Chapter 22 Flashcards
Where will a positive charge naturally move?
A positive charge will naturally move from a location of higher potential to a location of lower potential. In other words, a potential difference will cause a charge to move
Defn: Electric current
is the rate of charge flow, the amount of charge that flows past a point in a conductor per unit of time.
The symbol for current is I
Units for current: C/s = A (ampere)
- named after French physicist André Ampère (early 1800s)
I = Q/t
What does a source in a water pump do?
- The source in an electric circuit acts as a pump to maintain a potential difference so that charges will continue to flow. It is like the water pump in this diagram that maintains a pressure difference between the two sides so that water continues to flow.
A source:
Anything that produces a potential difference is called a source.
Examples of sources: are batteries, generators, solar cells, etc.
Electromotive force :
The maximum potential difference produced by a battery is called electromotive force and is given the symbol ε
Although it is what “pushes” the charges through the conductor, it is not really a force, and so we usually simply talk about “emf” rather than use the phrase “electromotive force”.
If I connected a wire between the positive and negative terminals of a battery, in what direction would the current flow through the wire? (What did people think 200 years ago compared to now?)
200 years ago, when electric circuits were first built, physicists knew nothing about electrons and protons. They simply knew that there were two types of charge, positive and negative, and described current as the flow of positive charge from the positive terminal of the battery to the negative terminal. This approach is called conventional current.
→ We now know that in reality it is electrons that move from atom to atom and so they flow from the negative terminal of the battery to the positive terminal. This is called electron current.
Despite the fact that the electron current model more accurately reflects reality, most often circuits are still described using the conventional current model. It is more practical. We describe electric forces and fields in terms of positive charges, and many things are easier to deal with if everything is treated as though it were positive charges moving. (e.g., As an electronic technician, I have lots of experience analyzing electric circuits. In circuit diagrams, the negative battery terminal is usually viewed as “ground”, or zero reference. Then all potential differences in a circuit are measured relative to this, and it is easiest to analyze circuits as positive charges flowing toward “ground”.
- Unless it is specifically stated that we are dealing with electron current, assume that we are using the conventional current model. But you must also be comfortable thinking about the directions that electrons actually flow.
Electron current
electrons that move from atom to atom and so they flow from the negative terminal of the battery to the positive terminal
Conventional current model:
It is more practical. We describe electric forces and fields in terms of positive charges, and many things are easier to deal with if everything is treated as though it were positive charges moving. (e.g., As an electronic technician, I have lots of experience analyzing electric circuits. In circuit diagrams, the negative battery terminal is usually viewed as “ground”, or zero reference. Then all potential differences in a circuit are measured relative to this, and it is easiest to analyze circuits as positive charges flowing toward “ground”.
Why does electrical energy actually have very little practical use?
As already discussed, charges flow because there is a potential difference between two points, and as they flow, they “lose” their electric potential energy as it is converted into other forms.
- This electrical energy actually has very little practical use (except in spark plugs, electric chairs, cattle prods, tazers, electric fences, and defibrillators).
- It is as the charges flow and the energy is converted into other forms that it really becomes useful.
- Electrical energy is useful for two main reasons:
It can easily be transported from one location to another. and 2 It can quite easily be converted into useful forms by various circuit devices.
How do most circuit devices convert electrical energy into heat, light, motion (via motors)?
Heat: - Heat results from conductors having resistance. As current flows, there are a lot of collisions between the moving electrons and atoms. In the collisions, much of the energy of the flowing electrons is transferred to the atoms, causing them to vibrate more.
Light: - How the energy is converted to light depends on the device. For incandescent bulbs, the filament is simply so hot that it glows brightly → 90% of the energy is given off as heat and only 10% as light. For fluorescent bulbs, sodium or mercury vapor lamps, or LED’s, the conversion is very different. To explain it involves quantum theory which we will talk about later in the year.
Motors: - e.g. hair dryer consists of a resistor element and a motor. We’ll learn how a motor works next year in Physics 12.
Sound: - Electrical current results in magnetic fields that interact with the magnetic fields of permanent magnets, causing the speaker to move back and forth, creating sound waves.
Power dissipated:
- The power dissipated by a circuit component is the current flowing through it multiplied by the potential difference across it.
Why do we usually write V rather than /\V in electric circuits?
In electric circuits, we usually just write V rather that /\V. We are still referring to a potential difference across the component. We also write E instead of AEp because we are always referring to the amount of energy converted.
How to know which side of the battery is positive and negative:
long end = positive short end = -
What will happen when a battery is connected to the end of a conductor?
- When a source such as a battery is connected to the ends of a conductor, current will flow.
What is Ohm’s Law also referred to as?
Although this is called “Ohm’s Law”, it is more correctly the definition of resistance! For each metal conductor, R is a constant that gives the relationship between the potential difference across the conductor and the current that flows as a result. (This relationship is not accurate for various other electronic devices.)
