12 Electric current Flashcards
what are charge carriers?
charge particles which flow through a conductor
what is current?
the rate of flow of charge in a wire or component
what is the formula for current?
I = Q/t
what is an insulator?
each electron is attached to an atom and cannot move away from the atom. no current can pass through
what is a metallic conductor?
most electrons are attached to atoms but some are delocalised and are the charge carriers.
what is a semiconductor?
the number of charge carriers increases with an increase of temperature so the resistance of a semiconductor decreases with temperature
what is potential difference?
the work done (or energy transfer) per unit charge
what is the formula for potential difference, work and Q?
V = W/Q
what is the emf of a source?
the electrical energy produced per unit charge passing through the source
what is the formula for electrical power, I and V?
P = IV
what is resistance?
the pd across the component / the current through it
what is the equation for resistance?
R = V/I
what does ohm’s law state?
the pd across a metallic conductor is proportional to the current through it, provided the physical conditions do not change
what is an equivalent statement for ohm’s law?
the resistance of a metallic conductor under constant physical conditions is constant
what is resistivity?
resistivity = RA / L
how do you determine the resistivity of a wire?
- measure diameter using a micrometer at several points along the wire to give a mean. calculate A.
- measure resistance of different lengths of wire and plot graph of R against L
- resistivity is gradient x A
what is a superconductor?
a material which has no resistivity at and below a critical temperature
what are superconductors used for?
- make high power electromagnets that generate strong magnetic fields for devices like MRI scanners and particle accelerators
- high power computers
what is the advantage of using a potential divider to measure the component characteristics?
the current and pd can be reduced to zero
what are the IV characteristics for a wire?
straight line through origin
- resistance is constant
what are the IV characteristics for a filament lamp?
curve with a decreasing gradient as the resistance increases as it becomes hotter
what does a thermistor at constant temperature show on an IV graph?
straight line
the higher the temperature, the steeper the line
at what pd does a diode conduct easily in the forward direction?
0.6 V and above
why does the resistance of metal wire increase with temperature?
the positive ions vibrate more when the temperature is increased so its harder for the charge carriers to pass easily through the metal
