⇒ Electronic engineers also need to know the simple ways that current and potential difference act in circuits

⇒ The 'rules' for these are universal and apply to simple circuits as well as the intricate circuits found in integrated circuit chips

⇒ These simple rules were first worked out by a German physicist called Gustav Kirchoff in 1845 and have become known as Kirchoff's First and Second Circuit Laws

⇒ At a circuit junction, the sum of the currents flowing into the junction equals the sum of the currents flowing out of the junction

⇒ This shows a circuit junction with two currents with two currents (I₁ and I₂) flowing into the junction and three currents flowing out of the junction (I₃, I₄ and I₅)

⇒ Kirchoff's First Circuit Law states: I₁ + I₂ = I₃ + I₄ + I₅

⇒ Conventional current flows from positive to negative, so the current flowing into the junction is measured by ammeter A₁

⇒ Current splits or recombines at a junction, so the current flowing out of the junction is measured by A₂ and A₃

• Kirchoff's First Circuit Law tells you that: A₁ = A₂ + A₃

⇒ Written more generally in mathematical notation the law can be summarised by:

⇒ Or, in other words, current is conserved at junctions

⇒ Looked at from a slightly different persepctive, as current is the rate of flow of charge (I = Q/t) then Kirchoff's First Circuit Law could be written in terms of charge: At a circuit junction, the sum of the charge flowing into the junction equals the sum of the charge flowing out of the junction (per second)

⇒ In a closed circuit loop, the sum of the potential differences is equal to the sum of the electromotive forces

⇒ This shows a single closed loop series circuit. In this case, there is one emf and two pds, and Kirchoff's second law states that: ε = V₁ + V₂

⇒ Or more generally, using mathematical notation, for any closed circuit loop: Σ ε = Σ V

⇒ If the circuit is extended to make a parallel circuit, this parallel circuit is effectively made up of two series circuits: ABCD and AEFD, so:

⇒ Kirchoff's second circuit law applies to any circuit, but in the case of parallel circuits, the circuit must be considered as a succession of individual series circuits with the same power supply

• This law is based on the conservation of energy: the energy per coulomb transferred to the charge by the battery, ε, is then transferred to other forms of energy by the charge as it flows through the circuit components

⇒ The standard AA battery is a common batery for a number of devices

⇒ The word battery is actually a misnomer: the AA batteries you put in devices are actually cells and several cells joined in series or parallel are called a battery

⇒ When identical cells are connected in parallel to form a battery, the emf of the resultant battery is just the emf of the individual cells

• Connecting cells in parallel is generally not a good idea as they are rarely identical, and one cell will force current back into the other, causing damage to the cells
• Cells in parallel have a higher capacity for storing and transferring electrical energy than single cells, but it is better to just buy and use a bigger single cell

⇒ Also see our notes on:

• Electrical Power in Circuits
• Internal Resistance and Electromotive Force
• Resistor Networks
• Potential Dividers