This document discusses series and parallel circuits. In a series circuit, all components are connected one after the other so there is only one path for electron flow. The total resistance is the sum of the individual resistances and the current is the same throughout. In a parallel circuit, each branch provides its own path so the voltage is the same across all branches but currents can differ depending on the branch resistances. The total current is the sum of the branch currents and the total resistance is lower than any single branch resistance.

1. Basic Electricity
Series and Parallel Circuits

2. Series Circuits
 When components are connected in
successive order.
 Only one path for electron flow.
 Current is the same for all series
components.

3. Series Circuits

4. Total R = sum of all series
resistances:
 RT = R1+ R2+ R3 ...+ etc.
 Where RT is the total resistance and R1,
R2, R3 are individual series resistances.

5. I = ET / RT
 RT is the sum of all resistances.
 ET is the voltage applied across the total
resistance.
 I is the current in all parts of the string.

6. Series IR Voltage Drops
 The IR voltage across each resistance is
known as an IR drop or a voltage drop.
 It reduces the potential difference available
for the remaining resistance in a series
circuit.
 V1, V2 etc are used for the voltage drops
across each resistor to distinguish them from
the applied voltage source ET. V1 = IT X R1,
V2 = IT X R2, etc
 ET = V1 + V2 + .... + etc

7. Total Power in Series Circuits
 The total power is the sum of the power
dissipated in each part of the circuit or:
 PT = P1 + P2 + ...+ etc
 Remember: 3 Power Formulas
 P = E x I
 P = I2 x R
 P = E2 / R

8. Effect of an open is a series
circuit
 Because the current is the same in each
part of a series circuit -
 An open results in no current for the entire
circuit.

9. Parallel Circuits
 Each parallel path is a branch with its own
individual current.
 Parallel circuits have one common voltage
across all branches, however -
 Individual branch currents can be different.

10. Parallel Circuits
R1 = 2Ω R2 = 4Ω

11. Voltage is equal across
parallel branches
 Since components are directly connected across
the voltage source, they must have the same
potential as the source.
 Therefore, the voltage is the same across
components connected in parallel.
 Components requiring the same voltage would
be connected in parallel.

12. Each branch I = E / R
 I1 = E / R1
 I2 = E / R2 and so on.
 If individual resistances are the same, then
individual branch currents would also be
the same.

13. Main-line IT = sum of branch
currents
 IT = I1 + I2 + ...+ etc

14. Resistances in parallel
 Total resistance across the main line can
be found by Ohm’s Law: Divide the
common voltage by the total current.
 RT = E / IT
 RT is always less than the smallest
individual branch resistance

15. Reciprocal resistance
formulae
 1 / RT = 1/R1 + 1/R2 + 1/R3 + ... etc
 This formulae works for any number of
parallel resistances of any value

16. If the values of R are the
same
 If all resistors in parallel are the same
value, then use this shortcut:
 The value of one resistor/total number of
resistors = Total resistance

17. If the there are only 2
resistors of differing values
 If there are only two resistors in parallel
and they are different in value, then use
this shortcut:
 R1 x R2/R1 + R2 = Total resistance

18. Power in parallel circuits
 Total power equals the sum of the
individual power in each branch.
 PT = P1 + P2 + ...+ etc
 In both series and parallel circuits the sum
of the individual values of power
dissipated in the circuit = the total power
generated by the source.

19. Effect of an open in a parallel
circuit
 An open in the main line results in no
current in all branches
 An open in a branch results in no current
for that individual branch - other branches
are not affected

20. Effect of a short circuit in
parallel
 A short circuit has practically zero
resistance
 A short results in excessive current