Lecture Ten Physics 100 Fall 2012

Series and Parallel Circuits

Bulbs present an obstacle

(or resistance) to the flow of current.

The current through a circuit (and so through the battery) changes when we change the number of bulbs in the circuit or how they are arranged.

For a given battery:

• The current produced is not always the same.• More resistance in the circuit, current in the

circuit.• Less resistance in the circuit, current.

For a given battery:

• The current is the circuit is not always the same.• More resistance in the circuit, less current in the

circuit.• Less resistance in the circuit, more current in the

circuit.• Does adding bulbs increase or decrease the

total resistance in the circuit?

More Bulbs Changes the Total Resistance in the Circuit

• Adding bulbs in series increases the resistance.• Adding bulbs in parallel decrease the resistance.

• Bulbs or other devices that obstruct current flow are called resistors.

• Resistors in SeriesR = R1 + R2

• Resistors in Parallel

• Bulbs or other devices that obstruct current flow are called resistors.

• Resistors in SeriesR = R1 + R2

• Resistors in Parallel

1 1 1

1 2R R R

1 1 1

1 2R R R

Resistors in Combination

RESISTANCE

Symbol: R

Unit: Ohm

Abbreviation for the Unit: Ω

Symbol in a circuit diagram:

For a given battery:

• The current is the circuit is not always the same.• More resistance in the circuit, less current in the

circuit.• Less resistance in the circuit, more current in the

circuit• What stays the SAME about the battery is its

“the strength of its push”.

The role of the battery

VOLTAGE

Symbol: V

Unit: Volts

Abbreviation for the Unit: V

Electric CurrentElectric current (or just current) is the rate at which

charges (electrons) flow through a wire.

The symbol for current is i

The unit for current is the Amp 1 Amp=1 C/s

Abbreviation for the units is A

For a given battery:

• The current is the circuit is not always the same.• More resistance in the circuit, less current in the

circuit.• Less resistance in the circuit, more current in the

circuit• What stays the SAME about the battery is its

“the strength of its push”.

VOLTAGE,CURRENT AND RESISTANCE

Voltage is the word we use for the “strength of the push”

V=IR

If Voltage of the battery stays constant, and the resistance of the circuit increases, the current must decrease.

Applying V=iR to circuits1. Calculate the total resistance in the circuit

using: Resistors in Series

R = R1 + R2

Resistors in Parallel

2. Then apply V=iR where V is the voltage of the battery.

2. Then apply

V=iR

1 1 1

1 2R R R

1 1 1

1 2R R R