the conduction band (and valence band) between the two sides of the junction. This called the potential barrier.
When the p-type side of the junction is connected to the positive terminal and the n-type to the negative terminal, the p-n junction is said to be forward biased. In this situation, if the potential difference is sufficiently large, electrons (and holes) can gain enough energy to cross the potential barrier and therefore the junction conducts.
When the n-type side of the junction is connected to the positive terminal and the p-type side to the negative terminal, the p-n junction is said to be reverse biased. In this situation, applying a potential difference to the junction increases the potential barrier so electrons (and holes) are unable to cross the potential barrier and therefore the junction will not conduct.
Diode
LED
Solar cell
A DIODE is a p-n junction that conducts when it is forward biased and does not conduct when it is reverse biased.
An LED is also a p-n junction that conducts when it is forward biased. In an LED, some electrons ‘fall’ from the conduction band into the valence band of the p-type semiconductor releasing their energy in the form of a photon of light.
In a SOLAR CELL, the p-n junction has a transparent coating and will react to light. It can be connected in either photovoltaic or photoconductive mode.
In PHOTOVOLTAIC MODE the solar cell has no bias. When photons of light with sufficient energy are incident on the junction they are able to give electrons enough energy to move from the valence band to the conduction band. This produces additional charge carriers in the junction, which are then able to cross the junction and a potential difference is maintained across the junction, even when a current is drawn.
In PHOTOCONDUCTIVE MODE the solar cell is connected in reverse bias. If it is dark the solar cell acts like a normal diode and will not conduct. If the solar cell is exposed to light, electrons are moved from the valance band to the conduction band. The current that passes through it is directly proportional to the irradiance.
