# S-Cool Revision Summary

## S-Cool Revision Summary

#### Kirchoff's First Law

**At any junction in a circuit, the sum of the currents arriving at the junction = the sum of the currents leaving the junction.**

In other words - charge is conserved. If this doesn't happen you'd either get a massive build-up of electrons at a junction in a circuit or you would be creating charge from nowhere! That's not going to happen.

#### Kirchoff's Second Law

**Here is the second principle:**

In any loop (path) around a circuit, the sum of the emfs = the sum of the pds.

In other words - energy is conserved. The total amount of energy put in (sum of the emfs) is the same as the total amount of energy taken out (sum of the pds).

*Note:* pd = V = IR so Σpd = ΣIR

#### Potential

As charge flows around a circuit it uses up its energy (its **electrical potential energy**) and turns it into other forms, such as heat and light.

Fortunately for us, electrons use **more** energy going through **larger** resistances and **less** energy going through **smaller** resistances. This means the larger the resistor, the greater the voltage needed across it for the same current to flow through it. In maths language, **the ratio of resistances gives you the ratio of voltages.**

**Let me show you with a simple example:**

Imagine the current leaves the cell with 3V. The current will use 1V (1JC^{-1}) in going through the 1Ω resistor and 2JC^{-1} in getting through the 2Ω resistor.

#### Potential Dividers

In this circuit you have a long piece of resistance wire.

As the charge passes through the wire it uses up its potential. Note this simple connection:-

**Halfway** through the wire it has used **half** the potential

**1/4** of the way through the wire it's used a **1/4** of the potential.

**1/8** of the way through the wire it's used an **1/8** of the potential.

**You can write this as a ratio:**

#### Using Potential Dividers to find EMFs

Potential dividers can be used to find the emf of a cell.

*Remember:*

This ratio can be used to measure all sorts of resistances, potential differences and emfs.