Electric circuits Internal resistance Intro: PHYSICS grade 11 and 12

Summary

In this lesson, Miss Martens explores electric circuits, focusing on internal resistance and electromotive force (EMF). She explains that while batteries have a constant internal resistance, it reduces the voltage available to the external circuit, creating 'lost volts' within the battery. Using simulations, she demonstrates the difference between EMF (measured when no current flows) and terminal voltage (measured when current flows), and introduces the formula connecting EMF, external resistance, and internal resistance. The video provides clear examples, practical explanations, and highlights how voltage drops inside a battery affect energy delivery, helping students grasp these critical concepts in circuit analysis.

Takeaways

• 🔋 Every battery has an internal resistance that affects the energy it supplies to an external circuit.
• ⚡ EMF (Electromotive Force) is the maximum energy per unit charge a battery can provide when no current flows.
• 📏 Previously, internal resistance and wire resistance were considered negligible in circuit calculations.
• 🔌 Terminal potential difference (V_external) decreases when current flows due to the battery's internal resistance.
• 💡 Lost volts (V_internal) represent the energy used within the battery to overcome its own resistance.
• 🧮 Ohm's Law can be applied separately for external and internal resistances: V = I × R.
• 🔹 Total EMF can be expressed as EMF = I × (R_external + r_internal).
• 📊 A voltmeter across battery terminals measures EMF when the switch is open and V_external when the switch is closed.
• 🔥 Energy lost due to internal resistance is typically converted into heat within the battery.
• 🔄 Total current remains the same throughout the circuit, affecting both external and internal voltage drops.
• 📝 V_external (V_terminal or V_load) is the voltage available for external components like resistors or bulbs.
• 🎯 The difference between EMF and V_external is key to understanding real-world battery performance in circuits.

Q & A

What is internal resistance in a battery?

Internal resistance is the inherent resistance within a battery that causes some of the battery's energy to be used internally rather than supplied to the external circuit. It results in a voltage drop when current flows.

How was internal resistance treated in earlier circuit lessons?

Earlier lessons often ignored internal resistance, assuming it and the resistance of connecting wires were negligible, and did not include it in circuit calculations.

What is the electromotive force (EMF) of a battery?

EMF is the maximum energy per unit charge that a battery can provide. It is measured in volts and represents the potential difference when no current is flowing.

How is the terminal voltage (V_external) different from EMF?

Terminal voltage is the voltage across a battery’s terminals when current is flowing. It is usually lower than the EMF due to the voltage drop caused by internal resistance.

What are lost volts or V_internal?

Lost volts, or internal voltage, are the difference between EMF and terminal voltage. They represent the energy used inside the battery to overcome its internal resistance.

What happens to the voltmeter reading when the switch in a circuit is open?

When the switch is open, no current flows, and the voltmeter reading across the battery terminals equals the EMF.

What happens to the voltmeter reading when the switch is closed?

When the switch is closed, current flows, and the voltmeter reading drops from the EMF to the terminal voltage due to the internal resistance of the battery.

How can Ohm’s law be applied to calculate V_external and V_internal?

Using Ohm’s law, V_external = I × R_external and V_internal = I × r_internal, where I is the total current, R_external is the external circuit resistance, and r_internal is the battery's internal resistance.

What is the relationship between EMF, internal resistance, and external resistance?

EMF equals the total current multiplied by the sum of internal and external resistances: ℰ = I × (R_external + r_internal).

Why does internal resistance cause a battery to heat up?

Internal resistance converts some of the battery’s energy into heat as current flows through it, which is why batteries can warm up when supplying current.

How can lost volts be calculated from EMF and terminal voltage?

Lost volts can be calculated as V_internal = EMF − V_external.

Why is the terminal voltage important for the external circuit?

The terminal voltage represents the actual voltage available to the external circuit, determining the energy supplied to devices like resistors, bulbs, or appliances.