4.3 Electrical circuits
4.3.1 Series and parallel circuits
Learners should be able to demonstrate and apply their knowledge and understanding of:
(a) Kirchhoff’s second law; the conservation of energy
(b) Kirchhoff’s first and second laws applied to electrical circuits
(c) total resistance of two or more resistors in series; R = R1 + R2 + .
(d) total resistance of two or more resistors in parallel;
(e) analysis of circuits with components, including both series and parallel
(f) analysis of circuits with more than one source of e.m.f.
You just need to add up the e.m.f.'s in the circuit to give you the total e.m.f. and then carry on as normal from there. The only issue is when one of the sources of e.m.f. is connected the other way round e.g. if you are charging a battery. In that case, you subtract that e.m.f.'s that are backwards.
4.3.2 Internal resistance
Learners should be able to demonstrate and apply their knowledge and understanding of:
(a) source of e.m.f.; internal resistance
(b) terminal p.d.; 'lost volts'
(c) (i) the equations E = I (R + r) and E = V=Ir
(ii) techniques and procedures used to determine the internal resistance of a chemical cell or other source of e.m.f.
4.3.3 Potential dividers
Learners should be able to demonstrate and apply their knowledge and understanding of:
(a) potential divider circuit with components Learners will also be expected to know about a potentiometer as a potential divider.
(b) potential divider circuits with variable components e.g. LDR and thermistor
(c) (i) potential divider equations e.g.
(ii) techniques and procedures used to investigate potential divider circuits which may include a sensor such as a thermistor or an LDR.
The first equation above is certainly true and does work but it can prove difficult to rearrange successfully. I would avoid it It is not necessary. You can always do V=IR twice, normally once on the whole circuit and then again on an individual resistor..
The second equation is the same as saying that the voltage divides in the same ratio as the resistances. I will say this a lot, and so should you. It is the answer to almost all of the wordy questions on this topic.