Direct Current and Ohm's Law

Electric Current

Electric current is the ordered motion of charged particles.

• Current I = q/t (ampere).
• Ohm's law for a section of a circuit: I = U/R.
• Resistance of a conductor: R = ρ·l/S.
• Ohm's law for a complete circuit: I = ε/(R + r), where ε is the EMF and r is the internal resistance of the source.

Connections

Connection	Total resistance	Quantity that is the same
Series	R = R₁ + R₂	current I
Parallel	1/R = 1/R₁ + 1/R₂	voltage U

• In a series connection, U = U₁ + U₂.
• In a parallel connection, I = I₁ + I₂.
• If two equal resistors are connected in parallel, the total resistance is half of one resistor.

Calculation

For two resistors in parallel, a convenient formula is: R = R₁·R₂/(R₁ + R₂). For example, for 10 Ω and 15 Ω, R = 150/25 = 6 Ω.

In a parallel connection, the total R is smaller than each individual resistance. In a series connection, however, it is larger.