Review of Electrical Terms: Ohm's Law, Kirchoff, resistance and Capacitance

Voltage is only ever a___________

difference in charge

By convention, we always discuss current in the _______________

direction of positive flow charge (even though both positive and negative ions are moving)

What are the requirements for electrode for neuron recording?

• 1 conducting tip

• tip must be very small (~1 micron→ to not damage the cell)

• measure Vm only at the tip (not accurate if it is measured throughout the length of the electrode)

• material must not damage the cell membrane (glass)

• the electrode should be filled with a conducting solution

What is Ohm’s law?

V=IR

units V is volts (V)

units R is ohms

Units I is amperes

Current and Charge are related by what equation?

I= dQ/dt

(difference in Charge over time)

When does a Votlage drop occur in a circuit?

Across a resistance (analogous as a drop in flow with constriction in a pipe)

current flows only in a ________

Closed loop

What is the concept of ground?

• it is easier to make measurements with only one lead

• ground is “0mV”

• everthing else is measured relative to ground

• all components of an experiment must be connected to ground

Ohms law concepts: more V for given R gives______

More I (current)

Ohms law concepts: More R for given V gives _______

less I (current)

if two resistors are in series:

all of the current that passes thorugh one [ressistor], MUST also pass through the other

What is Kirchoff’s voltage law? (series)

• sum of all voltages around a loop must equal zero

• if only 1 resistor, then the voltage drop across that resistor is equal to the coltage of the battery

What is Kirchoff’s Current Law? (Series)

Total current into a node must equal zero

Total current in must equal total current out

ie: Ib=I1=I2

Resistors in series____

add!

Req= R1+R2

Kirchoff’s voltage law? (parallel)

• the sum of all voltages around a loop must equal 0

• Vb=V1=V2

Kirchoffs Current Law? (parallel)

• the total current into a node must equal 0

• total current in must equal total current out

• Ib=I1+I2

Resistors in parallel___

the reciprocal of the equivalent resisitance is equal to the sum of the reiprocals of the individual resistance

(1/Req)=(1/R1)+(1/R2)

What are equivalent conductances?

• conductance is the inverse of resistance

• series: (1/geq)=(1/g1)+(1/g2)

• parallel: geq= g1+ g2

What are capacitators?

circuit component that stores charge

What are the characteristics of capacitators?

• a perfect resistor (insulators) between 2 conductive plates. No current flows through the insulator

• however, curretn is flowing in the circuit while the capacitator is being charged

• amount of current flowing onto one side of the caoacitator equal the amound of current leaving the other side of the capacitator

• an empty capacitator is easily charges and acts like a closed circuit

• when a capacitator is fully charges (has stored all of the charges it can), then it acts like a broken wire and no more current will flow in the circuit

• capacitator will store charge until the plates of the capacitators are connected

Amount of charge stored in a capacitator is defined as

Q=CV

Ic=dQ/dt —→ C* dv/dt (capacitance gives a circuit time dependence)

How to calculate equivalent capacitance?

• the closer together th plates, the greater the capacitance

• the larger the plates, the greater the capacitance

• in series: (1/Ceq)=(1/C1) +(1/C2)

• in parallel: Ceq= C1+ C2