Electricity Flashcards
What is electric current?
The flow of charge per unit time. I = deltaQ / deltaT (where Q is charge, in coulombs)
What is potential difference?
The energy transferred per unit charge between two points in a circuit. V = E / Q (where E is energy transferred, in Joules)
What is resistance?
A measure of how difficult it is for charge carriers to pass through a component. R = V/I
What does Ohm’s law state?
For an ohmic conductor, current is directly proportional to the potential difference across it when physical conditions are kept constant (i.e. temperature)
What would the current voltage graph look like for an ohmic conductor?
What would the gradient be on this graph?
Gradient = V/I = R
Gradient = resistance
How would a current-voltage graph for a semiconductor diode look like?
For current in the -ve direction, lots of voltage is needed for little current.
In the positive direction, only the threshold voltge is needed for current to easily flow.
What would a current-voltage graph look like for a filament lamp? Why?
Wire heats up as current increases, so resistance increases, so V/I increases, so gradient increases.
Ohm’s law is obeyed for low currents.
What assumption can be made about the resistance of ammeters?
Can be asumed to have 0 resistance, meaning they would not affect the measurement of the current of a circuit.
(R = V/I)
What assumption can be made about the resistance of voltmeters?
Voltmeters have infinite resistance, so no current can flow, therefore they measureof potential difference is exact.
(R = V/I)
if R is infinite, I is 0.
What is resistivity? What is its equation?
A measure of how easily a material conducts electricity.
p = RA / L
where R=resistance; A=cross sectional area; L=length
What can be said about the resistivity and resistance through a material of length 1 m and cross sectional area 1 m^2.
Resistivity and resistance will have the same value.
p = RA/L
p = R.(1/1)
p=R
What happens to the resistance of a metal conductor when temperature is increased?
Resistance increases.
This is because the atoms in the metal gain Ke, move more and so causes the charge carriers (electrons) to collide with the atoms more frequently.
This way current is decreased, and so resistance increases.
(R=V/I)
What happens to the resistance of a thermistor when temperature is increased? Why?
Thermistors are usually made from non-metals, therefore don’t have delocalised electrons.
An increase in temp. causes electrons to be emitted from atoms.
More electrons are free to move and so current increases.
Therefore resistance decresases
(R = V/I)
How would a temperature-resistance graph of a thermistor look like?
What can be one application of a thermistor?
trigger an event once the temperature changes.
i.e. Turn on the A.C. once temp. reaches a certain value.
How would a temperature-resistance graph of a superconductor look like?
What is a potential divider?
A circuit with several resistors in series connected across a voltage source used to produce a required fraction of the source potential difference.
Can a potential divider supply a variable potential difference? How?
Yes. By using a variable resistor.
