# Resistance and Ohm's Law

## In a nutshell

Resistance opposes current and makes it more difficult for charge to flow. Ohm's law relates potential difference, current and resistance. Conductors are materials in which current can easily flow. Insulators do not allow current to flow easily.

### Equations

#### Word Equation | #### Symbol Equation |

$potential \, difference=current \times resistance$ | $V=IR$ |

### Variable definitions

#### Quantity Name | #### Symbol | #### Unit Name | #### Unit |

$potential \, difference$ | $V$ | $volt$ | $V$ |

$current$ | $I$ | $amp\ (ampere)$ | $A$ |

$resistance$ | $R$ | $ohm$ | $\Omega$ |

## What is resistance?

Resistance is something which makes it difficult for charge to flow around a circuit. The higher the resistance, the harder it is for current to flow. Resistance is measured in Ohms, $\Omega$, and has the symbol $R$.

The circuit symbol for a resistor is a rectangle. | |

Resistance is caused by the collisions of the charge carriers, the electrons, with the ions in the wires and electrical components. When the electrons collide with the ions, they transfer some of their energy to heat.

Every electrical circuit will have some resistance involved as it is impossible to have no collisions. Wires have next to no resistance compared to most circuit components so are assumed to have no resistance when dealing with calculations and experiments.

## Ohm's Law

A German physicist noticed the relationship between current and resistance and came up with a relationship which is now called Ohm's law. Ohm's law is very important to remember in electric circuits and is given by:

$potential \, difference = current \times resistance \\ \ \\ V=IR$

Ohm's law can be used to find out missing values in circuits.

##### Example

*Calculate the voltage in the circuit shown in the diagram.*

$I=0.1A \\ \ \\ R=200 \Omega$

*Write down the equation you need to use:*

$V=IR$

*Then, substitute the values into the equation:*

$V=0.1 \times 200$

*Don't forget to include your units:*

$V=20\,V$

*The voltage of in the circuit is $\underline{20\ V}$*

## Conductors and insulators

#### Key word | #### Definition |

**Conductor** | Conductors are materials in which charge can easily flow. Therefore it is easy for an electrical current to pass through a conductor. |

**Insulator** | Insulators are materials in which charge can not easily flow. It is very difficult for electrical current to flow in an insulator. |

Whether or not a material is a conductor or an insulator depends on its molecular structure. Metals, such as copper and iron, have lots of free electrons which can be used as charge carriers and hence are very conductive. Materials such as plastic and glass do not have many free electrons in their structure which means it is very hard for electrical current to flow. Both can be extremely useful either by themselves or when used together.

##### Example

*Your phone charger cable has a conductive material inside to allow current to flow through it, however it is coated in an insulating material to stop the current flowing through you causing you to get a shock! *