6.1 Glossary of Circuit Terms and Laws
Symbol: Q
measured in: Coulombs (C)
Charge arises from a net difference between electrons and protons in a substance. In general, if there are more protons than electrons, the substance is postitively charged. If there are more electrons than protons, the substance is negatively charged. Since only electrons are really mobile, they are usually the charges considered in an electric circuit. There is more than one way of quantifying charge, but for the purpose of this unit, all that is needed is the coulomb.
A path through which current can move.
A closed circuit has no gaps in the intended path of the current. In other words, all components of the circuit are connected.
An open circuit is one in which there is not a continuous path in which the current can flow.
A series circuit is where only one path is available for current to take, for example, a single loop of wire. The current is the same at all points in a series circuit.
A parallel circuit is one that contains some kind of path (or wire) junction. This means that more than one path is available for the current, and that it must split up to cover both paths. The higher the resistance of a given path in a parallel circuit, the less current it will receive.
Symbol: I
Measured in: Ampères or Amps (A)
Current is defined as the movement of charges (electrons) in a circuit. It is the amount of charge flowing (past any point in the circuit) per unit time. In more mathematical terms this is expressed as
For current to flow, the circuit must be closed. In a circuit, current is said to flow through all points at the same time.
The direction of alternating current changes over time, alternating between negative and positive. Negative current simply means current in the other direction. The charges involved in this type of current do not experience any net displacement.
A.C. is more versatile, and is the best kind for transporting energy over long distances (e.g. power cables).
This type of current flows in one direction only, meaning that charges do get displaced. D.C. comes in two forms: "lumpy":
and "smooth":
While these are not actual classifications for direct current, they do help us to identify what the current is like. Direct current is a name given to any current flowing in one direction, thought it is D.C. of the "smooth" variety that is needed in electronics.
This is said to flow from the positive terminal of an electrical energy source (e.g. a battery, generator, or an outlet) to the negative terminal of the source. In fact, the electrons in the wire (the most commonly used charges)flow in the opposite direction, but this was not discovered until after a direction of current flow had already been agreed upon. The later discovery in no way affects current calculations, and so is considered irrelevant, and therefore is ignored.
Symbol: E
Measured in : Joules (J)
Electric energy is the work done in moving charges in a circuit. Like mechanical energy, it is simply the product of the power (work done per unit time) and time, and thus is expressed as :
Symbol: P
Measured in: Watts (W)
Power is the rate at which energy is used or converted, and can therefore be expressed as
By substituting other equations, three more useful equations can be derrived:
Symbol: E.M.F.
measured in: J/C, also known as Volts (V)
In order for charges to move around a circuit (as a current), they need something to help them move, or, a force to push them along. This force, provided by a power source is known as the E.M.F. The E.M.F. expresses the amount of energy given to each charge that passes through the power source, and therefore
These help to summarize two very important, yet simple, circuit ideas:
1. The Current Law: The sum of the currents flowing into a junction is equal to the sum of the currents flowing out.
2. The Voltage Law: The sum of the E.M.F.’s in a closed circuit is equal to the sum of the p.d.’s of the components.
The ratio of the p.d. across a circuit component to the current going through it is constant (provided that temperature and some other factors are as well). This ratio is known as the resistance of the component.
Symbol: V or p.d.
Measured in: J/C or Volts (V)
The potential diference is the amount of energy taken away from each charge as it passes through a component of a circuit (not including the power source).
Symbol: R
Measured in: V / A, also known as Ohms (W )
Ohmic resistors are ones that obey Ohm’s Law, and are defined by
Non-ohmic resistors have changing ratios, and are defined by their instantaneous resistance:
