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# Electric charge

## Definition of electric charge

An object carries an electric charge if it can exert a force on other charged objects.

Protons carry a positive ($\Tred{+}$) charge while electrons carry an equal negative ($\Tblue{-}$) charge. Neutrons have no charge.

The total or net charge of an object is equal to the sum of all the positive charges from protons and the negative charges from electrons.

The electric shock you sometimes get when you touch an object is caused by the transfer of charges from the object to your hand.

Any charged object (with a net positive or negative charge) carries an unequal number of protons and electrons. If an object has an imbalance of protons and electrons, it is said to be charged. The imbalance of charges is called static electricity.

If an object carries more electrons than protons, it is negatively charged.

A lithium ion with 3 protons, 4 neutrons and 2 electrons will have the same positive net charge as a single proton.

(Left to right) A positive proton, a neutral neutron and a negative electron (not to scale).

## Like charges and unlike charges

Two charged particles with the same sign are called like charges. Like charges repel each other.

Two protons (both positive) are like charges. They repel each other when brought close together. The same is true for two electrons (both negative).

Two charged particles with different (positive and negative) signs are called unlike charges. Unlike charges attract each other.

Particles of spray paint are all positively charged as they leave the paint can. This causes the paint particles to repel each other and spread out.

The surface to be painted is negatively charged. The paint particles tend to stick to the surface as they have unlike charges.

Two positive charges repel each other (above), while one positive and one negative charge attract each other (below).

## Symbol and units for electric charge

The most commonly used symbols for electric charge are $Q$ or $q$.

The SI unit of charge is the coulomb $(\text{C})$.

One coulomb is equivalent to the charge delivered by a current of one ampere in one second, so $1\text{ C}= 1\text{ A} \times 1 \text{ s}$.

The total charge in a new AAA battery, used in many small devices such as remote controls, is typically $3600\text{ C}$.

A lightning bolt rapidly transfers around $15 \text{ C}$ of charge from the sky to the ground.

The electric charge of a single proton or electron, which is denoted by the symbol $e$, is equal to $1.60\times 10^{-19}\text{ C}$.

Batteries store charge which can be used to power electrical appliances.