Measurement

Current

• Current I - Rate of charge flow.

V = Support for current

• Potential difference V. Units: Volts
• V = IR

Utility supply

India

• 220V RMS AC, at 50Hz. Single phase.

USA

120 volts RMS ± 6% at 60Hz.

Opposition to current

Components

• Impedence: Overall opposition to current.
  • $Z=\sqrt{X^2+R^2}$ as reactance X comes with a phase shift.
• Resistence R. Independent of frequency.
  • It is usually reasonable to assume that an input impedance is just resistance if the input signal has a low frequency (less than 1kHz).
• Reactance X dependent of frequency.
  • Capacitive Reactance - Decreases with frequency. $X_C={\displaystyle \frac{1}{2\pi fC}}$. DC has infinite $X_C$.
  • Inductive reactance $X_L=2\pi fL$. Increases with frequency. DC has zero $X_L$
  • $X=X_L-X_C$

Power

• $P=VI=V^2/R=I^{2}R$.
• This corresponds to the rate of heat generation + motion etc..
• To reduce power consumption, V being constant, reduce I.
  • In a parallel circuit, can turn off branches (as $I=I_1+I_2$ ).
  • In a series circuit, increase the resistance in the regulator/ rheostat (as $R=R_1+R_2$ ).