In this section, we focus on direct voltages (DC). All types of voltage sources are based on the separation of charges by providing a potential difference between a negative and a positive pole.
If the voltage source is connected to a load, which in the simplest case is an ohmic resistor, an electric current flows that is limited only by the total resistance. Typical voltage sources for direct current are batteries or accumulators, but also solar cells or the frequently mentioned fuel cells in hydrogen technology. In addition, there are thermoelectric voltages, which are generated by temperature differences between different metals.
If the open-circuit voltage is measured without a load, it is usually higher than under load. The voltage drop when connecting a load occurs because voltage sources also have an internal resistance $R_i$, which causes a voltage drop. The available voltage can be expressed using Ohm’s law and Kirchhoff’s loop rule as:
$$
U = U_0 - IR_i
$$
The current is then given by:
$$
I = \frac{U_0}{R_i + R_a}
$$
where $R_a$ is the resistance of the load.
By substituting and rearranging, one obtains the terminal voltage under load:
$$
U = U_0 \left(1 - \frac{R_i}{R_i + R_a}\right)
$$
Modern voltage sources usually have a very small internal resistance, so the voltage does not drop significantly even at larger currents.
A battery with an open-circuit voltage of 1.6 V is connected to a resistor of $27\,\Omega$. To what value does the voltage drop if the internal resistance of the battery is $1\,\Omega$?
1.51 V