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
What is the difference between direct current and alternating current ?
Direct current (DC) sources direct the flow of ions in one direction, without changes in polarity. It is typically generated by a battery. Alternating current sources periodically reverse their polarity: the UK mains current is sinusoidal, has a frequency of 50 Hz and voltage of 240 V.
What is the relationship between Current, capacitance and voltage ?
The current in a circuit containing a capacitor depends on the rate of change of voltage : I = C * dV/dt
What is i(t) in an RC circuit ?
i(t) = I(o) * exp(-t/RC)
What is τ ?
τ = RC
τ is the “ time constant” of the RC circuit
What are the equivalents of the different components of a cell in an RC circuit ?
Cell membrane = capacitance
Ion channels = resistances
E(ion) = battery for each ion
Are ion channels inserted in the membrane in series or in parallel ?
In parallel.
Will adding more channels increase or decrease the membrane resistance ? Why ?
Adding more channels will decrease the total membrane resistance, because resistances in parallel add as reciprocals.
What is the equilibrium potential for a certain ion given by ?
What is this the analogue of ion a electrical circuit ?
E(ion) = 58log ([ion]o/[ion]i) mV
E(ion) is the analogue of a battery and it is the Nernst potential for each permeable ion.
A synapse injects current into a post-synaptic cell when a released neurotransmitter acts on ionotropic receptors
- What effect will more or less open post-synaptic (but not in the site where NT is binding) channels have on the voltage change produced by the synaptic current?
- What will happen if the red channels close during the synaptic current?
- What effect will the membrane capacitance have on the rate of change of voltage?
- V = IR –> more channels = less membrane resistance (channels are resistances parallel) –> the voltage change will be smaller
- τ = RC –> more channels closed = higher membrane resistance –> higher τ –> longer time constant
- Small cells have small capacitance –> τ will be quite short (will load fast) Vs large cells will have larger capacitance and take much longer to load
