Electrical charge

Electrical current within a circuit is the flow of electrical charge over time. 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.

NOTE:

You may see this formula written as $Q = It$

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?

Answer:

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

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

$E\l\e\c\t\rical\ ch\a\r\g\e = 12xx1200=14400\ co\u\l\ombs$

Example 2

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

Answer:

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

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 standard unit of seconds, so:

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

therefore:

$Current=18400/3600=5.1\ amps$