Electricity (E) Flashcards
Current:
The rate of flow of positive charge carriers
What is current measured in?
Amps
equation relating time, charge and current
charge = current x time (Q=It)
What is conventional flow:
flow of charge from positive to negative
What is electron flow?
Negatively charged electrons flow from negative terminal of cell to positive.
What measures current?
An ammeter in series
What is potential difference?
the work done per unit charge in volts
Equation relating potential difference and work done:
potential difference = work done/charge
Resistance:
the opposition to current
The higher the resistance…
the lower the current
Why are wires often made of copper?
As they have very low resistance so are very good electrical conductors
equation for resistance, potential difference and current
resistance = potential difference / current
what is an ohm?
one volt per amp
State Ohm’s law:
For a conductor at a constant temperature, the current through it is proportional to the potential difference across it.
How can you investigate Ohm’s law?
adjusting a variable resistor to vary the current in the circuit. Measure voltage across fixed resistor as current varies. Draw graph.
How can you tell form a graph if a component obeys Ohm’s law?
straight line through the origin
Draw diode current voltage graph:
straight line on x axes until a little after origin on x axes it goes steeply upwards
Draw filament lamp current voltage graph:
initially follows ohmic component trend but as voltage gets too high, current flattens
Resistivity describes:
The extent to which a material opposes the flow of electric current through it
What happens as free electrons move through metal wire?
they collide with ions, transferring their energy upon collision
what is resistivity measured in?
ohm metres
higher resistivity=
higher resistance
What does higher temp do to resistance and why?
Higher temp means ions inside wire vibrate faster so there are more collisions, slowing them down so current decreases
What does a thermistor do?
As the temperature increases, the resistance decreases.
Draw symbol for thermistor:
—box with hockey stick through it—–
What is superconductivity?
If a material is below a temperature called the critical temperature, its resistivity disappears entirely. it is now a superconductor
What are superconductors useful for?
applications that require a large electric field or reductions of energy loss
name 4 uses of superconductors:
MRI scanners, transformers, motors, electromagnets
What does a light dependant resistor do?
As the light intensity increases the resistance decreases.
Resistors in series:
combined resistance of components = sum of all individual resistances (Rt = R1 + R2 + R3…)
Resistors in parallel:
reciprocal of combined resistance = reciprocal of individual resistances (1/R = 1/R1+ 1/R2 …)
Current in series:
current is same for all components
Current in parallel:
current is split across branches. total current in junction = total current out of a junction
Potential difference in series:
emd is shared amongst all components depending on their resistance
Potential difference in parallel:
voltage of all components in each branch = emf
Charge is never…
used up or lost
What is KIRCHHOFF’s first law?
sum of currents entering a junction = sum of currents coming out of a junction (I = I1 + I2 +….)
junction:
point where at least three circuit paths meet
branch:
point connecting two junctions
Energy is never…
used up or lost
What is KIRCHHOFF’s second law?
the total emf in a closed circuit equals the sum of the potential difference across each component (each closed circuit can be treated like a series circuit)
The sum of emfs in each closed loop =
emf of power supply
What is electrical power?
rate of change of work done
Power equation (wd and time) :
work done / time
What is work done?
Energy transferred
Power equation (current and voltage)=
current x voltage
Using Ohm’s law, how can you rewrite the power equation?
P=I^2R or P=V^2/R
How can you calculate energy transferred?
voltage x current x time
When two resistors are connected in series…
through K’s second law, the p.d across the power source divides between the two
Potential dividers:
circuits which produce an output voltage as a fraction of its input voltage
What are the three main purposes of potential dividers?
to provide variable potential differences, to enable specific pd to be chosen, to split pd of a power source between two or more componenets
V out =
(R2/R1+R2) x V in
