Mammoth Memory

Electrical charge

Electrical current within a circuit is the flow of electrical charge over time. Therefore the formula for calculating electrical charge is:

Electrical charge is measured by current multiplied by time

Electrical charge mnemonic

A bull charges into the IT department.



You may see this formula written as `Q = It`    Where:

Electrical charge is measured in coulombs (`C`)

Current is measured in amps (`A`)

And time is measured in seconds (`s`)

The internationally agreed symbols for this formula are:

`Q =` Charge

`I =` Current

`t =` time

To remember that charge is measured in coulombs think of this image:

Electrical charge is measured in coulombs

The bull charges into columns (Coulombs) of computers.


Example 1

A current of 12A flows for 20 minutes into an electric cooker. How much charge has the cooker used?


`E\l\e\c\t\rical\ ch\a\r\g\e = Currentxxtime`


But don’t forget that the time has to be converted to the SI unit.
So `20\ m\i\n\u\tes = 20xx60 = 1200\ s\e\c\o\nds` Therefore:

`E\l\e\c\t\rical\ ch\a\r\g\e = 12xx1200=1440\ co\u\l\ombs`



Example 2

If `18400 C` of charge flows through an air conditioning unit every hour, what current does it draw?


`E\l\e\c\t\rical\ ch\a\r\g\e = Currentxxtime`

Divide both sides by time to make current the subject,

`(E\l\e\c\t\rical\ ch\a\r\g\e)/(time) = (Currentxxcancel(time))/cancel(time)`

`Current=(E\l\e\c\t\rical\ ch\a\r\g\e)/(time)`



The time must be converted from hours to the SI units of seconds so 

`1\ hour = 60\ m\i\n\u\tes = 60xx60 = 3600\ s\e\c\o\nds`                                 


`Current=18400/3600=5.1\ amps`


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