Law of Conservation of Charge

• The sum of the currents entering any junction must equal the sum of the currents leaving that junction (junction or node rule). The rule should be used as necessary so that each current appears in at least one junction equation.

Law of Conservation of Energy

• The sum of the potential differences across all elements around any closed circuit loop must be zero. (loop rule)
• If going across a R in the same direction as the current, ΔV = -IR
• If going across R in the opposite direction as the current, ΔV = IR
• If going across a battery from negative to positive, ΔV = +ε
• If going across a battery from positive to negative, ΔV = -ε
• (Remember ΔV means final minus initial)
• Remember, the long side of the battery is positive and the short side is negative
• This rule should be applied as many times as is required to include each current at least one time in a loop rule equation.”

Additional Suggestions

Assign current direction – it does not matter what direction. If the current comes out to be negative, the wrong direction was chosen, but the magnitude of the current is correct. Solve simultaneous equations from loop rule and junction rule equations.

 

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