Section 14.2

Objectives  Describe how current divides in a parallel circuit.  Determine the voltage across and current through each branch of a parallel circuit.  Explain why circuit breakers and fuses are used in homes.

Parallel Circuits  A circuit with more than one path for the current to flow. Each path in the circuit is sometimes called a branch. The current through a branch is called the branch current. In parallel circuits, current splits at 1 or more branch points.

Kirchhoff’s Current Law  All current entering a branch point must exit again. Thus, in a parallel circuit, the current entering a branch must equal the current exiting the branch.

Voltage in a parallel circuit  Voltage is the same across each branch of a parallel circuit. Both bulbs see 3V from the battery since each is connected back to the battery by wires without any other electrical devices in the way.

Advantages 1. Each device in the circuit has a voltage drop equal to the full battery voltage. 2. Each device in the circuit may be turned off independently without stopping the current in the other devices in the circuit. This is why parallel circuits are used for most of the wiring in homes.

Current in a parallel circuit  In a parallel circuit, current is determined using the branch resistance and Ohm’s law.  Each branch is independent, so the current in one branch does not depend on what happens in other branches.  Total current is the sum of currents in each branch.

Calculating current & resistance  In a series circuit, adding a resistor increases the resistance for the entire circuit.  In a parallel circuit, adding a resistor provides another independent path for current to flow. As a result, more current flows for the same resistance. Because of this, when using Ohm’s law to calculate, you will see that resistance is less.

Short Circuits  A parallel path in a circuit with very low resistance.  This can happen by accidentally making a parallel branch with a wire.  The results could be dangerous as the low resistance of wires is combined with very high current, producing large amounts of heat and possibly starting fires.

Circuit breakers and fuses  Homes generally have many parallel circuits, each of which has its’ own fuse or circuit breaker which stops the flow of current if the current becomes to high.

Review  Page 326, #’s 1-4