Electrical Physics
The foundation of electrical physics is that there are two main types of charge, positive and negative. Like charges repel; opposite charges attract.
Definitions
Charge
Electric charge, , is a physical property of matter which causes a force to be created between itself and other particles.
One electron has a charge of .
Voltage
Voltage, , is a measurement of the difference in potential between two points in an electrical circuit. Specifically it is the change in potential energy per unit charge.
Current
Current, , is carried by charge carriers, such as electrons. Traditionally, charge is depicted as travelling from a positive to a negative terminal, though electrons actually travel the other way; this has no impact on calculations.
Power
Power, , is the rate at which energy is transformed by a circuit component.
Resistance
Resistance, , is the ratio of potential difference to current in a component.
Factors influencing resistance
A number of factors influence resistance. For example:
- Temperature: The greater the temperature, the higher the resistance. This is because temperature is directly proportional to mean kinetic energy. Therefore, the charge carriers will collide more often with the particles of the material they are travelling through, increasing resistance.
- Cross-sectional Area: The greater the cross-sectional area, the lower the resistance. This is because cross-sectional area increases the number of electrons per a unit length of the material that are available to carry current. This causes a decrease in resistance.
- Length: The greater the length, the higher the resistance. This is because the charge must move a greater distance.
Resistivity
Resistivity is a property of a material representing how strongly the material resists the flow of current. Lower resistivity corresponds to a higher flow of current.
where is cross-sectional area, is the resistance and is the length. (This may be used to calculate resistivity from measurements.)
Ohm's Law
Ohm's Law states that:
Voltage and Current are directly proportional given that temperature remains constant.
This may be restated as stating that resistance is constant when temperature is constant.
Ohmic and Non-Ohmic Devices
An ohmic device is one that 'follows' Ohm's Law, i.e. one such that the voltage and current are in direct proportion when measured at multiple locations.
A non-ohmic device is one that 'does not follow' Ohm'S Law, i.e. one where the voltage and current are not in direct proportion. Usually this means that for each point, the resistance at that point is higher than it would be in an ohmic device. Non-ohmic devices do not follow Ohm's Law because they heat up during use, increasing their resistance.
Efficiency
Efficiency, , is determined as follows:
Electrical Circuits
Series Circuits
A series circuit is a circuit in which each of the components are one after the other. For example, in the following diagram and are in series.
In a series circuit, the current through every component is equal, and the total voltage through the series circuit is found by summing the individual voltages. The "equivalent" resistance of the series circuit can be found by summing the resistances.
Parallel Circuits
A parallel circuit is a circuit in which each of the components are on a different 'branch'. For example, in the following diagram, and are in parallel.
In a parallel circuit, the sum of the currents through each component is equal to the total current through the circuit, and the voltage is equal on every branch. The "equivalent" resistance of the series circuit can be found by summing the reciprocals of the resistances, i.e.:
Household Circuits
Electrical power in a household is based on a potential difference between two main electrical wires: the Active wire and the Neutral wire. Appliances are almost always wired in parallel.
| Name | Colour | Voltage | Current | Purpose |
|---|---|---|---|---|
| Active | Red/Brown | 240 V rms | Full circuit current | Supply |
| Neutral | Black/Blue | 0 V | Full circuit current | Return charge flow |
| Ground | Green/Green-Yellow | 0 V | Normally none | Protection |
Safety devices
- Ground wire: The ground wire exists to reduce the risk of an electrical shock. In the event that an appliance casing becomes 'live', i.e. the active wire touches the case, the casing becomes part of the circuit. Without a ground wire, any person who touched it would likely be electrocuted. The ground wire exists to offer a path of least resistance for electricity to flow through instead of through a person if a person accidentally touched a live casing, reducing the likelihood (and severity) of electrical shock.
- Fuse: A fuse is a material that melts in the event of excess current flowing through a circuit, breaking the circuit. If a circuit had excess current without a fuse flowing through it, this is likely a symptom of a short circuit occurring; a short circuit may cause wires to melt or pose a fire hazard. Fuses are less widely used than circuit breakers (see below) as they require replacement every time they are tripped.
- Circuit breaker: A circuit breaker is a switch containing a bimetallic strip - a strip made of two metals of different conductivities which are joined together. When heated the bimetallic strip bends towards the less conductive metal. If a current greater than a safe current flows, the strip bends downwards and releases a trip lever, breaking the circuit. (See above for the explanation of why excess current is hazardous, or is a symptom of hazard.)
- RCD: A Residual Current Device detects leakages in currents by detecting tiny differences in current between the active and neutral wires. When detected, the circuit is opened to prevent any further current flow. A leakage in current may indicate that a human has touched a live appliance and current is flowing through them, so RCDs reduce risk of electrocution.
- Double Insulation: Double insulation involves providing two layers of insulative material between active and neutral wires and the housing of an appliance, reducing the chance that the appliance will become live (this will only occur if the outer housing is damaged.) This reduces risk of electrocution from touching a live appliance.
- Surge protection: Prevents spikes in voltage or current by diverting excess current to the earth / ground wire. This protects individual appliances.