phy resistance (urgent)

1. Current in the circuit = emf/total resistance (Ohm's Law)
i.e. current = 12/(1000 + rb)
voltage across voltmeter = current x resistance of voltmeter = [12/(1000 + rb)] x 1000
but it is given that voltage across voltmeter = 11.9 v
hence, 11.9 = [12/(1000 + rb)] x 1000
which the is equation given in your solution.

2. Apply Ohm's Law to the whole circuit,
emf = current x total resistance of circuit
6 = 0.6 x (8 + rb)
i.e. 6/(8 + rb) = 0.6
which is the equation given in your solution.

你呢兩題係ohm's law嘅應用

第一題先計總電流︰電壓錶指示11.9v，內阻係1kohm，咁流過嘅電流係 11.9/1000=11.9ma，由於係串聯電路，流過電池嘅電流都係11.9ma。
題目指出電錶電壓低於電池電壓0.1v，其原因係電池內阻影響，當電池內阻大時產生嘅電壓降亦加大，令電池端點電壓下降。
咁電池內阻可以點計呢，已知電流係11.9ma，電壓係0.1v，內阻係0.1/0.0119=8.4ohm。

第二題同第一題類似，不過你要先計電阻嘅電壓降，你畫電流值應該係0.6a，咁電阻嘅電壓降係0.6x8=4.8v，咁電池嘅端電壓唔見咗1.2v，照返上題就計到內阻。