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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. \(X_C = \dfrac{1}{2 \pi f C} \). 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

\(P = VI = V^2/R = I^2R\).
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 \(R = R_1 + R_2\) ).