# Basics of Electricity

## Basics of Electricity Revision

**Basics of Electricity**

In this section we will begin to look at the topic of **electricity**. Many of these terms will be familiar to you from GCSE but you will require a more in-depth understanding for A-level.

**Circuit Symbols**

Circuits can be drawn using symbols to give a **visual representation** of what the desired circuit should include. Symbols allow scientists to visualise a circuit without a language barrier as symbols are universal. Here is some of the** circuit symbols **you will come across in this unit:

**Electric Current**

**Electric current**, which is measured in **a****mperes** \text{(A)} is defined as “the rate of flow of **charge**”.

As an equation, this can be written as:

I=\dfrac{\Delta Q}{\Delta t}

- I=
**current**in amperes \text{(A)} - \Delta Q= the
**charge**in coulombs \text{(C)} - \Delta t= the
**time**in seconds \text{(s)}

**Conventional current** is defined as the flow of charge from the positive terminal to the negative terminal within a **circuit**. However, this definition was given before scientists had a full understanding of electricity and is actually opposite to the flow of **electrons** which flow from negative to positive.

To measure **current**, an **ammeter** is often used. For measuring very small **currents**, a **microammeter** may be used. These pieces of equipment can only work if they are inserted in **series** into a circuit.

**Potential Difference**

**Potential difference** (p.d) is defined as “the **work done** per unit of **charge**” and is measured in **Volts** \text{(V)}. This can be written as an equation in the form:

V=\dfrac{W}{Q}

- V= the
**potential difference**in volts \text{(V)} - W=
**work done**in joules \text{(J)} - Q=
**charge**in coulombs \text{(C)}

The **potential difference **is a result of the **power** source which causes one end of the **circuit** to become positively charged, whilst the other end of the **circuit** becomes negatively charged.

To measure **potential difference**, a **voltmeter** is used and should always be connected in **parallel** across the component needed to be measured. The **terminal pd**, which is the **potential difference** the circuit is supplied with, can be measured by putting the **voltmeter** in **parallel** across the terminals of the power source.

**Resistance**

**Electrical resistance **can be defined as “the opposition to motion of the **charge carriers**” or “the opposition to **current**”.

This can be written as an equation in the form:

R=\dfrac{V}{I}

- R=
**resistance**in ohms (\Omega) - V=
**p****otential difference**in volts \text{(V)} - I=
**current**in amperes \text{(A)}

All components in a circuit have a **resistance** including the wires (in exam questions we considered the wires to have zero resistance) and these resistances contribute to the **total resistance** of the circuit. The amount of resistance determines how much **current **is able to pass through the **circuit**.

One **Ohm** (\Omega) can be defined as one **volt** per **ampere** (\text{VA}^-1) as an alternative unit.

## Basics of Electricity Example Questions

**Question 1:** Give the definitions for current, potential difference and resistance.

**[3 marks]**

Current – the **rate of flow of charge.**

Potential Difference – the **work done per unit of charge.**

Resistance – the **opposing force to current** in a circuit.

**Question 2:** Give an alternative (non-SI) unit to the Ampere for current.

**[1 mark]**

I=\dfrac{Q}{t} so the alternative unit for current would be coulombs per second or \textbf{Cs}\bold{^{-1}}.

**Question 3:** Explain how you would measure the current, potential difference and resistance in a circuit.

**[3 marks]**

To measure current, an **ammeter must be used in series.**

To measure potential difference, a **voltmeter must be used in parallel** to the component you are trying to measure the potential difference of.

Resistance is not measured directly, it needs to be calculated using the equation \bold{R=\dfrac{V}{I}}.

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