# 3.5 Energy Conservation in Circuits

## Energy Conservation in Circuits

#### The Electrical Voltages Rule (Kirchhoff's Second Law)

• Energy is never used up or lost in a circuit, since everything follows the Law of Conservation of Energy
• The electrical voltages rule is defined as:

The sum of the e.m.f.s in a closed circuit loop is equal to the sum of the potential differences around that loop

• Each closed circuit loop can be treated like a series circuit
• A typical circuit might have a setup where  E1 + E2 = V1 + V2 where:
• E1 and E2 represent the e.m.f.s in the closed loop
• V1  and V2 represent the potential differences in the closed loop

The sum of the voltages is equal to the total e.m.f from the batteries

• In a series circuit, the voltage is split across all components depending on their resistance
• The sum of the voltages is equal to the total e.m.f of the power supply

• In a parallel circuit, the voltage is the same across each closed loop
• The sum of the voltages in each closed circuit loop is equal to the total e.m.f of the power supply:

The sum of the p.ds in each closed loop is equal to the total e.m.f of the power supply

• A closed-circuit loop acts as its own independent series circuit
• Each loop separates at a junction
• A parallel circuit is made up of two or more of these loops

Each circuit loops acts as a separate, independent series circuit

• This makes parallel circuits incredibly useful for home wiring systems
• A single power source supplies all lights and appliances with the same voltage
• If one light breaks, voltage and current can still flow through for the rest of the lights and appliances

#### Exam Tip

The Electrical Voltages Rules is sometimes known as Kirchhoff's Second Law.

Drawing the loops in different colours, as in the example above, can be a helpful way of identifying the different loops.

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Joanna obtained her undergraduate degree in Natural Sciences from Cambridge University and completed her MSc in Education at Loughborough University. After a decade of teaching and leading the physics department in a high-performing academic school, Joanna now mentors new teachers and is currently studying part-time for her PhD at Leicester University. Her passions are helping students and learning about cool physics, so creating brilliant resources to help with exam preparation is her dream job!