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Circuits
AP PHYSICS
CAUTION!
 Do not leave any circuit connected longer than
necessary to observe bulb brightness. Leaving a
circuit connected for too long can ruin the battery
(and is not environmentally friendly).
Stuff to do
 Make a bulb light with 1 bulb, 1 battery, and 1 wire
 What do we need to make the bulb light?
 What doesn’t work?
 Complete Circuits
Stuff 2 do
 Conductors and Insulators
 Look inside a bulb – what is connected to which
other parts?
 How is this similar to a complete circuit idea from
before?
Stuff 3 do
 Flashlights
 Switches
What does it all mean?
 Electrons are travelling through the conductors
 The more electrons pass by every second, the more
brightness we see in the bulb. We call this
CURRENT = rate of charge past a point
 How can we tell how much current there is?
Series and Parallel
 Put 2 bulbs in a row connected to the same # of
batteries. How does the brightness change?
 What will happen with 3 in a row?
 Why do you think this happens?
 Bulbs in a row are called SERIES
Parallel
 Put two bulbs in PARALLEL (as shown on the
board). What happens to their brightness compared
to before?
 What about the total current? Can we tell for sure?
How do we test this?
 The splits in the wires before parallel bulbs are called
JUNCTIONS
Total Current
 See section 3…
 Models of electric circuits!
Circuit Parts
 Battery – Voltage – Potential Difference
 Wire – just a connection, path of conductors
 Bulb – Resistance and Brightness (current)
 Other Resistors
Diagrams
 Circuit Diagrams:
 Battery
 Wire
 Bulb
 Capacitor
 Resistor (other than a bulb)
 Diode/LED
More Stuff
 Fuses
 AC/DC
 Current - Drift velocity vs.


velocity of an electron
http://www.stmary.ws/highschool/physics/home/animations3/electricity/electrons_conductivity.html
meters
 Charge is said to flow THROUGH a circuit
 Current is said to be THROUGH a bulb
 Voltage is said to be placed ACROSS a circuit
 Voltage can be calculated ACROSS a battery
 Why? (hint: what are the definitions of the two
words)
 How do we measure current and voltage?
Meters
 What are we measuring?
 Ammeter - # electrons passing by per second
 Voltmeter – potential difference between 2 points
Ohm’s Law
 George Simon Ohm –
 1789 – 1854
 Taught math
 Most physicists did not use
math, just observed
 He combined observations
and math
 Ohm’s Law V = IR
Questions
 How much current is drawn by a lamp that has a
resistance of 60 Ω when a voltage of 12 V is
impressed across it?
 What is the resistance of an electric frying pan that
draws 12 A when connected to a 120 V circuit?
Kirchhoff’s First Rule
 Kirchhoff’s First Rule:
 The total current out of a node is equal to the total current into
the node.




(current in = current out)
i1 + i2 = i3 + i4 +i5
The algebraic sum of the currents at a node is zero.
i1 + i2 – i3 –i4 –i5 = 0
Resistances
 Resistors in parallel add together like this:
 1/RT = 1/R1 + 1/R2 + 1/R3 + ….
 Resistors in a row (series) add together
 RT = R1 + R2 + R3 +…
Rules for Circuit Analysis
 Redraw the circuit – resistors in series and parallel
 Simplify – combine each set of series and parallel
 Find Req for the entire circuit
 Find I total – current though the battery
 Use Kirchoff’s Law to find other I values