Experimental Skills — Electrical & Optical Experiments (Medium)

Quick Recap — Electrical & Optical Experiments

• Metre bridge: unknown $X=R\dfrac{l}{100-l}$ (balance length $l$ cm); best sensitivity near the middle. Potentiometer: compares emfs $\dfrac{\varepsilon_1}{\varepsilon_2}=\dfrac{l_1}{l_2}$; internal resistance $r=R\dfrac{l_1-l_2}{l_2}$.
• Ohm's law: the slope of a $V$–$I$ graph is the resistance.
• Mirror/lens: use $\dfrac{1}{v}+\dfrac{1}{u}=\dfrac{1}{f}$ (mirror) or $\dfrac{1}{v}-\dfrac{1}{u}=\dfrac{1}{f}$ (lens); a $\dfrac{1}{v}$ vs $\dfrac{1}{u}$ graph gives $f$ from the intercept.
• Refractive index: real/apparent depth, or prism $n=\dfrac{\sin\frac{A+\delta_m}{2}}{\sin\frac{A}{2}}$.

Worked mini-example. A metre bridge balances at 40 cm with a known 6 $\Omega$ in the right gap: $X=6\times\dfrac{40}{60}=4\ \Omega$.