Introduction to electricity Flashcards
What is the difference between conductors and insulators ?
Good conductors cannot store charges whereas good insulators can.
Are insulators usually elements or molecules ?
Molecules.
Give examples of conductors and insulators.
Insulators : Rubber, glass, wood, dry air
Conductors : Silver, copper, aluminum, iron, mercury, carbon, water
Wat is the unit of measure of charge (Q) ?
What is the charge of one electron (or univalent) ?
Coulombs (C).
1 e- –> 1.6e-19 C
How many electrons is one coulomb ?
6.25e18 electrons
What is the charge of one mole of univalent ions defined as ?
As the Faraday constant :
F = N(A)e(0) = 6.022e23 x 1.602e-19 = 96.500 coulombs per mole
Define electrical current literally and mathematically.
Electrical current = the flow of electrons or ions from one place to another –> rate of change of charge with time : I = dQ/dt
What is current measured in ?
Current is measured in Amperes (A) : 1 Ampere is defined as the flow of 1 Coulomb of charge in 1 second
Given 1C =6.24e18 charges and the current through a sodium channel is 1e-12 A, how many sodium ions will pass through the channel if it stays open for 1 ms ?
Current A = 1e-12 A = 1e-12 C/s = 1e-15 C/ms
1e-15 C/ms * 6.24e18 (the number of ions in 1 C) = 6,240 sodium ions.
What is necessary for charge to move ?
How is this necessity measured ?
Where does this unit come from ?
Charge only moves if there is a potential difference between two points
Potential difference is measured in volts (V)
Alessandro Giuseppe Antonio Anastasio Volta (1745-1827)
What does resistance measure ?
The ease with which a charge moves through a conductor.
What is the symbol of a resistor ?
It resembles a type of resistor called “wire wrapped resistor”, a wire wrapped around a ceramic core.
State Ohm’s law.
Where does it come from ?
V = IR
Georg Simon Ohm (1787-1854)
What are the units of resistance ?
The ohm.
What is the reciprocal of resistance ?
How else can Ohm’s law thus be expressed ?
Conductance (G = 1/R).
I = GV
What are the units of conductance ?
Where does it come from ?
The siemens (S). Ernst Werner von Siemens (1816-1892).
What are the 3 different ways in which the elements of an electrical circuit can be arranged ?
Series, parallel or series-parallel.
What is the differences between resistances in series and in parallel ?
Resistances in series add, resistances in parallel add a their reciprocal.
How can charge be stored within an electrical circuit ?
What usually allows this ?
The arrangement of an insulator sandwiched between two conductors allows the storage of charges in an electrical circuit.
Such a device is called a capacitor.
How does a capacitor work ?
When a capacitor is connected to a battery electrons build up on one plate, repelling electrons from the other plate.
Once the capacitor is fully charged up, the electron flow
stops and the charge is stored on the plates.
What is the relationship between the charge stored in a capacitor and the applied voltage ?
The charge stored in a capacitor is proportional to the applied voltage : Q= CV
C is the capacitance, indicating how much charge can be stored for a given charging voltage
What is capacitance measured in ?
Where does this unit come from ?
The capacitance is measured in farad (F) : when 1 F is the capacitance of an element that can store 1 C of charge given a 1 V potential difference
Michael Faraday FRS (1791 –1867)
What are the factors affecting capacitance ?
Give the formula supporting this.
Plate area, plate spacing and dielectric material. C = εε(o)A/d C = capacitance ε = absolute permittivity of dielectric ε(o) = polarisability of free space
How do capacitors is series and in parallel differ ?
Parallel : C(total) = C1 + C2 + C3 + … + Cn
Series : 1/C(total) = 1/C1 + 1/C2 + … + 1/Cn
