6.3 Electromotive force and internal resistance
2 min read•august 7, 2024
(EMF) and are key concepts in understanding how sources work in electric circuits. They explain why real and can't maintain a constant voltage under all conditions, unlike ideal voltage sources.
These concepts are crucial for analyzing practical circuits and predicting their behavior. By understanding EMF and internal , we can better design and troubleshoot electrical systems, from simple battery-powered devices to complex .
Electromotive Force (EMF) and Voltage Sources
EMF and Ideal Voltage Sources
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- Electromotive force (EMF) represents the energy per unit charge available from a source to produce an electric
- Measured in volts (V)
- Can be generated by various sources (batteries, , generators)
- An provides a constant voltage regardless of the current drawn from it
- Has zero internal resistance
- Maintains a fixed potential difference across its terminals
Real Voltage Sources and Terminal Voltage
- have internal resistance that affects the voltage delivered to the circuit
- As current increases, the decreases due to the across the internal resistance
- The terminal voltage is the voltage measured across the load connected to the source
- The terminal voltage () is related to the EMF (), current (), and internal resistance () by the equation:
- This equation shows how the terminal voltage is reduced by the voltage drop across the internal resistance
Internal Resistance and Circuit Laws
Internal Resistance
- Internal resistance is the resistance within a voltage source that opposes the flow of current
- Represents the source's inability to supply an unlimited amount of current
- Causes a voltage drop within the source, reducing the terminal voltage
- The internal resistance can be modeled as a resistor in series with an ideal voltage source
- The consists of an ideal voltage source with EMF and a resistor with resistance
Kirchhoff's Voltage Law and EMF
- (KVL) states that the sum of voltages around any closed loop in a circuit is zero
- When applying KVL, the EMF is treated as a rise in potential, while the voltage drop across the internal resistance and other circuit elements is treated as a drop in potential
- In a closed loop containing a voltage source, the EMF is equal to the sum of the voltage drops across the internal resistance and the external circuit elements
- Mathematically, , where is the sum of the voltage drops across the external circuit elements
- When analyzing circuits with multiple voltage sources, KVL is applied to each closed loop, considering the EMFs and voltage drops across all elements in the loop
Key Terms to Review (18)
Ammeter: An ammeter is an electrical instrument used to measure the current flowing through a circuit. It is typically connected in series with the circuit, allowing it to provide a direct reading of the current in amperes. Understanding how an ammeter works is crucial when examining electromotive force and internal resistance, as it provides insight into the relationship between voltage, current, and resistance within an electrical system.
Batteries: Batteries are devices that store and convert chemical energy into electrical energy through electrochemical reactions. They consist of one or more electrochemical cells, which produce a flow of electric current when connected in a circuit. Batteries play a crucial role in providing the necessary voltage and current to power various electronic devices, as well as establishing a potential difference, which is fundamental to the operation of electric circuits.
Current: Current is the flow of electric charge, typically measured in amperes, that moves through a conductor as a result of an electric potential difference. It plays a crucial role in how circuits operate and is essential for understanding the behavior of electrical devices. Current is driven by electromotive force and can be influenced by factors like resistance, which affects how much charge flows for a given voltage.
Electromotive force: Electromotive force (emf) is the energy provided per unit charge by a source of electrical energy, like a battery or generator, when it drives current through a circuit. This concept is crucial for understanding how energy is transformed and transferred in electrical systems, particularly when considering factors like internal resistance that can affect the actual voltage available to a load.
Emf = ir + ir: The equation 'emf = ir + ir' represents the relationship between electromotive force (emf), current (i), and resistance (r) in an electrical circuit. This formula indicates that the total emf in a circuit is equal to the sum of the voltage drops across the external resistance and the internal resistance of the power source. Understanding this equation helps to analyze how energy is distributed in a circuit and how internal resistance affects overall performance.
Equivalent circuit: An equivalent circuit is a simplified representation of an electrical circuit that retains the essential features and behaviors of the original circuit while making analysis easier. This concept is particularly useful for understanding how various components, like batteries and resistors, interact when they are combined in series or parallel configurations. It allows for easier calculations of voltage, current, and resistance, making it a vital tool in circuit analysis.
Generators: Generators are devices that convert mechanical energy into electrical energy through electromagnetic induction. They operate based on the principle that moving a conductor through a magnetic field induces an electromotive force (emf), allowing for the generation of electric current. This process is crucial in understanding how electric power is produced and distributed, especially when considering aspects like internal resistance that can affect efficiency.
Ideal voltage source: An ideal voltage source is a two-terminal device that provides a constant voltage output regardless of the current drawn from it. This means that it can maintain its voltage under any load condition, with no internal resistance affecting its performance. This idealized concept is essential for understanding how real voltage sources behave in circuits, especially when considering electromotive force and internal resistance.
Internal resistance: Internal resistance is the opposition to the flow of electric current within a power source, such as a battery or cell. It plays a crucial role in determining the overall efficiency and performance of the power source, affecting the voltage output under load. High internal resistance can lead to significant voltage drops and decreased power delivery when the device is used in practical applications.
Kirchhoff's Voltage Law: Kirchhoff's Voltage Law (KVL) states that the sum of the electrical potential differences (voltages) around any closed circuit loop must equal zero. This law emphasizes the principle of conservation of energy within electrical circuits, connecting potential energy and electric potential difference, and is critical for analyzing how voltage is distributed in circuits, understanding electromotive forces, and considering the impact of internal resistances.
Power Distribution Networks: Power distribution networks are electrical systems that deliver electricity from transmission systems to individual consumers. These networks are crucial for managing the flow of electrical energy, ensuring it reaches homes and businesses efficiently and reliably while maintaining the balance between supply and demand.
Real voltage sources: Real voltage sources are electrical devices that provide a specific voltage to a circuit but are not ideal due to the presence of internal resistance. This internal resistance affects the output voltage and current supplied by the source, causing the actual voltage delivered to a load to differ from the nominal voltage. Understanding how real voltage sources operate is essential for analyzing circuits accurately, especially when considering factors like electromotive force and internal resistance.
Resistance: Resistance is a measure of the opposition to the flow of electric current in a circuit, quantified in ohms (Ω). It plays a crucial role in determining how much current will flow for a given voltage according to Ohm's law, and it influences the performance of electrical components and circuits, particularly in terms of energy dissipation as heat.
Solar cells: Solar cells, also known as photovoltaic cells, are devices that convert light energy directly into electrical energy through the photovoltaic effect. This process involves the absorption of photons from sunlight, which then generates electron-hole pairs in a semiconductor material, leading to a flow of electric current. Solar cells play a crucial role in renewable energy technology, making them significant in discussions about electromotive force and internal resistance.
Terminal voltage: Terminal voltage is the voltage across the terminals of a battery or power source when it is connected to a circuit and delivering current. It reflects the actual voltage available to drive current through the load, taking into account both the electromotive force (emf) of the source and any internal resistance that may reduce the voltage available at the terminals.
Voltage: Voltage, also known as electric potential difference, is the measure of electric potential energy per unit charge between two points in an electric field. It represents the work done to move a charge from one point to another and is a key factor in understanding how electrical energy is transferred within circuits and systems. Voltage influences the flow of current, determines the behavior of capacitors, and plays a crucial role in electromotive forces generated by sources like batteries.
Voltage drop: Voltage drop refers to the reduction in voltage across a component or conductor in an electrical circuit, caused by the resistance of the material through which the current is flowing. This concept is crucial for understanding how electrical energy is distributed and consumed in various circuit configurations, affecting the overall efficiency and functionality of electrical systems.
Voltmeter: A voltmeter is an instrument used to measure the electrical potential difference, or voltage, between two points in an electric circuit. It is essential for understanding how electromotive force operates in a circuit and how internal resistance can affect voltage readings. By connecting a voltmeter across components, one can assess not only the voltage supplied by a source but also the impact of any internal resistance present within that source or circuit.