# Internal Resistance

### Explanation

All electrical cells have some amount of resistance inside of them - whether unintentionally, due to the natural resistance of the materials the cell is made out of, or intentionally, for safety reasons in certain high-voltage applications. This is known as **internal resistance**.

The symbol for internal resistance is

**internal resistance**.

**load resistance**.

### Electromotive Force

The electromotive force (EMF) of a circuit is the **energy transferred into the circuit per unit of charge**. It is equal to the **voltage across a cell, excluding the internal resistance**.

Since EMF is a voltage, we can use the voltage formula,

### Terminal Potential Difference

The terminal potential difference, or terminal voltage, is the **voltage across the entire power supply, including its internal resistance**. This is also equal to the potential difference across the load - since both form identical circuits!

### Lost Volts

The actual productive voltage of a power supply with internal resistance will be lower than its EMF, because some voltage is lost on the internal resistance. This lost voltage is known as **lost volts**.

Since the ratio of voltage across components in a series circuit is equal to the ratio of the components' respective resistance (as it is a potential divider circuit), the greater the resistance of the load, the less the lost volts are.

#### Formula

is the EMF, in volts. is the terminal p.d., in volts. is the **current through the circuit**, in amperes.is the internal resistance of the power supply.