A physicist’s insights

# Always in flux

### The discoveries about electrical circuits of researcher Gustav Robert Kirchhoff shape grid planning to this day.

#### What are current and resistance?

Do you recall the physics lesson when you learnt about current, voltage and resistance? Current is the directed motion of electrical charge carriers, namely electrons, through a conductor. Voltage is the driving force behind this motion. When electrons pass through a conductor they bump into the atoms of the conductor material. In the process part of the electrical energy is converted into heat. This impeding of the current’s flow is known as resistance. Its strength depends among other things on the material of which the conductor is made and its length: the shorter the conductor the less the electrons are impeded and the smaller the amount of electrical energy lost as heat.

#### How does current behave at a network node?

One of the pioneers of electrical theory was Gustav Robert Kirchhoff. In 1845 he proposed a law governing the behaviour of current at a junction in a direct current network. Experts refer to this as a parallel circuit (see above graphic), at which the current I1 divides into subcurrents, here called I2 and I3. This occurs in inverse proportion to the resistance of the respective conductors, here termed R2 and R3. This means that wherever the resistance is lower (R2) more current (I2) flows.

#### How does current flow in a meshed power grid?

To put it simply, the current in the grid follows the line of least resistance. It divides at network nodes in accordance with the respective resistances of the subsequent sections. Kirchhoff’s Law allows these current flows to be calculated. The interconnected grids in Germany and Europe operate on alternating current. Rather than resistance these are subject to impedance, from the Latin for “inhibit” or “impede”.). However, here too Kirchhoff’s Law applies, provided the elements in the circuit may be described as concentrated components. The current splits in accordance with the impedance relationships of parallel sections of the network. In our example (see above graphic), most of the current flows through section 2.

To this day these physical laws govern the operation and expansion of the transmission network.

#### Pioneer of electrical theory:

Gustav Robert Kirchhoff (1824 – 1887)