Chapter 10 Electrical Circuits
1. State Kirchhoff's First Law and explain its significance in circuit analysis.
Answer: Kirchhoff's First Law, also known as the junction rule or current law,
states that the algebraic sum of currents entering any junction (or node) in an
electrical circuit is equal to the algebraic sum of currents leaving that junction. In
simpler terms, the total current flowing into a point must equal the total current
flowing out of that point. This law is a direct consequence of the conservation of
electric charge. It is fundamental for analyzing complex circuits by allowing us to
establish relationships between currents at different points.
2. Explain Kirchhoff's Second Law and its underlying principle.
Answer: Kirchhoff's Second Law, also known as the loop rule or voltage law,
states that the algebraic sum of the electromotive forces (EMFs) in any closed loop
of an electrical circuit is equal to the algebraic sum of the potential differences
(voltages) across all the components in that loop. This law is based on the principle
of conservation of energy. As a charge moves around a closed loop, any energy
gained from EMF sources must be entirely used up by the potential drops across
the resistive components.
3. Define a 'node' in the context of electrical circuits and describe its role.
Answer: In an electrical circuit, a node (or junction) is a specific point where two
or more circuit components, such as resistors, capacitors, inductors, or voltage
sources, are connected. Nodes serve as crucial connection points where current can
branch out or combine. Analyzing the currents entering and leaving nodes, as
dictated by Kirchhoff's First Law, is essential for understanding the overall current
distribution within the circuit.
4. What is a 'branch' in an electrical circuit, and how does it relate to nodes?
Answer: A branch in an electrical circuit represents a single circuit component (or
a combination of components) that is located between two nodes. It is the pathway
through which current flows between these connection points. Multiple branches
can exist between the same two nodes, especially in parallel circuits, providing
alternative routes for current.
,5. Describe a 'loop' in an electrical circuit and its importance in applying
Kirchhoff's Laws.
Answer: A loop in an electrical circuit is any closed path that starts at a particular
node and travels through a sequence of branches, eventually returning to the same
starting node without passing through any node more than once. When applying
Kirchhoff's Second Law (the voltage law), we analyze the sum of EMFs and
potential differences around such closed loops to determine the voltage
relationships between different components within the circuit.
6. Explain the key characteristics of a series circuit regarding current and the
number of paths for current flow.
Answer: A series circuit is characterized by having only one single path for the
electric current to flow. This means that all the components in the circuit are
connected end-to-end, forming a single loop from the EMF source back to its
terminal. Consequently, the current is the same at all points throughout the series
circuit, as there are no alternative pathways for the charge to take.
7. Discuss the disadvantages of using series circuits in practical applications.
Answer: Series circuits have several limitations that make them less suitable for
many practical applications. These include:
Component Failure: If one component in a series circuit fails (e.g., a bulb
burns out or a resistor breaks), the entire circuit is broken, and all other
components cease to function because the current flow is interrupted.
Voltage Division: The total voltage supplied by the EMF source is divided
among all the components in the series circuit. This means each component
receives only a fraction of the total voltage, which might not be sufficient for
some devices to operate optimally.
Current Dependency: The current is the same through all components,
which might not be ideal if different components require different operating
currents.
8. Describe the key features of parallel circuits, focusing on current paths and
current distribution.
Answer: Parallel circuits are designed to provide more than one path for the
electric current to flow between the nodes connected to the EMF source. This
arrangement means that the current supplied by the source splits and flows through
,each of the parallel branches. Consequently, the current is generally not the same
at all points in a parallel circuit; the amount of current flowing through each
branch depends on the resistance of that particular branch. The charge flowing
through each branch is inversely proportional to the resistance of that branch,
according to Ohm's Law (I=V/R), as the voltage across each parallel branch is the
same.
9. Define 'internal resistance' of a power source and explain its effect on the
circuit.
Answer: Internal resistance (r) is the inherent resistance within a real power
source (such as a battery or generator) to the flow of electric current. It represents
the opposition to current flow due to the materials and construction of the source
itself. When current (I) flows through the power source, a potential difference (Ir),
known as the lost volts, is developed across this internal resistance. This means
that the terminal potential difference (the voltage available across the external
circuit) is always less than the electromotive force (EMF) of the source by the
amount of the lost volts (Vterminal=EMF−Ir).
10. Outline the experimental procedure to investigate the internal resistance
of a power source.
Answer: An experiment to investigate the internal resistance of a power source
typically involves the following steps:
1. Set up a circuit consisting of the power source connected in series with an
ammeter, a variable resistor, and a switch. Connect a voltmeter in parallel
across the terminals of the power source to measure the terminal potential
difference (V).
2. Close the switch and record the initial readings of the voltmeter (V) and the
ammeter (I).
3. Adjust the variable resistor to obtain a range of different current values (I)
flowing through the circuit. For each adjustment, record the corresponding
terminal potential difference (V).
4. Plot a graph of terminal potential difference (V) on the y-axis against the
current (I) on the x-axis.
5. The graph will typically be a straight line with a negative slope. The
gradient of this V/I graph is equal to the negative of the internal
resistance (−r). The y-intercept of the graph represents the electromotive
force (EMF) of the power source.
, 11. Define 'terminal potential difference' and explain how it differs from the
EMF of a power source.
Answer: The terminal potential difference (terminal PD) is the actual voltage
measured across the external load resistance connected to a power source when
current is flowing. It represents the potential difference that is available to drive
current through the external circuit. The terminal PD is less than the
electromotive force (EMF) of the power source because some of the energy
provided by the source is used to overcome its own internal resistance. This
voltage drop across the internal resistance is the reason for the difference between
the EMF (the total potential difference the source can provide) and the terminal PD
(the potential difference actually delivered to the external circuit).
12. Describe the function of a potential divider circuit and its key components.
Answer: A potential divider is a simple electrical circuit designed to divide a
larger input voltage into a smaller, specific output voltage. Its key components are
typically two or more resistors connected in series across the input voltage
supply. The output voltage is taken across one (or a combination) of these resistors.
The ratio of the resistances determines the fraction of the input voltage that appears
as the output voltage. Potential dividers are commonly used to provide specific
voltage levels for different components within a larger circuit.
13. Identify the physical quantity that is conserved in Kirchhoff's First Law.
Answer: The physical quantity that is conserved in Kirchhoff's First Law (the
junction rule) is electric charge. The law essentially states that charge cannot be
created or destroyed at a junction; the rate at which charge flows into the junction
must equal the rate at which it flows out.
14. Identify the physical quantity that is conserved in Kirchhoff's Second Law.
Answer: The physical quantity that is conserved in Kirchhoff's Second Law (the
loop rule) is energy. The law reflects the fact that as a unit charge moves around a
closed loop, the total energy gained from the EMF sources must equal the total
energy lost due to potential drops across the resistive components.
15. Explain the function of a sensing circuit using LDRs and thermistors,
providing examples of their applications.
1. State Kirchhoff's First Law and explain its significance in circuit analysis.
Answer: Kirchhoff's First Law, also known as the junction rule or current law,
states that the algebraic sum of currents entering any junction (or node) in an
electrical circuit is equal to the algebraic sum of currents leaving that junction. In
simpler terms, the total current flowing into a point must equal the total current
flowing out of that point. This law is a direct consequence of the conservation of
electric charge. It is fundamental for analyzing complex circuits by allowing us to
establish relationships between currents at different points.
2. Explain Kirchhoff's Second Law and its underlying principle.
Answer: Kirchhoff's Second Law, also known as the loop rule or voltage law,
states that the algebraic sum of the electromotive forces (EMFs) in any closed loop
of an electrical circuit is equal to the algebraic sum of the potential differences
(voltages) across all the components in that loop. This law is based on the principle
of conservation of energy. As a charge moves around a closed loop, any energy
gained from EMF sources must be entirely used up by the potential drops across
the resistive components.
3. Define a 'node' in the context of electrical circuits and describe its role.
Answer: In an electrical circuit, a node (or junction) is a specific point where two
or more circuit components, such as resistors, capacitors, inductors, or voltage
sources, are connected. Nodes serve as crucial connection points where current can
branch out or combine. Analyzing the currents entering and leaving nodes, as
dictated by Kirchhoff's First Law, is essential for understanding the overall current
distribution within the circuit.
4. What is a 'branch' in an electrical circuit, and how does it relate to nodes?
Answer: A branch in an electrical circuit represents a single circuit component (or
a combination of components) that is located between two nodes. It is the pathway
through which current flows between these connection points. Multiple branches
can exist between the same two nodes, especially in parallel circuits, providing
alternative routes for current.
,5. Describe a 'loop' in an electrical circuit and its importance in applying
Kirchhoff's Laws.
Answer: A loop in an electrical circuit is any closed path that starts at a particular
node and travels through a sequence of branches, eventually returning to the same
starting node without passing through any node more than once. When applying
Kirchhoff's Second Law (the voltage law), we analyze the sum of EMFs and
potential differences around such closed loops to determine the voltage
relationships between different components within the circuit.
6. Explain the key characteristics of a series circuit regarding current and the
number of paths for current flow.
Answer: A series circuit is characterized by having only one single path for the
electric current to flow. This means that all the components in the circuit are
connected end-to-end, forming a single loop from the EMF source back to its
terminal. Consequently, the current is the same at all points throughout the series
circuit, as there are no alternative pathways for the charge to take.
7. Discuss the disadvantages of using series circuits in practical applications.
Answer: Series circuits have several limitations that make them less suitable for
many practical applications. These include:
Component Failure: If one component in a series circuit fails (e.g., a bulb
burns out or a resistor breaks), the entire circuit is broken, and all other
components cease to function because the current flow is interrupted.
Voltage Division: The total voltage supplied by the EMF source is divided
among all the components in the series circuit. This means each component
receives only a fraction of the total voltage, which might not be sufficient for
some devices to operate optimally.
Current Dependency: The current is the same through all components,
which might not be ideal if different components require different operating
currents.
8. Describe the key features of parallel circuits, focusing on current paths and
current distribution.
Answer: Parallel circuits are designed to provide more than one path for the
electric current to flow between the nodes connected to the EMF source. This
arrangement means that the current supplied by the source splits and flows through
,each of the parallel branches. Consequently, the current is generally not the same
at all points in a parallel circuit; the amount of current flowing through each
branch depends on the resistance of that particular branch. The charge flowing
through each branch is inversely proportional to the resistance of that branch,
according to Ohm's Law (I=V/R), as the voltage across each parallel branch is the
same.
9. Define 'internal resistance' of a power source and explain its effect on the
circuit.
Answer: Internal resistance (r) is the inherent resistance within a real power
source (such as a battery or generator) to the flow of electric current. It represents
the opposition to current flow due to the materials and construction of the source
itself. When current (I) flows through the power source, a potential difference (Ir),
known as the lost volts, is developed across this internal resistance. This means
that the terminal potential difference (the voltage available across the external
circuit) is always less than the electromotive force (EMF) of the source by the
amount of the lost volts (Vterminal=EMF−Ir).
10. Outline the experimental procedure to investigate the internal resistance
of a power source.
Answer: An experiment to investigate the internal resistance of a power source
typically involves the following steps:
1. Set up a circuit consisting of the power source connected in series with an
ammeter, a variable resistor, and a switch. Connect a voltmeter in parallel
across the terminals of the power source to measure the terminal potential
difference (V).
2. Close the switch and record the initial readings of the voltmeter (V) and the
ammeter (I).
3. Adjust the variable resistor to obtain a range of different current values (I)
flowing through the circuit. For each adjustment, record the corresponding
terminal potential difference (V).
4. Plot a graph of terminal potential difference (V) on the y-axis against the
current (I) on the x-axis.
5. The graph will typically be a straight line with a negative slope. The
gradient of this V/I graph is equal to the negative of the internal
resistance (−r). The y-intercept of the graph represents the electromotive
force (EMF) of the power source.
, 11. Define 'terminal potential difference' and explain how it differs from the
EMF of a power source.
Answer: The terminal potential difference (terminal PD) is the actual voltage
measured across the external load resistance connected to a power source when
current is flowing. It represents the potential difference that is available to drive
current through the external circuit. The terminal PD is less than the
electromotive force (EMF) of the power source because some of the energy
provided by the source is used to overcome its own internal resistance. This
voltage drop across the internal resistance is the reason for the difference between
the EMF (the total potential difference the source can provide) and the terminal PD
(the potential difference actually delivered to the external circuit).
12. Describe the function of a potential divider circuit and its key components.
Answer: A potential divider is a simple electrical circuit designed to divide a
larger input voltage into a smaller, specific output voltage. Its key components are
typically two or more resistors connected in series across the input voltage
supply. The output voltage is taken across one (or a combination) of these resistors.
The ratio of the resistances determines the fraction of the input voltage that appears
as the output voltage. Potential dividers are commonly used to provide specific
voltage levels for different components within a larger circuit.
13. Identify the physical quantity that is conserved in Kirchhoff's First Law.
Answer: The physical quantity that is conserved in Kirchhoff's First Law (the
junction rule) is electric charge. The law essentially states that charge cannot be
created or destroyed at a junction; the rate at which charge flows into the junction
must equal the rate at which it flows out.
14. Identify the physical quantity that is conserved in Kirchhoff's Second Law.
Answer: The physical quantity that is conserved in Kirchhoff's Second Law (the
loop rule) is energy. The law reflects the fact that as a unit charge moves around a
closed loop, the total energy gained from the EMF sources must equal the total
energy lost due to potential drops across the resistive components.
15. Explain the function of a sensing circuit using LDRs and thermistors,
providing examples of their applications.
