Electricity Flashcards
Define Current
? Rate of flow of charge
Define Voltage
? Energy per coulomb
Define resistance
How hard is it for current to flow
Kirchhoff’s first 2 laws
Current into a junction is equal to current out of a junction
Voltage across each of the resistors will add up to the equal voltage across the battery
Parallel - the voltage on each route adds up to the voltage across the battery
Thermistor
? Temperature dependent resistor
Inversely proportional
Equation for current
Q=It
Define 1 Ampere
The current flow when 1 coulomb of charge passes a point each second
Define potential difference
? Work done per coulomb of charge (V)
What properties does an ideal voltmeter havee
Infinite resistance, no current is drawn from the circuit
Equation for WD and PD
W=QV/ E=QV
Ohms Law
V=IR
? for a wire at constant temperature, the voltage across is proportional to the current
What is an ohmic conductor
? a conductor that obeys ohms law at constant temperature
Equation to calculate resistivity
R = pL/A
Define superconductor
? all object of a material that has zero resistance, only if below its critical temperature
‘High temperature’ superconductor = critical temperature of >77K(-196)
Semiconductors
Components for which the resistance changes depending on external conditions
Define power
Rate of energy transfer
Equation for Power
Power =WD/t
P=IV
=I^2R =V^2/R
IV graph of a diode
Semiconductor diode - when looking at the current-voltage graph of this component you
must consider its forward and reverse bias. The forward bias of a diode is the direction in
which it will allow current to flow easily past the threshold voltage, which is the smallest
voltage needed to allow current to flow. In the direction of the reverse bias, the resistance of
the diode is extremely high meaning that only a very small current can flow.
IV graph of a Filament lamp
Filament lamp - This component contains a length of metal wire, which heats up as
current increases, therefore the resistance of this component increases as current
increases. At low currents the metal wire will not heat up significantly therefore for very low
currents, Ohm’s law is obeyed. However, as the current increases (in either direction), the
graph begins to curve due to the increasing resistance.
Electromotive force
? Voltage acorss a power supple
Internal resistance
Opposition to the flow of charge within a cell
Equation relating internal resistance and electromotive force
e = IR + Ir
e = E/Q
Sum of resistance in a series circuit
R1 + R2 + R3 …
Sum of resistance in parallel circuit
1/Rt = 1/R1 + 1/R2 …
Equation relating energy and current and Voltage
E =VIT
Why is variable resistor used in a potential divider
Used to change the voltage supplied to a suitable voltage without using a transformer. However this method wastes power
IV of a diode
● Semiconductor diode - when looking at the current-voltage graph of this component you
must consider its forward and reverse bias. The forward bias of a diode is the direction in
which it will allow current to flow easily past the threshold voltage, which is the smallest
voltage needed to allow current to flow. In the direction of the reverse bias, the resistance of
the diode is extremely high meaning that only a very small current can flow.
what happens to the resistance of a metallic conductor when temperature rises
Resistance increases
Application of thermistors as temperature sensors
Accurate and cost effective
Application of superconductors
Power cables - reduce energy loss through heating
Strong magnetic fields - medical applications
Practical in determining resistivity of a wire
Measure the diameter d of the wire and record its diameter.
Set up the apparatus as shown in the diagram on the next slide.
Adjust the length l to 0.100m using the crocodile clips
Read and record the current I on the ammeter and the voltage V on the voltmeter. Calculate the resistance R by using R = V/I and record this.
Increase l by 0.100m and repeat this, increasing it by 0.100m each time up to 0.800m.
