Lecture 3

Question 1

Voltage (V)

Answer 1

A voltage is a difference in electrical potential energy between two points

• Voltage is energy that is capable of moving charges though a conductor (e.g. a wire, an electrolyte solution)
• In excitable cells, voltage/potential across the cell membrane is established via differences in charged ion concentrations

Question 2

Current (I)

Answer 2

Movement of charges through a conductor (eg. a wire)

Question 3

Resistors (R)

Answer 3

Poor conductors

• Resistors “resist” current flow
• Resistors “consume” voltage • Generate voltage drops
• The unit for resistance is the Ohm (Ω), named after German physicist Georg Simon Ohm
• In cells, resistors are ion channels

Question 4

Ohms Law

Answer 4

The current I through a conductor (with resistance R) between two points is directly proportional to the voltage V across the two points

V=IR

• Voltage provides a “push force” to move charges through the closed circuit
• The rate at which charges move (i.e. the current) depends on the amount of push on the charges (the voltage) and the capacity of the resistor to restrict current flow (i.e. the resistance)

Question 5

Kirchoff’s Law

Answer 5

Used when two resistors are in series.

1. The amplitude of current entering any junction (J) is equal to the amplitude of current leaving that junction
   • R1 and R2 pass the same amount of current
2. The sum of all voltages around a loop is
   equal to zero

• Resistors in series “divvy up” the battery’s voltage between them

Question 6

Conductance (g)

Answer 6

Conductance (g) is the reciprocal of resistance

• g = 1/R → Ohm’s law: V = I/g

Question 7

Capacitors

Answer 7

Capacitors consist of two conducting plates
separated by an insulator

The cell membrane acts like a capacitor

Capacitors accumulate charges when a voltage is applied
• The ability of a capacitor to hold charge is described
as follows:
C = q/V
• Where charge (q) is in coulombs
• Unit for capacitance is coulombs/volt or farads (F)

Question 8

Charging a Capacitor

Answer 8

When there is resistance, a capacitor takes time to charge

When there is NO resistance, charging and discharging a capacitor happens instantly.

Question 9

Capacitors Voltage Decay

Answer 9

unlike a battery, a capacitor’s voltage depletes as charges are lost

• This equation describes the decay in a capacitor’s voltage over time:
V(t) = V0e-t/RC
(V0 is the battery’s voltage or VMax for the capacitor)