Internal Resistance in Batteries Start the car headlights

Internal Resistance in Batteries

Start the car headlights dim Why? Inference: Voltage from batteries is not constant, but depends somehow on current. https: //www. youtube. com/watch? v=Z 7 q. LVu 3 O 6 p. Y seconds: 11 -16

Anatomy of a battery Batteries are comprised of • Anodes Allows build-up of electrons Commonly zinc • Cathodes Eager to accept electrons Commonly carbon Separated by electrolyte Typically a weak acid suspended in gel

Batteries provide energy Battery’s rated emf Current through circuit Internal resistance is typically abbreviated with lower-case r

Example (part A) A device with a 65 - resistor is connected to the terminals of a battery whose emf is 12. 0 V and whose internal resistance is 0. 5 . Calculate the current in the circuit. Voltage to circuit, terminal voltage, current, and internal resistance? Voltage to circuit, current, resistance? substitute Isolate emf Isolate current Substitute known values Solve

Example (part B) A device with a 65 - resistor is connected to the terminals of a battery whose emf is 12. 0 V and whose internal resistance is 0. 5 . Calculate the terminal voltage. Voltage to circuit, current, resistance? Substitute known values Solve

Example (part C) A device with a 65 - resistor is connected to the terminals of a battery whose emf is 12. 0 V and whose internal resistance is 0. 5 . Calculate the power dissipated by the device. Power, current, voltage Power, current, resistance Substitute known values Solve

Example (part D) A device with a 65 - resistor is connected to the terminals of a battery whose emf is 12. 0 V and whose internal resistance is 0. 5 . Calculate the power dissipated by the internal resistance in the battery. Power, current, voltage Power, current, resistance Substitute known values Solve Batteries get warm! And get warmer with larger currents.

Graphical model voltage current

With non-zero voltage from source, we expect non-zero current to circuit. The y-intercept on voltage vs. current graph gives a clue that there is internal resistance in the voltage source. voltage Graphical model (cont’d) current

Finding internal resistance w/ graph Suppose you have known resistors and measure the current and voltage. How could you use a graph to determine internal resistance? Voltage, emf, current, internal resistance Rewrite as y = mx + b NOTE: I like the logic, but am puzzled by the implications. Negative resistance? Hrmph.

Finding internal resistance w/ graph Suppose you have known resistors and measure the current. How could you calculate internal resistance? Emf, current, resistance, internal resistance Isolate inverse of current Rewrite as y = mx + b