Introduction to electricity

Question 1

What is the difference between conductors and insulators ?

Answer 1

Good conductors cannot store charges whereas good insulators can.

Question 2

Are insulators usually elements or molecules ?

Answer 2

Molecules.

Question 3

Give examples of conductors and insulators.

Answer 3

Insulators : Rubber, glass, wood, dry air

Conductors : Silver, copper, aluminum, iron, mercury, carbon, water

Question 4

Wat is the unit of measure of charge (Q) ?

What is the charge of one electron (or univalent) ?

Answer 4

Coulombs (C).

1 e- –> 1.6e-19 C

Question 5

How many electrons is one coulomb ?

Answer 5

6.25e18 electrons

Question 6

What is the charge of one mole of univalent ions defined as ?

Answer 6

As the Faraday constant :

F = N(A)e(0) = 6.022e23 x 1.602e-19 = 96.500 coulombs per mole

Question 7

Define electrical current literally and mathematically.

Answer 7

Electrical current = the flow of electrons or ions from one place to another –> rate of change of charge with time : I = dQ/dt

Question 8

What is current measured in ?

Answer 8

Current is measured in Amperes (A) : 1 Ampere is defined as the flow of 1 Coulomb of charge in 1 second

Question 9

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 ?

Answer 9

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.

Question 10

What is necessary for charge to move ?
How is this necessity measured ?
Where does this unit come from ?

Answer 10

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)

Question 11

What does resistance measure ?

Answer 11

The ease with which a charge moves through a conductor.

Question 12

What is the symbol of a resistor ?

Answer 12

It resembles a type of resistor called “wire wrapped resistor”, a wire wrapped around a ceramic core.

Question 13

State Ohm’s law.

Where does it come from ?

Answer 13

V = IR

Georg Simon Ohm (1787-1854)

Question 14

What are the units of resistance ?

Answer 14

The ohm.

Question 15

What is the reciprocal of resistance ?

How else can Ohm’s law thus be expressed ?

Answer 15

Conductance (G = 1/R).

I = GV

Question 16

What are the units of conductance ?

Where does it come from ?

Answer 16

The siemens (S).
Ernst Werner von Siemens (1816-1892).

Question 17

What are the 3 different ways in which the elements of an electrical circuit can be arranged ?

Answer 17

Series, parallel or series-parallel.

Question 18

What is the differences between resistances in series and in parallel ?

Answer 18

Resistances in series add, resistances in parallel add a their reciprocal.

Question 19

How can charge be stored within an electrical circuit ?

What usually allows this ?

Answer 19

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.

Question 20

How does a capacitor work ?

Answer 20

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.

Question 21

What is the relationship between the charge stored in a capacitor and the applied voltage ?

Answer 21

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

Question 22

What is capacitance measured in ?

Where does this unit come from ?

Answer 22

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)

Question 23

What are the factors affecting capacitance ?

Give the formula supporting this.

Answer 23

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

Question 24

How do capacitors is series and in parallel differ ?

Answer 24

Parallel : C(total) = C1 + C2 + C3 + … + Cn

Series : 1/C(total) = 1/C1 + 1/C2 + … + 1/Cn

Question 25

What is the difference between direct current and alternating current ?

Answer 25

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.

Question 26

What is the relationship between Current, capacitance and voltage ?

Answer 26

The current in a circuit containing a capacitor depends on the rate of change of voltage : I = C * dV/dt

Question 27

What is i(t) in an RC circuit ?

Answer 27

i(t) = I(o) * exp(-t/RC)

Question 28

What is τ ?

Answer 28

τ = RC

τ is the “ time constant” of the RC circuit

Question 29

What are the equivalents of the different components of a cell in an RC circuit ?

Answer 29

Cell membrane = capacitance
Ion channels = resistances
E(ion) = battery for each ion

Question 30

Are ion channels inserted in the membrane in series or in parallel ?

Answer 30

In parallel.

Question 31

Will adding more channels increase or decrease the membrane resistance ? Why ?

Answer 31

Adding more channels will decrease the total membrane resistance, because resistances in parallel add as reciprocals.

Question 32

What is the equilibrium potential for a certain ion given by ?
What is this the analogue of ion a electrical circuit ?

Answer 32

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.

Question 33

A synapse injects current into a post-synaptic cell when a released neurotransmitter acts on ionotropic receptors

1. 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?
2. What will happen if the red channels close during the synaptic current?
3. What effect will the membrane capacitance have on the rate of change of voltage?

Answer 33

1. V = IR –> more channels = less membrane resistance (channels are resistances parallel) –> the voltage change will be smaller
2. τ = RC –> more channels closed = higher membrane resistance –> higher τ –> longer time constant
3. 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