Circuits

- Electrical components connected in different

loops of the same circuit are connected in parallel.

3 light bulbs connected to

a battery in a parallel circuit.

The same parallel circuit

as a circuit diagram.

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.

More current flows through the smaller resistance. More

charges take the easiest path.

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

across the resistor in a loop is equal to the total input

voltage. (Kirchoff’s voltage 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.

In a parallel circuit, the point where the current a

separates is called a junction.

Kirchhoff's Current Law

The sum of the currents entering a junction is

equal to the sum of the currents leaving the

junction.

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 into

The junction equals

7 amps (1A + 2A + 4A).

The current leaving the

Junction is 7 amps (7A)

The current entering the

junction is 6 amps ( 5A + 1A).

The current leaving the

Junction is 6 amps (4A + 2A)

The diagram above represents current flowing in

branches of an electric circuit. What is the current

at point B? 13 A

3. When resistors are connected in parallel, the

total resistance of the circuit decreases.

The more branches you add to a parallel circuit, the lower

the total resistance becomes.

4. The total resistance of a parallel circuit is always

less than the value of the smallest resistor.

Formula for Total Parallel Resistance

The inverse of the total resistance of the circuit (also called

effective or equivalent resistance) is equal to the sum of the

inverses of the individual resistances.

For 2 resistors,

RT = R1 x R2

R1 + R2

which means…

Power In Parallel Circuits

Total power in a parallel circuit is found by adding up

the powers of all the individual resistors, the same

as for series circuits.

Connecting lights and appliances in parallel makes them operate

independently. If one goes off, the other can still operate.

measuring currentSERIES CIRCUIT

PARALLEL CIRCUIT

• current is the same

at all points in the

circuit.

2A 2A

2A

• current is shared

between the components

2A2A

1A

1A

fill in the missing ammeter readings.

?

?

4A

4A

4A

3A?

?

1A

?

3A

1A

1A

The circuit is no longer complete, therefore current can not flow

The voltage decreases because the current is decreased

and the resistance increases.

The current remains the same. The total resistance drops in a

parallel circuit as more bulbs are added

The current increases.

measuring current

Electric current is measured in amps (A) using

an ammeter connected in series in the circuit.

A

measuring current

AA

This is how we draw an ammeter in a circuit.

SERIES CIRCUIT PARALLEL CIRCUIT

measuring voltage

The ‘electrical push’ which the cell gives to the current

is called the voltage. It is measured in volts (V) on a

voltmeter

V

measuring voltage

V

This is how we draw a voltmeter in a circuit.

SERIES CIRCUIT PARALLEL CIRCUIT

V