In Pokémon episode Electric Shock Showdown Brock says
"It's over. Raichu ran out of electricity."
How can an Electric Pokemon run out of electricity? Does it mean Water-types can run out of the water and Fire-types can run out of the fire?
In the Pokémon anime, as in the Pokémon games, there is not what you would call a high degree of internal consistency regarding the "physics" of the powers wielded by Pokémon. Laws of thermodynamics and conservation of mass are not rigorously applied. That being said, there is a common-sensical logic to the world, followed or ignored to whatever extent serves the story or game mechanics.
According to this logic, Electric-type Pokémon can produce electricity, as fire-type Pokémon can produce flames and Ice-type Pokémon can produce snow and the aurora borealis, but this doesn't mean they can produce these substances in unlimited quantities, just as terrestrial Poison-type animals can only produce so much venom without taking some time to replenish themselves.
Concerning Charmander, an archetypical Fire-type Pokémon, the Pokédex in Pokémon Pearl says:
The fire on the tip of its tail is a measure of its life. If healthy, its tail burns intensely.
In HeartGold, the Dex says:
The flame on its tail shows the strength of its life force. If it is weak, the flame also burns weakly.
And in FireRed:
From the time it is born, a flame burns at the tip of its tail. Its life would end if the flame were to go out.
Charmander's fire-generating ability is (at least partially) interdependent with its health. Its affiliation with the Fire type doesn't guarantee unlimited fire-production ability.
In the mechanics of the games, Pokémon's ability to use their various powers is expressed as a resource, "Power Points" (almost always referred to as PP). Each time a move is used, its PP goes down; when the PP of a move reaches 0, it can't be used again until the player takes steps to refresh the Pokémon. If all a Pokémon's moved are reduced to 0 PP, it can only use the self-damaging Struggle attack.
The climactic battle in "Electric Shock Showdown" seems to follow roughly the same lines: Lt. Surge's Raichu eventually "runs out of PP" for its Electric-type Thunderbolt attack, leaving it vulnerable to Ash's Pikachu. This kind of "war of attrition" strategy is only feasible because, in the anime as well as in the games, a Pokémon's elemental power is not an inexhaustible resource.
In the anime we see that Pokemon can run out of their "type" if they overuse it. We see at least one other example of this in Volcanion And The Mechanical Marvel when Volcanion runs out of water.
Volcanion: Explosions are my speciality
Ash: Hold on, you're out of water. Part of the fortress runs on steam, so it has water tanks
And in Hocus Pokemon Weezing attacks Pikachu with a sludge-attack move for about a minute. It runs out of sludge before Pikachu runs out of electricity and is defeated
Meowth: It's run out of sludge!