CH8 Flashcards
What is meant by quantisation of charge?
It means that electric current comes in discreet, invisible units. The smallest unit of electric charge that exists in nature is the charge of an electron
How much is the charge of an electron?
1.60x (10 )-19 C
What is the charge of one electron and one proton?
Electron: - 1 elementary charge
Proton: 1 elementary charge
What is the Millikan experiment?
Carried out in 1909. It confirmed that electric charge is quantized and the charge of a single electron was found to be -1.59 x (10)-19
Describe the Millikan experiment (or the oil-drop experiment)
He analysed the motion of electrically charged oil droplets between two oppositely charged parallel plates. The oil particles experienced gravitational force, air resistance and upthrust. By adjusting the electrical field between the plates he could make the droplets rise, fall or stay suspended. By comparing these measurements with the known properties of air he was able to determine the charge on each droplet. He found that they were all multiples of a fundamental unit of charge, which he identified as the charge of single electron.
What is electric charge?
A physical property
How is charge measured?
In coulombs (C). C is defined as the electric charge flowing past a point in one second when there is electric current of 1 Ampere
How is electric current defined?
It is defined as the rate of flow of charge
How is electric current calculated?
I = Delta Q/ Delta t
Where I is the electric current in Ampere, Delta Q is the charge transferred in C and Delta t is the time in seconds
What is the net charge of an object?
Q = +- ne
where Q is the net charge in C, n is the number of electrons either added or removed and e is the elementary charge
What is meant by conventional current?
It is from the positive terminal to the negative terminal
What are liquids that conduct electricity called?
Electrolytes
What is the flow of currents in liquids?
A flow of ions
What are usually electrolytes?
Ionic solutions
Example of ionic solution
Salt (NaCl) dissolved in water. The salt separates into positively charged sodium ions (cations) Na+ and negatively charged chlorine ions (anions) Cl-
What does happen to Na+ ions when they reach the cathode?
They accept an electron
What does happen to Cl- ions when they reach the anode?
They donate an electron
What is an ammeter?
Used to measure current
What is a positive charged ion called?
Cation
What is a negative charged ion called?
Anion
What is a positive electrode called?
Anode
What is a negative electrode called
Cathode
What is meant by Number Density?
Number Density is the number of free electrons per cubic meter of material
How are materials classified based on number density?
Materials are classified into three groups:
- Insulators (eg wax and rubber)
- Semiconductors (eg silicon and germanium)
- Conductors (eg. Lead, copper and zinc)
What does the number density of a material determines?
The number density determines how well a material will conduct an electric current. The greater the number the better a material conduct electricity
What order of density number have conductors and semiconductors?
(10)28 m-3
(10)17 m-3
Can semiconductors carry the same current as conductors?
Yes they can but in order to do so their electrons need to do much faster (as their density number is lower). This results in an increase of temperature which needs to be taken into account in design (eg computer processors are made of silicone so computers need carefully designed cooling systems)
What causes the random movement of electrons?
Repeated collisions between positive ions and free electrons
How fast do charge carriers move?
They move slowly
If charge carriers move slowly, then why do lumps turn on almost immediately when you press the switch button?
Because all the free electrons start moving almost at once
What is the equation of electric current which take into account number density and mean drift velocity?
I = Anev
Where: I is the electric current in the conductor in Amp,
A is the cross sectional area of the conductor in m2
e is the elementary charge (1.60 x 10-19 C)
n is the number density
v is the mean drift velocity of the charge carriers in ms-1
What is the relationship between mean drift velocity and the cross sectional area of the wire?
The mean drift velocity is inversely proportional to the cross sectional area of the wire.
What is mean drift velocity?
Drift velocity is the average velocity of the charge carriers travelling through the conductor
