Download A wire of length L and radius r has a resistance R. What is the

Notes: Intro to Circuits
Key Parts of a Circuit
Circuit: A complete path of conductors that electrons can flow through
Electric Current: Electric current is simply the flow of electrons through a conductor. The symbol for current is I.
How we measure electric current:
I
Q
t
Equation for current with the units 1 A  1 amp  1
C
s
I is the current in amperes, Q is the charge in Coulombs, and t is the time in seconds.
Direction of the Current
Key Ideas:


Current flows from the positive end of the
battery to the negative end of the battery
Electric Current (I) shows the direction of the
positive charge.
Do positive charges actually move?
Which direction do the electrons flow?
Building a Circuit
We often draw circuits using specific symbols.
Element/Object
Current
Symbol
An arrow
(not always shown)
Measured in…
Amps (A)
Voltage Source
Volts (V)
Resistor
Ohms (Ω)
Light bulb
Ohms (Ω)
Capacitors
Capacitance (F)
Switch
Not measured- just a place where we
break the path of the conductors, so
the flow of electricity will stop
Rules for drawing circuits
1.
2.
3.
4.
Straight lines for wires.
Try to keep everything in straight lines and right angles
Use symbols not pictures
If two lines cross, it means those wires are connected
Ohm’s Law
Memorize This!
𝑉 = 𝐼𝑅
A potential difference across a conductor causes current to flow. The flow of the current is opposed by the
resistance of the circuit. The resistance “uses up” some of the energy in the current and converts it to heat
Practice Problem
A waffle iron draws 7.1 A when plugged into a 120 V circuit. What is its
resistance?
Resistance
Memorize This!
R is the resistance,  is the resistivity of the conductor, l is the length of the
conductor, and A is the cross sectional area of the conductor.
This equation is only good for wires that have a constant cross-section.
The resistivity, ρ, depends on what the object is made of and, occasionally,
temperature. If they don’t discuss temperature, you can assume it is determined
by what resistor is made of.
Practice Problem: Basic Resistance Calculation
What is the resistance of a copper wire, which has a diameter of 1.00 mm and a
length of 52 m?
AP Question: Comparing Resistances
A wire of length L and radius r has a resistance R. What is the resistance of a second wire made from the same material that has a
length L/2 and a radius r/2?
(A) 4R (B) 2R (C) R (D) R/2 (E) R/4
AP Practice: Resistance Ratios
Two conducting cylindrical wires are made out of the same material. Wire X has twice the length and twice the diameter of wire Y.
What is the ratio Rx/Ry
(A) 1/4 (B) ½ (C) 1 (D) 2 (E) 4
Practice Problem: Drawing a Wire
A wire of resistance Ro is drawn through a draw plate until its radius has been halved. What is the new resistance of the
wire?
Power
Previously in physics, we learned
𝑃=
𝑊 𝐸
=
𝑡
𝑡
The units for power are Watts (W).
We usually talk about power in two ways:


The power a battery provided to the circuit
The power a resistor or device dissipates- loses to the outside environment.
Memorize This!
𝑷 = 𝑽𝑰
𝟐
𝑷=𝑰 𝑹
𝑷=
𝑽𝟐
𝑹
Only the first equation will be given to you on the AP exam
AP Practice: Easy Power
When lighted, a 100–watt light bulb operating on a 110–volt household circuit has a resistance closest to
(A) 10–2 Ω (B) 10–1 Ω (C) 1 Ω (D) 10 Ω (E) 100 Ω
Practice Problem: Incandescent Light Bulb
A incandescent light bulb is rated with 60 Watts, 120 V. What is the resistance
of this light bulb?