D.C. circuits

You will be able to

• Use E = V + Ir to explain why the terminal p.d. falls as the current increases.
• Determine the e.m.f. (intercept) and internal resistance (minus the gradient) from a graph of V against I.
• Show that the power delivered to the load is a maximum when the load resistance equals the internal resistance.

You will be able to

• Apply Kirchhoff's first law (ΣI into a junction = ΣI out) as a consequence of conservation of charge.
• Apply Kirchhoff's second law (ΣE = ΣIR round a loop) as a consequence of conservation of energy.
• Determine the currents in a two-loop network, including their directions from the signs of the answers.

You will be able to

• Use V_out = V_in × R₂ / (R₁ + R₂) for a potential divider, including one with a thermistor or an LDR.
• Explain how a potentiometer compares potential differences by a null method.
• Determine an unknown e.m.f. from balance lengths, E₁ / E₂ = l₁ / l₂.