Circuits

1. Identify a parallel circuit.

2. Determine the voltage across each parallel branch.

3. Determine the current across each parallel branch.

4. Apply Kirchoff’s current law

4. Determine total parallel resistance.

5. Apply Ohm’s law in a parallel circuit.

6. Use a parallel circuit as a current divider.

7. Determine power in a parallel circuit.

Objectives

Twice as many cars can travel on a double road, three times as many on a three-lane road and so on.

One could say that these two roads are parallel to each other in that there is more than one path for the cars to follow.

This analogy can be applied to parallel circuits.

Parallel circuit – a circuit that provides separate conducting paths for the current.

In a parallel circuit, each current path is called a branch.

The presence of branch lines means that there are multiple pathways by which charge can traverse the external circuit.

In parallel circuits, each branch shares a direct connection to a battery.

3 light bulbs connected toa battery in a parallel circuit.

The same parallel circuitas a circuit diagram.

More current flows through the smaller resistance. Morecharges take the easiest path.

As charges move through the resistors (represented by thepaddle wheels) they do work on the resistor and as a resultlose electrical energy.

By the time the charges make it back to the battery, they havelost all their energy.

The total current is equal to the sum of the currents in the branches.

Itotal = I1 + I2 + I3 + ...

Rules for Parallel Circuits

1. The voltage is equal across all components in the circuit. All components share the same voltage. The voltage drops of each branch equals the voltage rise of the source.

The voltage across R1 is equal to the voltage across R2 which is

equal to the voltage across R3 which is equal to the voltage

across the battery.

As with series circuits, the sum of the potential differences as you go around the loop is zero.  This is true no matter which branch you look at. (Kirchoff’s 2nd Law)

2. The current divides into separate branches such that the current can be different in every branch.

The total current is equal to the sum of the individual branch currents.

It is still the same amount of current, only split up into more than one pathway.

3. When resistors are connected in parallel, thetotal resistance of the circuit decreases.

The more branches you add to a parallel circuit, the lowerthe total resistance becomes.

4. The total resistance of a parallel circuit is alwaysless than the value of the smallest resistor.

In a parallel circuit, the point where the current a separates is called a junction.

Kirchoff’s Current LawThe sum of the currents entering a junction isequal to the sum of the currents leaving thejunction.

In this example you will notice 8 Amps and 1 Amp entering the junction while 7 Amps and 2 Amps leave.  This makes a total of 9 Amps entering and 9 Amps leaving.

A B

The current going intoThe junction equals7 amps (1A + 2A + 4A).

The current leaving theJunction is 7 amps (7A)

The current entering thejunction is 6 amps ( 5A + 1A).

The current leaving theJunction is 6 amps (4A + 2A)

The diagram above represents current flowing inbranches of an electric circuit. What is the currentat point B? 13 A

Formula for Total Parallel Resistance

The inverse of the total resistance of the circuit (also called effective resistance) is equal to the sum of the inverses ofthe individual resistances.

Power In Parallel Circuits

Total power in a parallel circuit is found by adding upthe powers of all the individual resistors, the sameas for series circuits.

Fuses are always connected in series with the component(s) to be protected, so that when the fuse blows (opens) it will open the entire circuit and stop current through the component(s).

The fuse opens the circuit my melting a thin metal filament inside the casing.

Once a fuse is blown, it must be replaced.

Fuse - a device used in electrical systems to protect against excessive current.

NEVER replace a fuse with a fuse rated for higher current than the recommended fuse.

Glass Cartridge Fuses

Plug Fuses

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Automotive fuses are a class of fuses used to protect thewiring and electrical equipment for vehicles.

They are generally rated for circuits no higher than 24 voltsdirect current, but some types are rated for 42 volt electricalsystems.

The color of thefuse is an indicationof its rating.

Automotive Fuses

Circuit Breakers

A circuit breaker automatically shuts off the power to the circuit in the event of a dangerous electrical overload or short circuit.

A circuit breaker can also be used manually to disconnect a circuit from incoming power so that you can repair or upgrade your receptacles, outlets, and fixtures.

Circuit breakers are much easier to fix than fuses.

When the power to an area shuts down, the homeowner can look in the electrical panel and see which breaker has tripped to the "off" position.

The breaker can then be reset to the "on" position and power will resume again.

If the breaker continues to trip after you flip it, you may have a faulty breaker, a wiring problem, or there may be an issue with an electrical device that relies on that breaker. You should leave the breaker off and consult an electrician.

A main circuit breakers shuts off power to the whole house. Individual circuit breakers connect to circuits throughout the house.

GFCI breakers are designed to protect people from electrical shock, rather than prevent damage to a building's wiring.

The GFCI constantly monitors the current in a circuit's neutral wire and hot wire. When a surge in current is detected on the how wire, the GFCI breaks the circuit, preventing electrocution.

Since it doesn't have to wait for current to climb to unsafe levels, the GFCI reacts much more quickly than a conventional breaker.