A positive externality of consumption occurs when the external impact on society is positive, such as when electric vehicles are consumed and CO2 emissions fall.
A negative externality of consumption occurs when the external impact on society is negative, such as when the consumption of alcohol increases anti-social behavior.
A positive externality of production occurs when the external impact on society is positive, such as when managed pine forests produce timber and increase CO2 absorption.
A negative externality of production occurs when the external impact on society is negative, such as when the production of electricity increases air pollution.
Why are public goods under-provided by a free market?
Public goods are under-provided by a free market because there is less opportunity for sellers to make economic profits from providing these goods/services.