Circuits
Chapter 17: Current and Resistance
Current:
· Most modern day appliances require electricity to operate.
· The electricity generates a ______which runs through the appliance.
· The current is made out of charged particles
· Current is defined as the ______to the direction of flow.
o For example: if the charges are flowing through a wire, the area is the cross-sectional area of the wire
Equation for Current:
Where: I = current (amps, A)
DQ = change in charge or how much charge flows (Coulombs, C)
Dt = time (seconds)
Example: The amount of charge that passes through the filament of a certain lightbulb in 2.00 sec is ______. Find (a) the current in the bulb and (b) the number of electrons that pass through the filament in ______sec.
Electric Current Flow
· In almost all real circuits it is ______(- charges) that are flowing. We call this kind of current “______”.
· However, for simplicity we assume that all charge carriers are ______. We draw current arrows in the direction that such charges would move….that is, ______.
· Historically this is because ______guessed wrong about the charge carriers in a metal being positive.
Two conditions for an electric current to flow between two points:
1. There must be a conducting path between the points along which the charges can move.
WHY?
2. There must be a difference of electric potential (volts) between the two points.
WHY?
______is the measure of ______moving within a conductor.
Why do the charges move?
______, or amps, measures the amount of current that flows. Amps is named after a 19th-century French physicist, Andre-Marie Ampere, and abbreviated ___.
Voltage
Voltage, or ______, is measured in volts, named after a physicist named Volta, and abbreviated V.
Examples: AAA, AA, C, D are all 1.5 V a car battery is 12 V. Power lines are several thousand volts.
Common Misconception:
MYTH: TRUTH:
Electrical Resistance
· A ______is anything that electricity cannot travel through ______.
· When electricity is forced through a ______, often the energy is changed into another ______, such as ______.
· The reason a light bulb glows is that the electricity is forced through tungsten, which is a resistor. The ______.
· Resistance is sort of like the ______associated with ______or electrical current.
Types of Conductors
· Good conductors, like most ______, allow current to flow ______
· Poor conductors, like most ______, ______the flow of current to a ______extent.
· ______ like very cold niobium-tin, are special substances that allow current to flow with ______;
· ______, like silicon, are either good or poor conductors ______.
Ohm’s Law
Resistance quantifies how much current you get across something per volt applied.
Resistance has units of Volts/Amp, which get another name, ohms, represented by the Greek letter omega Ω.
Example: How much voltage difference does it take for ______amps to flow through a resistance of 8 Ω?
The resistance of a conductor that obeys Ohm’s law depends upon three factors:
- The material of which it is composed; the ability to carry an electric current varies more than almost any other physical property of matter.
- Its length (L): The longer the conductor, the greater its resistance.
- Its cross-sectional area (A). The thicker the conductor the less its resistance.
r = resistivity of the material. This is a characteristic property of the material and it varies with temperature
Example: Calculate the resistance of a piece of aluminum that is ______long and has a cross sectional area of 10-4m2. (r = 2.82e-5Wm)
Example 2: Hard rubber on the other hand has a r = ______. Repeat the above calculation for hard rubber.
Resistance and Temperature
The resistivities of nearly all substances vary with temperature. In general, metals ______in resistivity with an increase in temperature while ______in resistivity.
a is the temperature coefficient of resistivity of the material. This is a characteristic property of the material. A negative coefficient indicates the resistivity decreases with increasing temperature.
An alternative formula, used to calculate the resistance of a material at any temperature is given by….
In this equation, resistivity is referenced to some standard resistivity ro at some standard temperature To.
In terms of resistance …
Example: A metal wire has a resistance of 10.00 W at a temperature of 20oC. If the same wire has a resistance of 10.55 W at 90oC, what is the resistance of the wire when its temperature is -20oC?
Power
Power = volts x amps = Joules/second = Watt or W
Example: What is the current flow to a ______hairdryer plugged into a 120V socket?
Example: Lets say you want to protect your new speakers by putting a fuse in line with the speakers. Assume you have _____ speakers and that they are _____. What size (amps) fuse should you use so that it just blows at 100 Watts?
Example: Two light bulbs operate from _____, but one has a power rating of _____, and the other has a power rating of 100W. Which bulb has the higher resistance?
Chapter 18: Electric Circuits
Circuit Symbols:
Battery: Resistor: Light-bulb: Switch: Wire:
Three general types of circuits:
1. ______:There is a complete loop with wires going from one side of the battery through a resistor(s) to the other side of the battery.
2. ______- There is not a complete loop.
3. ______- There is a complete loop, but it does not contain any resistors.
Examples:
There are two ways to put a circuit together:
- Series - Resistors are considered to be in series if the current must go through all of the resistors in order.
- Parallel - Resistors are considered to be in parallel if the current is shared between multiple resistors.
As more identical resistors R are added to the parallel circuit shown, the total resistance between points P and Q ______
When one bulb is unscrewed, the other bulb will remain lit in which circuit…
Example: A 25W bulb and a 100W bulb are connected in series. Which bulb will glow brighter?
Note: light bulbs are really resistors. The power rating on an American light bulb is how many watts of power a light bulb will give off if it is the only bulb in an American 120 V circuit. Since there are two bulbs in the circuit, the power ratting isn’t very helpful.
(A) Calculate the resistance for each resistor shown.
B) Calculate the total resistance of the circuit.
C) Calculate the current through each resistor.
D) Calculate the power used by each resistor.
E) Calculate the voltage across each resistor.
The circuit below consists of two identical light bulbs burning with equal brightness and a single 12V battery. When the switch is closed, the brightness of bulb A…
Charge flows through a light bulb. Suppose a wire is connected across the bulb as shown. When the wire is connected…