Circuits
Current
Resistance & Ohm’s Law
Resistors in Series, in Parallel, and in combination
Capacitors in Series and Parallel
Voltmeters & Ammeters
Resistivity
Power & Power Lines
Fuses & Breakers
Bulbs in Series & Parallel
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Circuits - Physics - Lecture Slides, Slides of Physics
In these Physics Lecture Slides, following major aspects of physics have been discussed :Circuits, Resistance, Ohm’S Law, Resistors in Series, Capacitors in Series and Parallel, Voltmeters, Ammeters, Resistivity, Power Lines, Fuses
Typology: Slides
2012/2013
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Circuits
Current
Resistance & Ohm’s Law
Resistors in Series, in Parallel, and in combination
Capacitors in Series and Parallel
Voltmeters & Ammeters
Resistivity
Power & Power Lines
Fuses & Breakers
Bulbs in Series & Parallel
Electricity
The term electricity can be used to refer to any of the
properties that particles, like protons and electrons, have as a
result of their charge. Typically, though, electricity refers to
electrical current as a source of power. Whenever valence
electrons move in a wire, current flows, by definition, in the
opposite direction. As the electrons move, their electric
potential energy can be converted to other forms like light,
heat, and sound. The source of this energy can be a battery,
generator, solar cell, or power plant.
Charge Carriers & Current
A charge carrier is any charged particle capable of moving. They are usually ions or subatomic particles. A stream of protons, for example, heading toward Earth from the sun (in the solar wind) is a current and the protons are the charge carriers. In this case the current is in the direction of motion of protons, since protons are positively charged. In a wire on Earth, the charge carriers are electrons, and the current is in the opposite direction of the electrons. Negative charge moving to the left is equivalent to positive charge moving to the right. The size of the current depends on how much charge each carrier possesses, how quickly the carriers are moving, and the number of carriers passing by per unit time.
wire electrons (^) I
protons (^) I
A circuit is a path through which an electricity can flow. It often consists of a wire made of a highly conductive metal like copper. The circuit shown consists of a battery ( ), a resistor ( ), and lengths of wire ( ). The battery is the source of energy for the circuit. The potential difference across the battery is V. Valence electrons have a clockwise motion, opposite the direction of the current, I. The resistor is a circuit component that dissipates the energy that the charges acquired from the battery, usually as heat. (A light bulb, for example, would act as a resistor.) The greater the resistance, R , of the resistor, the more it restricts the flow of current.
A Simple Circuit
Building Analogy Correspondences
Battery ↔ Elevator that only goes up and all the way to the top floor
Voltage of battery ↔ Height of building
Positive charge carriers ↔ People who move through the building en masse (as a large group)
Current ↔ Traffic (number of people per unit time moving past some point in the building)
Wire w/ no internal resistance ↔ Hallway (with no slope)
Wire w/ internal resistance ↔ Hallway sloping downward slightly
Resistor ↔ Stairway , ladder, fire pole, slide, etc. that only goes down
Voltage drop across resistor ↔ Length of stairway
Resistance of resistor ↔ Narrowness of stairway
Ammeter ↔ Turnstile (measures traffic without slowing it down)
Voltmeter ↔ Tape measure (for measuring changes in height) Docsity.com
Current and the Building Analogy
In our analogy people correspond to positive charge carriers and a hallway corresponds to a wire. So, when a large group of people move together down a hallway, this is like charge carriers flowing through a wire. Traffic is the rate at which people are passing, say, a water fountain in the hall. Current is rate at which positive charge flows past some point in a wire. This is why traffic corresponds to current.
Suppose you count 30 people passing by the fountain over a 5 s interval. The traffic rate is 6 people per second. This rate does not tell us how fast the people are moving. We don’t know if the hall is crowded with slowly moving people or if the hall is relatively empty but the people are running. We only know how many go by per second. Similarly, in a circuit, a 6 A current could be due to many slow moving charges or fewer charges moving more quickly. The only thing for certain is that 6 coulombs of charge are passing by each second.
Current flows from the positive terminal of the battery, where + charges are at high potential, through the resistor where they give up their energy as heat, to the negative terminal of the battery, where they have zero potential energy. The battery then “lifts them back up” to a higher potential. The charges lose no energy moving the a length of wire (with no internal resistance). Similarly, people walk from the top floor where they are at a high potential, down the stairs, where their potential energy is converted to waste heat, to the bottom floor, where they have zero potential energy. The elevator them lifts them back up to a higher potential. The people lose no energy traveling down a (level) hallway.
Battery & Resistors and the Building (cont.)
e l e v a t o r top floor hallway: high U grav
V
R
bottom floor hallway: zero U grav
flow of + charges
+
-
flow of people
Resistance
Resistance is a measure of a resistors ability to resist the flow of current in a circuit. As a simplistic analogy, think of a battery as a water pump; it’s voltage is the strength of the pump. A pipe with flowing water is like a wire with flowing current, and a partial clog in the pipe is like a resistor in the circuit. The more clogged the pipe is, the more resistance it puts up to the flow of water trying to flow through it, and the smaller that flow will be. Similarly, if a resistor has a high resistance, the current flowing it will be small. Resistance is defined mathematically by the equation:
V = I R
Resistance is the ratio of voltage to current. The current flowing through a resistor depends on the voltage drop across it and the resistance of the resistor. The SI unit for resistance is the ohm, and its symbol is capital omega: Ω. An ohm is a volt per ampere:
1 Ω = 1 V / A The Voltage Lab^ (scroll down)
The definition of resistance, V = I R , is often confused with Ohm’s law, which only states that the R in this formula is a constant. In other words, the resistance of a resistor is a constant no matter how much current is flowing through it. This is like saying a clog resists the flow of water to the same extent regardless of how much water is flowing through it. It is also like saying a the width of a staircase does not change: no matter what rate
Georg Simon Ohm 1789-
Ohm’s Law
people are going downstairs, the stairs hinder their progress to the same extent. In real life, Ohm’s law is not exactly true. It is approximately true for voltage drops that aren’t too high. When voltage drops are high, so is the current, and high current causes more heat to generated. More heat means more random thermal motion of the atoms in the resistor. This, in turn, makes it harder for current to flow, so resistance goes up. In the circuit problems we do we will assume that Ohm’s law does hold true.
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If Ohm’s law were always true, then as V across a resistor increases, so would I through it, and their ratio, R (the slope of the graph) would remain constant.
In actuality, Ohm’s law holds only for currents that aren’t too large. When the current is small, not much heat is produced in a real, so resistance is constant and Ohm’s law holds (linear portion of graph). But large currents cause R to increase (concave up part of graph).
I
V
Ohmic Resistor
Ohmic vs. Nonohmic Resistors
Real Resistor
I
V
Resistors in Series: Building Analogy
To go from the top to the bottom floor, all people must take the same path. So, by definition, the staircases are in series. With each flight people lose some of the potential energy given to them by the elevator, expending all of it by the time they reach the ground floor. So the sum of the V drops across the resistors the voltage of the battery. People lose more potential energy going down longer flights of stairs, so from V = I R , long stairways correspond to high resistance resistors.
The double waterfall is like a pair of resistors in series because there is only one route for the water to take. The longer the fall, the greater the resistance.
3 steps
Elevator (battery)
R 1
R 2
R 3
R 1
R 2
I
V
R 1
R 2
R 3
Equivalent Resistance in Series
I
V (^) R eq
If you were to remove all the resistors from a circuit and replace them with a single resistor, what resistance should this replacement have in order to produce the same current? This resistance is called the equivalent resistance , R eq_._ In series R eq is simply the sum of the resistances of all the resistors, no matter how many there are:
R eq = R 1 + R 2 + R 3 + · · ·
Mnemonic: Resistors in Series are Really Simple.
6 V
- Find R eq
2. Find I total
- Find the V drops across each resistor.
0.5 A
2 V, 1 V, and 3 V (in order clockwise from top) (^) Solution on next slide
Series Sample
6 V
- Since the resistors are in series, simply add the three resistances to find R eq: R eq = 4 + 2 + 6 = 12
- To find I total (the current through the battery), use V = I R : 6 = 12 I. So , I = 6/12 = 0.5 A
Series Solution
- Since the current throughout a series circuit is constant, use V = I R with each resistor individually to find the V drop across each. Listed clockwise from top: V 1 = (0.5)(4) = 2 V V 2 = (0.5)(2) = 1 V V 3 = (0.5)(6) = 3 V Note the voltage drops sum to 6 V. Docsity.com