Chapter 8 Charge and Current Flashcards
What is the unit for current?
Amperes
What is the definition for current and what is the equation?
It is the number of coulombs of charge per second passing a point
I = ΔQ / ΔT
What is electric charge?
It is a physical property and comes in two forms, positive and negative
What is the measure of electric charge? and what is the quantity of 1 coulomb
Coulombs
1 coulomb is the amount of charge that passes in 1 second when there is a current of 1 amp
What is the charge of an electron? and a proton?
Electron: -1.6x10^-19 Coulombs
Proton: 1.6x10^-19 coulombs
How do you calculate the charge on ions?
Take the relative charges (e.g. +1 or -2 etc) and multiply by 1.6x10^-19 to get the charge in coulombs
How is the charge on an object calculated?
Using the equation: Q = ±ne
Q = Charge
n = number of electrons added or removed
e = the elementary charge
What is the elementary charge and why is it elementary?
It is the smallest possible charge that something can have. It is the same as the charge of an electron, 1.6x10^-19
This is possible because charge is quantised
Explain charge quantisation
It means that the charge has fixed values which are integers of e where e is the elementary charge.
You cannot have any value of charge.
What are charge carriers and what are some examples
Charge carriers are anything that carries a charge. Examples include electrons or ions. When current flows through a liquid, the charge carriers are often ions in the liquid
What is the structure of a metal?
It is a regular crystalline structure of positive ions surrounded by a sea of delocalised electrons.
How does current flow through a metal?
When a metal is connected to different ends of a battery the free electrons move through the metal towards the positive end of the battery. NOT ALL ELECTRONS MOVE ONLY THE FREE ONES
What causes a large current?
More electrons moving through the wire at the same time
Electrons moving faster through the wire
What is the direction of conventional current vs electron flow
Conventional current flows positive to negative and electron flow is from negative to positive
What are electrolytes?
They are solutions that conduct electricity. They have ions that act as charge carriers that move through the solution and transfer charge
When conduction through an electrolyte what is the:
a) anode
b) cathode
a) anode is the positively charged end
b) cathode is the negatively charged end
What happens to the positive ions in an electrolyte solution
The positive ions move towards the cathode and gain an electron. The negative ions move towards the anode and lose an electron.
How is electric current measured?
Using an ammeter, it is placed in series in the circuit. It should have the lowest possible resistance.
What is the law of the conservation of charge
Electric charge cannot be created or destroyed and so the total electric charge in the universe is constant
What is Kirchhoff’s first law? what is the equation?
The sum of the currents into a point (junction) is equal to the sum of the currents out of that point
ΣI in = ΣI out
What is the number density?
It is the number of free charge carriers per unit volume
What is the number density of:
a) conductors?
b) semiconductors?
c) insulators?
a) Larger than 10^28 m^-3
b) around 10^17 m^-3
c) Small
What is the problem with using semiconductors to transfer electricity?
The electrons have to move faster so the semiconductor heats up more
How fast do charge carriers move? and why do lights turn on instantly
They move slowely but they all move at once so the light turns on instantly
What is the equation for mean drift velocity?
I = Anev I = Current A = Cross sectional area e = The elementary charge v = mean drift velocity
What is the mean drift velocity?
This is the mean speed of charge carriers
What is the affect of changing the cross sectional area on mean drift velocity?
The smaller the cross sectional area the faster the electrons move in order for the current to be the same as a wire with a larger cross sectional area because there is less space for the electrons to pass.
