Higher Electricity Definitions

a.c. – current which constantly changes direction and instantaneous value with time.

Peak and rms V – the peak voltage is greater than the rms value and can be calculated using an equation.  If an a.c. and d.c. supply have to be compared the a.c. values rms is the d.c. equivalent.

Frequency – the number of waves per second.

Potential Difference – the energy given to each coulomb of charge between two points.

Current – the amount of charge transferred per unit time (Q=It).

Resistance – the opposition to the flow of current.

Power – a measure of the energy used per second (E=Pt).

Electromotive Force (EMF) – the energy (number of joules) given to each coulomb of charge as it passes through the supply.

Internal Resistance – the resistance of the power supply.

Short Circuit – when the load resistor is bypassed in the circuit i.e. the internal resistance is the only resistance.

Open Circuit – when there is a break in the circuit and no current is able to flow.

Capacitance – the number of coulombs per volt (Q=CV).

Conductor – in conductors, the highest occupied band is not completely full and overlaps with the valence band. This allows the electrons to move between bands and therefore conduct. This band is known as the conduction band. Conductors have a low resistance.

Insulator – in an insulator, the highest occupied band (called the valence band) is full. The first unfilled band above the valence band is the conduction band. For an insulator, the gap between the valence band and the conduction band is large and at room temperature there is not enough energy available to move electrons from the valence band into the conduction band where they would be able to contribute to conduction. There is no electrical conduction in an insulator as it has a high resistance.

Semiconductor – in a semiconductor, the gap between the valence band and conduction band is smaller than an insulator and at room temperature there is sufficient energy available to move some electrons from the valence band into the conduction band allowing some conduction to take place. An increase in temperature increases the conductivity, lowers the resistance, of a semiconductor.

Doping – the addition of impurity atoms to reduce the resistance of a semiconductor.

Ptype – when a semiconductor is doped with an element from group 3.  This reducing the overall resistance by decreasing the band gap via making the valance band bigger. Most free change carriers are positive but overall charge of material is neutral.

Ntype – when a semiconductor is doped with an element from group 5.  This reducing the overall resistance by decreasing the band gap via making the conduction band bigger. Most free charge carriers are negative but overall charge of material is neutral.

Forward bias – when the ptype material is connected to the positive side of the supply and the ntype material is connected to the negative side of the supply.  This decreases the size of the depletion layer formed at the junction, decreasing the electric field and allows electrons to flow through the conduction band.

Reverse bias – when the ntype material is connected to the positive side of the supply and the ptype material is connected to the negative side of the supply.  This increases the size of the depletion layer formed at the junction, increasing the electric field and will not allow conduction to take place.

Photovoltaic mode – when no bias is applied and the photodiode acts as a power source.  This is due to a potential difference being created when photons enter the layer.

LED – when a photodiode is connected in forward bias a current is allowed to flow.  This allows electrons to move from the conduction band of the ntype material into the conduction band of the ptype material.  Photons of light are emitted when electrons fall from the conduction band to the valence band at either side of the junction.

Solar cell – a pn junction designed to produce a potential differences through the absorption of photons in photovoltaic mode.

Photovoltaic – absorption of photons in a pn junction to raise electrons from the valence band to the conduction band which are then able to move towards ntype and create a potential difference.