Cells, EMF, terminal voltage & internal resistance | Electric current | Physics | Khan Academy

Summary

The video script delves into the concepts of Electromotive Force (EMF), terminal voltage, and internal resistance within a cell, aiming to clarify the distinctions and relationships between these electrical terms. It begins with a practical scenario of connecting bulbs to a battery and observing the dimming effect as more bulbs are added, illustrating the impact of increased current draw on voltage. The script then defines EMF as the energy transferred per coulomb by the battery, typically indicated on the battery itself. However, due to internal resistance within the battery, not all of this energy is gained by the charge as some is lost as heat. This heat loss, dependent on the current and internal resistance, results in a lower terminal voltage—the actual energy per coulomb available at the battery's terminals. The script further explains that as current increases (or the charge moves faster), more heat is generated, leading to a greater drop in terminal voltage. Conversely, at zero current, there is no heat loss and the terminal voltage equals the EMF. Finally, the script presents the equation connecting these variables: Terminal Voltage (VT) equals EMF minus the product of current (I) and internal resistance (R), encapsulating the essence of how these factors interact within a battery.

Takeaways

• 🔋 **EMF Defined**: Electromotive force (EMF) is the energy transferred per coulomb by the battery, often indicated on the battery itself (e.g., 1.5V or 9V).
• 🚫 **EMF vs. Force**: Despite its name, EMF is not a force but a measure of energy.
• 🔌 **Terminal Voltage**: Terminal voltage is the actual voltage or energy gained by a coulomb as it travels from the negative to the positive terminal of the battery.
• ⚡ **Internal Resistance**: The internal resistance of a battery is the resistance encountered by the charge as it moves through the battery's chemicals, which can cause energy loss as heat.
• 💡 **Effect of Current**: As more bulbs (or loads) are connected in parallel, more current is drawn from the battery, leading to a drop in terminal voltage due to increased heat loss from internal resistance.
• 🌡️ **Heat Generation**: The movement of charge through the battery's medium generates heat, which is a form of energy loss that reduces the terminal voltage from the EMF.
• 🚀 **Speed and Heat**: Faster movement of charge through the battery's medium results in more heat generation and thus a greater energy loss, lowering the terminal voltage.
• 🔥 **Current and Heat Loss**: The greater the current drawn from the battery, the more heat is lost due to internal resistance, resulting in a lower terminal voltage.
• 🔁 **Charge Speed and Voltage**: If the charge moves very slowly through the battery, less heat is lost, and the terminal voltage approaches the EMF.
• ⏹️ **Zero Current**: At zero current, there is no heat loss, and the terminal voltage equals the EMF.
• 📐 **Ohm's Law Application**: The voltage drop across the battery's internal resistance (due to current flow) can be calculated using Ohm's law (V = I * R), which helps to determine the terminal voltage.

Q & A

What happens when you connect more bulbs in parallel to a battery?

As more bulbs are connected in parallel, the overall current drawn from the battery increases, leading to a decrease in the brightness of the bulbs due to the higher current demand and the internal resistance of the battery.

What is the Electromotive Force (EMF) of a battery?

The EMF, often indicated on the battery, is a measure of the energy transferred by the battery per coulomb of charge. It is not a force but an energy term, typically expressed in volts (e.g., 1.5V or 9V).

How does the internal resistance of a battery affect the terminal voltage?

The internal resistance of a battery causes some of the energy transferred to the charge to be lost as heat. This results in the terminal voltage being less than the EMF, especially when higher currents are drawn from the battery.

Why does the voltage across a battery drop when more current is drawn?

The voltage drop occurs because as more current flows through the battery's internal resistance, more energy is lost as heat. This heat loss reduces the net energy available to the external circuit, hence the terminal voltage decreases.

What is the relationship between the speed at which charge moves through a battery and the terminal voltage?

The faster the charge moves through the battery, the more heat is generated due to the internal resistance, leading to a greater energy loss and thus a lower terminal voltage.

How does the brightness of bulbs connected to an inverter change when more bulbs are turned on?

As more bulbs are turned on and connected to an inverter, the current drawn from the inverter increases, causing a drop in voltage and a decrease in brightness due to the internal resistance and heat loss.

What is the significance of the term 'internal resistance' in the context of a battery?

The internal resistance is the opposition to the flow of electric current inside the battery. It is significant because it determines the amount of energy lost as heat when current flows through the battery, affecting the terminal voltage.

What is the terminal voltage of a battery?

The terminal voltage is the actual voltage available to an external circuit when a battery is connected to a load. It is the net energy gained by a coulomb of charge as it moves through the battery, minus the energy lost due to the internal resistance.

How does the EMF of a battery differ from its terminal voltage?

The EMF is the total energy provided by the battery per coulomb of charge, while the terminal voltage is the actual voltage experienced by the external circuit, which is less than the EMF due to the energy lost as heat because of the internal resistance.

What happens to the terminal voltage when the current through a battery is zero?

When the current is zero, there is no heat loss due to internal resistance, so the terminal voltage equals the EMF of the battery, as all the energy provided by the battery is available to the external circuit.

Can you provide the mathematical equation that connects the EMF, internal resistance, current, and terminal voltage of a battery?

Yes, the equation is: VT = E - I * R, where VT is the terminal voltage, E is the EMF, I is the current, and R is the internal resistance of the battery.