Pc Emerson-Thomas wanted first go in the van because it was fitted with a new radio, the jury was told.
Prosecutor Mark Spackman said: "The incident began in what can only be described as a petty squabble over who was driving the police van that evening.
"The sergeant told Pc Bartlett to take the van but the defendant was a bit upset about it and he asked Pc Bartlett if he could have it.
"Pc Bartlett simply got up and picked up the keys from the board then sat down at a table to drink a cup of tea.
"He went to pick up the cup of tea and Pc Emerson-Thomas punched his arm causing it to spill over the table and himself."
The trial heard Pc Bartlett picked up a paper towel to clean up the spilled tea.
But Pc Emerson-Thomas was accused of using other towels to wipe the tea down his colleague's arm.
Mr Spackman said: "Emerson-Thomas then shoved the paper towels into Pc Bartlett's face."
The incident happened at Fairwater police station in Cardiff
The court was told that other officers had told them to stop but Pc Emerson-Thomas then lashed out at Pc Bartlett with his fist and hit his eyebrow.
"It was a hard punch and knocked the contact lens out of his eye," said Mr Spackman.
"There was a cut which may have been caused by the signet ring Emerson-Thomas was wearing."
Pc Emerson-Thomas was sent home from the station and he has been suspended since.
The court heard that the men had worked together for five months but there had never been any animosity. They had also known each other for four years. More
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