You need three things in order to make a complete circuit:
The conductor runs a circular path from the power source, through the resistor, and back to the power source. The power source moves the existing electrons in the conductor around the circuit. This is called a current. Electrons move through a wire from the negative end to the positive end. The resistor uses the energy of the electrons around the wire and slows down the flow of electrons. A battery is one way to generate electric current. Inside the battery, chemical reactions take place. One reaction (at the negative end of the battery) creates loose electrons; the other (at the positive end) uses them up. To recharge the battery, the chemical reactions must be reversed to move the electrons in the opposite direction. In this activity:
In order to increase the electrical current, we must speed up the movement of electrons; we do this in the model by adding extra energy in the form of extra smarties. Students will feel warmer as they speed up, which mimics what takes place along a wire in a real circuit. This physical reaction can be used to build in a safety feature in a circuit: if there is a sudden surge of charge and the wire heats up to a certain temperature, a wire could melt, stopping the current. This is basically how a fuse works. A fuse uses a metal wire that melts at a certain temperature, corresponding to the pre-determined limit for the circuit.
What makes an electric charge move? You know that work has to be done to lift an object because the Earth's gravitational field is pulling the object down. In a similar way work must be done to move a charged particle in an electric field. The amount of work required to move a charge between to points or the work per unit of charge is called the 'electric potential difference' between the two points. The unit of potential difference is called the volt. Potential difference can be either positive or negative depending on the movement of the charge. To move charges we need a device that can do work. Such devices include: batteries, generators, thermocouples and batteries. How do electrons move along a wire? Electrons do not move along a wire like cars on a highway. Actually, Any conductor (thing that electricity can go through) is made of atoms. Each atom has electrons in it. If you put new electrons in a conductor, they will join atoms, and each atom will deliver an electron to the next atom. This next atom takes in the electron and sends out another one on the other side. What is electromotive Force (EMF)? Electromotive force, also called emf (and measured in volts) is the voltage developed by any source of electrical energy such as a battery or generator. It is generally defined as the electrical potential for a source in a circuit. A device that supplies electrical energy is called electromotive force or emf. Emfs convert chemical, mechanical, and other forms of energy into electrical energy. The word "force" in this case is not used to mean mechanical force, measured in newtons, but a potential, or energy per unit of charge, measured in volts. What are Conductors? In metals such as copper, silver and aluminum the electrons are not tightly bound to the atoms. They are called "free electrons". This makes them good conductors. Condu tors are materials that allow electricity to flow easily. When a negative charge is brought near one end of a conductor electrons are repelled. When a positive charged object is placed near a conductor electrons are attracted the the object.  ABOVE - COPPER ATOM -- SINGLE VALENCE ELETRON IS LOOSELY BOUND Metals contain free moving delocalized electrons. When electric voltage is applied, an electric field within the metal triggers the movement of the electrons, making them shift from one end to another end of the conductor. Electrons will move toward the positive side. -- Copper is a good conductor because the outer most electrons from the nucleus are weekly bound and repulsive, such that a small perturbance, like a potential difference between two ends of a wire, can knock the valence electrons from an atom free, which then perturb the neighboring valence electrons and so on resulting in a cascade disturbance of moving charges or current throughout the material. The energy required to free the valence electrons is called the band gap energy because it is sufficient to move an electron from the valence band or outer electron shell, into the conduction band where upon the electron may move through the material and influence neighboring atoms. The above following diagram illustrates this concept. What are Insulators? Insulators are materials where the electrons are not able to freely move. Examples of good insulators are: rubber, glass, wood, What is a Battery and How does it Work? A battery converts chemical energy into electrical energy by a chemical reaction. Usually the chemicals are kept inside the battery. It is used in a circuit to power other components. A battery produces direct current (DC) electricity (electricity that flows in one direction, and does not switch back and forth as is with (AC) alternating current). For more information on Batteries see: How does a Battery Work? Generators A generator usually means a machine that makes electrical energy. It has a generator head with wires, spinning inside a magnetic field. The resulting electromagnetic induction makes electricity flow through the wires. Hybrid electric vehicles carry a generator powerful enough to make them go. The biggest generators don't go anywhere; they stay in their power station. Thermocouples Thermocouple, TC for short, is a device that converts heat directly into electricity. A thermocouple can also work in reverse - using an electric current to transform into heat as well as cold. Test your Understanding:
We're talking about metals here. Typically, an object of metal does not consist of molecules. It consists of metal atoms, all grouped together. This is shown in the picture below: The red circles are electrons. As you can see, you can't really say to what atom an electron 'belongs'. These electrons form the connections between the atoms - so they belong to two atoms. Now, when a current starts flowing, these electrons indeed move. When a current flows, energy is transferred. Since the atoms can't move easily, the electrons have to move. You can see this in the unit Ampere of current as well: 1 ampere equals 1 Coulomb per second. The Coulomb (C) is the unit of charge (Q). 1 Ampere means 1 Coulomb of charge passes a certain point in 1 second. This charge is produced by the electrons that actually flow from object one to object two. When we're talking about DC current (normal battery-powered application, for example), these electrons will not return to their source. Consider this circuit: At the beginning, there's a difference in charge between the negative and the positive pole: the negative pole has a surplus of electrons. This creates a force (voltage), and since there's a link between the two poles (the wire and the bulb), the electrons start to flow. The electrons move from the negative pole through the bulb to the positive pole, until there's no difference in charge anymore (or it's that little that it won't cause a current to flow). You can now see that these electrons did not return to their source: they started at the negative pole and ended at the positive pole. We call this a closed path because there's a circle: current starts at the battery and ends at the battery. There's confusion because the battery actually exists of two objects: the positive and the negative pole. Look at this circuit (which is basically the same, but with a capacitor instead of a battery and a resistor instead of a bulb): Current flows from the right side of the capacitor (negatively charged, electrons surplus) through the resistor to the left side of the capacitor (positively charged, electrons shortage). Here, the capacitor plates are separated, so you can easily see that it actually isn't a closed path. We just call it a closed path, because current starts and ends at the capacitor. Since the electrons don't really have to return to their base, you can now understand that electrons can flow into the earth as well. This is also what happens with lightning. Electrons flow from the clouds to the earth (or the other way around, I wouldn't know), just to neutralize the difference in charge. |
When there’s no way for the electron to flow or no complete flow of electron from source to load.
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