P17. Electricity
Simple circuits
A simple circuit contains the minimum things needed to have a functioning electric circuit. A simple circuit requires three (3) things:
Despite being considered simple, they are useful and quite common. They exist as flashlights, doorbells, and many kitchen appliances. From a Physics point of view they display many of the rules that govern the behavior of electricity in circuits.
- A source of electrical potential difference or voltage. (typically a battery or electrical outlet)
- A conductive path which would allow for the movement of charges. (typically made of wire)
- An electrical resistance (resistor) which is loosely defined as any object that uses electricity to do work. (a light bulb, electric motor, heating element, speaker, etc.)
Despite being considered simple, they are useful and quite common. They exist as flashlights, doorbells, and many kitchen appliances. From a Physics point of view they display many of the rules that govern the behavior of electricity in circuits.
Flowing electrons- electrical current
Electric current is very similar to a flowing river. The river flows from one spot to another and the speed it moves is the speed of the current. The size of the current flow is related more to the size of the river than it is to the speed of the river. A river carries more water each second than a stream, even if both flow at the same speed. With electricity, current is a measure of the amount of charge transferred over a period of time. Current is a flow of electrons, or individual negative charges. When charge flows, it carries energy that can be used to do work. Scientists measure current with units called amperes.
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In this animation you should notice the following things:
- The battery or source is represented by an escalator which raises charges to a higher level of energy.
- As the charges move through the resistor (represented by the paddle wheel) they do work on the resistor and as a result, they lose energy.
- By the time each charge makes it back to the battery, it has lost all the energy given to it by the battery.
- As the charges move through a wire, they lose no energy (assuming the wires are short and are a good conductor).
- The potential drop ( - potential difference) across the resistor is the same as the potential rise ( + potential difference) across the battery. This demonstrates that a charge can only do as much work as was done on it by the battery.
- The charges are positive so this is a representation of Conventional Current (the apparent flow of positive charges)
- The charges are only flowing in one direction so this would be considered direct current ( D.C. ).
Circuit diagrams
We use circuit symbols to draw diagrams of electrical circuits, with straight lines to show the wires. The diagram shows some common circuit symbols:
The idea of a circuit diagram is to use circuit symbols instead of drawing each component in the circuit. Always try to make the wires straight lines, and don't be tempted to make them wiggly.
The whole point is to make it easier to see what is connected to what. Here you can see how the symbols for a cell (not a battery!) and a lamp look in a circuit diagram
The whole point is to make it easier to see what is connected to what. Here you can see how the symbols for a cell (not a battery!) and a lamp look in a circuit diagram