Electrical charge and circuits

In a nutshell

Current is the rate of flow of electrical charge through a circuit. Current will only flow through a component if there is a potential difference across that component, and it is in a complete (closed) circuit.

Equations

Word equation	symbol equation
$charge \space flow = current \times time$	$Q = I \times t$

Variable definitions

quantity name	symbol	unit name	unit
$charge \space flow$	$Q$	$coulomb$	$C$
$current$	$I$	$ampere$	$A$
$time$	$t$	$second$	$s$

Current

Current is defined as the rate of flow of charge, which is caused by the flow of electrons in metals. The formula for current is:

$current = \frac{charge \space flow}{time}$

$I = \frac{Q}{t}$

From this relation, it as also possible to calculate the charge flowing through a point. If a current ( $I$ ) flows past a point in a circuit for a length of time ( $t$ ), then the charge flow ( $Q$ ) can be calculated by the following formula:

$charge \space flow = current \times time$

$Q = I \times t$

The current flowing through a component depends on the potential difference across it, as well as the resistance of the component.

Example

A battery charger provides a current of $0.5\ A$ to a mobile phone over a period of $6\ hours$ . How much charge does the battery charger pass to the phone's battery over this time?

First, write out the quantities needed and make sure they are in the correct form:

$I = 0.5 \space A$

$t = 6 \space hours = 21\space600 \space s$ 

Next, write down the equation you need to use:

$Q = I \times t$

Then, substitute the values into the equation:

$Q = 0.5 \times 21\space600$

Don't forget to include your units:

$10\space800 \space C$

The battery charger passes $\underline{10,800\ C}$ of charge to the phone's battery.

Circuit symbols

There are many circuit symbols which are needed to draw circuit diagrams. Below is a table containing the important circuit symbols:

Component name	Circuit symbol
Cell
Battery
Open switch
Closed switch
Filament lamp
Fuse
LED
Resistor
Variable resistor
Ammeter
Voltmeter
Diode
LDR
Thermistor
Motor