Download Lecture 10

Lecture Ten
Physics 100
Fall 2012
 Series and Parallel Circuits
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.
Bulbs present an obstacle
(or resistance) to the flow of
current.
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.
Resistors in Combination
• Bulbs or other devices that obstruct current flow
are called resistors.
• Resistors in Series
R = R1 + R2
• Resistors in Parallel
1 1 1
 
R R1 R2
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 Current
Electric 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 circuits
1. Calculate the total resistance in the circuit
using:
Resistors in Series
R = R1 + R 2
Resistors in Parallel
1 1 1
 
R R1 R2
2. Then apply V=iR where V is the voltage of
the battery.