13.10: How Series and Parallel Circuits Differ
pg. 571
Key Concepts:
5. Connecting loads in series and parallel affects the current, potential
difference, and total resistance.
- Using your knowledge of current, voltage and resistance, you can predict
patterns within circuits.
Loads in Series
- A series circuit has only one path in which the electrons can flow.
- Potential difference occurs instantaneously once current begins to flow.
Current through Loads in Series
- The current flow in a series circuit with one load is greater then a circuit
with two loads.
- As more loads are added there is an increase in the total resistance of the
circuit.
Figure 1: From Ohm’s relationship, the current
in a circuit depends on the resistance in that circuit.
Sample Problem: Comparing the Current in Two Series Circuits
The same type of lamp is used in two series circuits. The first circuit has two
identical lamps (Figure 2). The second circuit has three identical lamps
(Figure 3). The potential difference across the battery is 10 V. The circuit
with two lamps has a resistance 0f 10 Ω. The circuit that has three lamps has
a total resistance of 15 Ω. Use the total resistance given to calculate the
current through each circuit.
Figure 2
Figure 3
Voltage across Loads in Series
- A battery contains potential energy. It converts chemical potential energy
into electrical energy.
- As electrons leave the battery and enter a circuit it creates current.
- As electrons pass through the circuit potential energy is converted to
kinetic energy.
- A circuit with one lamp, the potential energy is converted to light or heat
energy.
- With one load the voltage drop across the load will be equal to the voltage
across the battery.
- A circuit with two lamps, the energy leaving the battery is equal to its
potential energy.
- With two lamps, only have of the potential energy is converted to light or
heat.
- The voltage will drop across each lamp.
- The total voltage drop across the each identical load is equal to the voltage
drop across the battery.
V load = V source
# of loads
Sample Problem: Calculating Voltage in Series Circuit
A series circuit contains three identical lamps (Figure 4). The potential
difference of the battery is 30 V. Calculate the potential difference across
each lamp.
Figure 4
Loads in Parallel
- In a parallel circuit there is more then one path for electrons to flow.
Current through Loads in Parallel
- When two identical loads are connected in a parallel circuit, the current
will split in two.
- When three identical loads are connected in a parallel circuit, the current
will be divided into thirds.
- Mathematical Equation used to calculate current through a load within a
parallel circuit.
I load =
I source
# of loads
Figure 5: The more loads that are connected
in parallel, the more paths electrons have to follow.
Sample Problem: Calculating Current in Parallel Circuit
The total resistance in the circuit in Figure 5 (b) is 2 Ω. The potential
difference of the battery is 18 V. Calculate the current through each lamp.
(Calculate current first)
Figure 5(b)
Voltage across Loads in Parallel
- The resistance in loads connected in parallel is less then a series circuit
with the same number and type of loads.
- Since there are more then one path for electrons to follow there is less
resistance across the load in any one path.
- In a parallel circuit, the voltage drop across a load is equal to the voltage
drop across the battery. This is different in comparison to a series circuit.
Sample Problem: Calculating the Voltage in a Parallel Circuit
A parallel circuit contains three identical lamps (Figure 6). The current
coming out of the energy source is 2.5 A. The total resistance of the circuit is
6.0 Ω. Calculate the voltage across the energy source and across each lamp.
Figure 6
Resistance, Current, and Voltage in Circuits
Table 1: The relationships of Loads in Series and Parallel Circuits
Quantity
Series Circuit
Parallel Circuit
Total resistance of
Increases
decreases
circuit (RT)
Current through loads (I I source decreases as
I source splits among
more loads are added
loads based on the
load)
number of branches in
parallel
Voltage across loads (V V source splits based on
Voltage of each parallel
the number of loads
branch is the same as V
load)
source
Evidence of Learning …. Students can
- describe how lamps or loads can be placed in a series circuit and parallel
circuit.
- explain that the current in a series circuit flows though one circuit and
lessens as the loads are increased.
- understand that the current in a parallel circuit flows through many
different paths.
- understand that the more identical loads that are connected in series, the
voltage drop across each load decrease.
- understand that the voltage drop across a parallel load will be the same as
the voltage drop across the battery.
Check Your Learning
Questions 1 – 7, page 575
Summary:
- In parallel circuit, the total resistance is less than if the loads were
connected in series.
- The more loads that are connected in series, the lower the current will be in
the circuit.
- The more loads that are connected in parallel, the lower the current will be
through each branch.
- In a series circuit with identical loads, the voltage drop across the energy
source is split equally among the loads.
- In a parallel circuit, the voltage drop across each parallel branch will be the
same as the voltage drop across the energy source.