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Module 4 - Chapter 8 - Charge and Current - Coggle Diagram
Module 4 - Chapter 8 - Charge and Current
Charge and current
Electric current
In metals, electrons carry the charge
In fluids, ions carry the charge
Rate of flow of charge
Convectional current - rate of flow of charge from the positive to the negative
Electron flow - opposite direction to conventional current, negative to positive
Electric charge
Physical property
Like charges repel, opposite charges attract
Measuring charge
Measured in coulombs
Electrial charge flowing past a point in one second where there is an electric current of one ampere
A partical with electric charge is a charge carrier
Moving charges
Flow of charge
charge carries don't have to be electrons
In liquids, charge carries are ions
metals
Metals have a lattic of positive ions, surrounded by a number of free electrons
Positive ions aren't free to move, but they vibrate aorund fixed points, they vibrate more vigorously as the temperature increases
A small number of electrons are free to move
Electrons will move if one end of the metal wire is positive and the other is negative
The greater the rate of charge flow, the greater the current
A greater current
Greater number of electrons moving past a point each second
Same number of electrons moving faster through the metal
Conventional current
Flows from positive terminal to a negative one
direction of electric currents is treated as from positive to negative regardless of the direction of the charge carriers
Electrons flow from negative to positive
Electron flow is in the opposite direction to conventional current
Electric current in electrolytes
Liquids that carry electricity are called electrolytes
Electrolytes are either molten ionic compounds or ionic solutions
Pure water is an insulator, but water from taps is an ionic solution that contains dissolved ions
If a postive electrode and a negative electrode are placed in the solution, ions are attratced to electrodes
Movement of ions is a flow of charge, which is an electric current
Measuring electric current
An ammater measures electric current, it is always placed in series at the point you want to measure the current
An ideal ammeter has zero resistance
Kirchhoff's first law
Conservation
Charge must be conserved
In any interaction, the total charge before and after must be the same
Conservation of charge states that charge can neither be created nor destroyed
The total amount of electric charge in the universe is constant
1st law
For an point in an electrical circuit, the sum of currents into that point is equal to the sum of currents out of that point
This law is based on conservation of charge, where charge is the product of current and the time
Charge cannot be destroyed, so the charge entering a point in a given time must equal the total number of charge carriers leaving that point during that time
Mean drift velocity
Number Density
number of free electrons per cubit metre of material
The higher the number density, the greater the number of free electrons per m^3, so the better the electrical conductor
Semiconductors have a much lower number density than metals, so in order to carry the same current, the electrons need to move much faster, this increases the temperature of the semiconductors
Drift velocity - the average velocity of the electrons in the direction of the wire
v = I / nAe
I = Current [A]
L = Length of wire
A = Cross sectional area [m^2]
e = charge of electron
n = number density [m^-3]
How fast do charge carries move?
Electrons move very quickly in random directions
Electrons move slowly in one direction
Effect of changing cross-sectional area
Narrower the wire, the greater the drift velocity must be in order for the current to be the same
Electons flow faster in narrow wires to maintain the same current
Mean drift velocity is inversely proportional to the cross-sectional area of the wire