Electricity Flashcards
Electric current
The rate of flow of charge in the wire or component.
Convention for the direction of current in a circuit
From positive (+) to negative (-).
Charge flow
ΔQ = I Δt
Insulator
Each electron is attached to an atom and cannot move away from the atom. When a pd is applied across an insulator no current passes through it becuase no electrons can move.
Metallic conductor
Most electrons are attached to atoms but some are delocalised - the delocalised electrons are the charge carriers in the metal. When a pd is applied across the metal, these conduction electrons are attracted towards the positive terminal of the metal.
Semiconductor
The number of charge carriers increases with an increase of temperature. The resistance of a semiconductor therefore decreases as its temperature is raised.
Potential difference
The work done per unit charge, V = W/Q.
The emf of a source of electricity
The electrical energy produced per unit charge passing through the source.
Electrical power, P
P = I V
Resistance
R = V/I
Ammeter
Used to measure the current through a component, must be placed in series.
Voltmeter
Used to measure the pd across a component, must be placed in parallel with the component.
Ohm’s law
The pd across a metallic conductor is proportional to the current through it, provided the physical conditions do not change.
Resisitivity
ρ = RA/L
Superconductor
A wire or device made of material that has zero resistivity at and below a critical temperature that depends on the material.
