LDR circuits and parallel networks

Last time

Before we start: last time, in Kirchhoff's laws and cells in parallel, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Reading the light

Streetlights switch on at dusk by themselves. Somewhere a light-dependent resistor senses the dimming light, and its resistance rises. In a circuit with several resistors and a battery, one changing resistance can alter every current and voltage. Here is how to follow the changes.

The LDR

The resistance of a light-dependent resistor decreases as the light intensity increases. More light releases more charge carriers in the semiconductor, so more carriers per unit volume, and less resistance. To answer a what-happens question, always start with the LDR, then total resistance, then the battery current.

An LDR across a cell

A cell of e.m.f. four point nine volts is connected straight across an LDR of two hundred and thirty ohms. The voltmeter reads four point seven eight volts. The power in the LDR is V squared over R: four point seven eight squared over two thirty, zero point zero nine nine watts. The lost volts are zero point one two. The current is V over R. So r is zero point one two times two thirty over four point seven eight: five point eight ohms. A possible systematic error in the digital voltmeter is a zero error or calibration error.