Electronic Circuit Elements

Question 1

Current

Answer 1

I = ΔQ/Δt.

Current is the rate of charge flow through the cross-section of a conductor (wire).

Question 2

Battery, electromotive force, voltage

Answer 2

Electromotive force (emf) is really not a force, but a potential difference, with the unit voltage.

A battery is a source of emf.

If the battery has no internal resistance, then potential difference across the battery = EMF.

If the battery has internal resistance, then potential difference across battery = EMF - voltage drop due to internal resistance.

Question 3

Terminal potential

Answer 3

Voltage across terminals of battery; EMF - IR_internal

Question 4

Internal Resistance

Answer 4

Internal resistance of a battery is like a resistor right next to the battery connected in series.

Question 5

Ohm’s Law

Answer 5

V = IR

Question 6

Resistors In Series

Answer 6

I_series = I₁ = I₂ = I₃.

All resistors in series share the same current.

Vseries = V₁ + V₂ + V₃

Question 7

Resistors in Parallel

Answer 7

V_parallel = V₁ = V₂ = V_3.

All resistors in parallel share the same voltage.

I_parallel= I₁ + I₂ + I₃

Question 8

Resistivity

Answer 8

ρ = RA/L

For higher resistivity, keep wire with high resistance and high area and low length.

Question 9

Concept of parallel-plate capacitor

Answer 9

C = Q/V = εA/d.

Greater capacitance is created by a greater charge on plates (Q) for a given voltage (V), greater plate area (A), or smaller distance between plates (d).

V = Ed, where V is voltage across capacitor, E is electric field between capacitor, and d is the distance between capacitor plates

Question 10

Energy of a Charged Capacitor

Answer 10

U = Q²/2C = ½QΔV = ½C(ΔV)²

U is the potential energy of the charged capacitor, Q is charge stored (magnitude of either +Q or -Q on one of the plates), C is capacitance.

Question 11

Capacitors in Series

Answer 11

1/Ceq = 1/C1 + 1/C2 + 1/C3

Question 12

Capacitors in Parallel

Answer 12

Ceq = C1 + C2 + C3

Question 13

Dielectric

Answer 13

Dielectric = nonconducting material.

Inserting a dielectric between the plates of a capacitor increases the capacitance by either increasing Q (if V is constant) or decreasing V (if Q is constant).

V = V₀/κ

C = κC₀

Question 14

Discharge of a capacitor through a resistor

Answer 14

During the discharge of a capacitor, the capacitor acts as a battery and drives current flow, which decreases with time as the capacitor discharges.

Question 15

Conductivity theory: concentration of electrolytes

Answer 15

Conductivity is affected by electrolyte concentration:

No electrolyte, no ionization, no conductivity.

Optimal concentration of electrolyte, greatest conductivity due to greatest mobility of ions.

Too much electrolyte, ions are too crowded, less ion mobility, less conductivity.

Question 16

Conductivity Theory: Temperature

Answer 16

Conductivity is affected by temperature:

In metals, conductivity decreases as temperature increases.

In semiconductors, conductivity increases as temperature increases.

At extremely low temperatures (below a certain critical temperature typically a few degrees above absolute zero), some materials have superconductivity - virtually no resistance to current flow, a current will loop almost forever under such conditions.

Question 17

Power in Circuits

Answer 17

P = IV = I²R

Question 18

To minimize P dissipated by the wires….

Answer 18

minimize I by maximizing V. This is why power lines transfer electricity at high voltage.

Question 19

Root-mean-square current of AC

Answer 19

I_rms = I_max/√2 = 0.7 I_max

Question 20

Root-mean-square voltage of AC

Answer 20

V_rms = V_max/√2 = 0.7 V_max