Adjustable & Balanced Dividers | CAIE 9702 Paper 2 Notes

Paper 2

Physics9702

Adjustable & Balanced Dividers

A sliding contact changes the fraction of a uniform resistance wire included in the output.

Two dividers can be compared at their midpoints. A zero detector reading means those midpoint potentials are equal.

A slider selects a fraction of the wire

One resistor, three connections. The contact splits it into a top part and a bottom part. Slide the contact to the bottom and Vout is 0. Slide it to the top and Vout is the full Vin.

A uniform wire has resistance proportional to length. With the output measured from one end to the slider, moving the slider farther from that end increases the output p.d.

At one endpoint the output is 0 V; at the other it is the full input p.d. These endpoint checks catch a reversed fraction.

A null reading compares two ratios

Two dividers, one supply. The galvanometer compares the two middle points. When it reads zero, the two chains split the supply in the same ratio.

When the detector reads zero, no current passes through it and the two midpoint potentials are equal. For the labelled balanced bridge:

\frac{R_1}{R_2}=\frac{X}{Y}

Symbols

$R_1, R_2$ = resistances in one divider (\Omega)
$X, Y$ = resistances in the other divider (\Omega)

Your turn9702/22/M/J/25 Q6(c)

Explain why a galvanometer connected between the midpoints of two potential dividers reads zero at balance. [2] (9702/22/M/J/25 Q6)

Show worked answer

The two midpoints are at the same potential, so the p.d. across the galvanometer is zero and no current passes through it.