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)
What happens to the resistance of a metal conductor when temperature is increased?
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.
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.
What is a superconductor?
A material which, below a certain temperature, has zero resistance.
The critical temperature depends on the material and is usually close to 0K.
How would a temperature-resistance graph of a superconductor look like?
What are some aplications of superconductors?
Power cables - reducing energy loss through heat to 0.
Strong magnetic fields - so that they don't require a constant power source.
What is the total resistance in series?
Rt = R1 + R2 + R3 + ...
What is the total resistance in parallel?
1/Rt = 1/R1 + 1/R2 + 1/R3 + ....
What is power? What equations can be used to calculate it?
The energy transferred over time.
P = E/t
P = V.I. = J/C . C/S = JC/CS = J/S
I = V/R so P = V.(V/R) = V^2/R
V = I.R. so P = I.R.I = I^2.R
What is the equation for energy transferred?
E = P.t
E = IVt
What can be said about the current in a series circuit?
The current is the same everywhere in the circuit.
What is the p.d. difference of the battery in a series circuit?
Battery p.d. = sum of the voltages across all elements of te circuit.
What can be said about the current in a parallel circuit?
total current = su of the currents in each branch.
(current splits at divisions)
What can be said about the potential difference in a parallel circuit?
Potential difference across each branch is the same.
What is the total voltage for batteries joined in series?
Vt = V1+V2+V3+...
What is the total voltage for batteries for identical cells joined in parallel?
Vt = V1 = V2 = V3 = ...
What does Kirchoff's first law state?
Total current flowing into a junction is equal to the current flowing out of that junction (charge is conserved) in DC circuits.