Chapter 6: Electricity Flashcards
What are the two main properties of matter?
mass and charge
What is the charge of an individual particle?
1.6 x 10^-19 C (coulomb)
Can you interact with other particles if you have charge?
yes, a charged particle can interact with other particles via a magnetic field
What is current?
The rate of flow of charge
the rate that charge is displaced from one place to another
What is the formula for current ?
I=Q/t
What is the direction of the movement of charge?
from negative to positive
What is the direction of conventional current?
from positive to negative
(opposite to actual charge flow)
(it describes the way a positive charge carrier would travel)
What is potential difference?
the p.d between two points is the work done (energy transferred) in moving a unit charge
to make charge flow in a circuit, energy needs to be transferred to charge (this energy supplied by power supply)
What is the formula for potential difference?
V=W/Q
W=work done
How is resistance created in a circuit?
when electrons collide with atoms and lose energy
What is internal resistance?
it is the resistance created INSIDE the power source when electrons collide with atoms inside the power source and lose energy
What is load resistance?
it is the TOTAL resistance of all the components in the external circuit
otherwise known as ‘external resistance’
What is e.m.f?
electromotive force
it is the amount of electrical energy the battery produces AND transfers to each coulomb of charge
What is the formula for e.m.f?
ε = Energy/Charge
e.m.f is measured in Volts
What is the potential difference across the load resistance?
it is called TERMINAL potential difference (V)
it is the energy transferred when one C of charge passes through the load resistance
If there was no internal resistance, what would terminal p.d equal to?
the terminal p.d would be the same as the electromotive force
What are lost volts?
it is the ENERGY wasted per coulomb overcoming the internal resistance
its symbol is: v
How is a current formed?
by putting a potential difference across an electrical component, then a current will flow
What is resistance?
a measure of how difficult it is for current to flow, it is measured in ohms (Ω)
opposition that a substance offers to a flow of charge
How can you describe resistance at a particle level?
resistance is the result of inelastic collisions between charge carriers and other electrons or ion/atoms in material
inelastic collision=transfers energy to surroundings during collision
What is the formula for resistance?
R=V/I
What is Ohm’s law and what is an ohmic conductor?
Ohm’s law: as long as the physical conditions stay the same, the current through an ohmic conductor is directly proportional to the p.d across it
an ohmic conductor: a conductor that follows ohm’s law
What is a physical factor that resistance is dependent on?
temperature
a higher temp. means that atoms will vibrate more and the electrons (charge carriers) will be scattered more and more inelastic collisions, so more energy lost
What is the resistivity of a material?
it is how difficult it is for current to flow through a material
What three factors does resistance depend on?
length: longer the wire, more difficult it is to get a current to flow through it
area: wider the wire, the easier it is for electrons to pass along it
resistivity: it depends on the material and external features (e.g-temp.)
What is the formula for resistivity?
resistivity = R * A / L
R=resistance
A=area
L=length
What makes a conductor better?
if it has more charge carriers (electrons)
What is a semi-conductor?
- some free electrons but a lot less than a conductor
- if energy is supplied to it (e.g-temp. increase), then MORE charge carriers will be released which reduces resistivity
- moderate resistance (higher temperature, lower resistance)
What effect can increasing temperature have on resistivity of semi-conductors?
it can reduce resistivity, as more charge carriers released
How can semi-conductors be used?
they can make great sensors for detecting changes in the environment such as temp. and light
What is a superconductor?
a conductor that has ZERO resistance
How can you lower resistivity?
by cooling the material down, so less vibrations, less electron scattering and less energy wasted
How can a superconductor be made?
by cooling some specific materials down to below their CRITICAL TEMPERATURE, their resistivity becomes zero
What is the problem with making a superconductor?
most metal’s critical temperature is very very low (like -263°C ). getting things that cold is very tricky and expensive
What are possible uses of superconductors?
- power cables that transmit electricity without wasting any power
- strong electromagnets
- electronic circuits that work fast with minimal energy loss, no resistance to slow the current down
How do you get the resistance of an I-V graph?
by taking the reciprocal of the gradient
How much resistance does the ideal voltmeter have and why is it called ideal?
it is called ideal because it is almost impossible to achieve
an ideal voltmeter should have infinite resistance
How much resistance should the ideal ammeter have?
it should have no resistance
so that there would be no potential difference across them
What is the I-V graph like for an ohmic conductor?
an ohmic conductor, under the same physical conditions should have a current that is directly proportional to voltage
graph: straight line that goes through zero
What is the I-V graph like for a filament bulb and explain?
curve that starts steep but gets shallower
the filament is a coil of wire that increases its temperature when current flows through it, so resistance increases
more current= more temp=more resistance=current decreases
What happens when a current flows through a metal conductor?
some of the electrical energy is transferred into heat energy, causing the particles in the metal to vibrate even more, these vibrations make it harder for the current to pass
What are diodes made from and how does the current flow?
semi-conductors
only let current flow in one direction
What is forward and reverse bias?
forward bias is the way that the current is allowed to flow in a diode
reverse bias is the way that the current cannot flow
What does the I-V graph look like for a diode?
flat line on negative side and exponential growth on positive side when the voltage is 0.7 V
very very high resistance on negative side
How can you measure diameter of wire?
by using a micrometer
What is the graph for determining the resistivity?
resistance * area on y-axis length on x-axis OR resistance on y-axis length/area on x-axis
Explain the I-V characteristics of a filament lamp
ohm’s law initially obeyed, and there is a straight line
at a given voltage,current heats up the filament
as filament heats up, resistance increases (more vibrations)
so gradient of line decreases
resistance is the reciprocal of the gradient
in terms of electron flow, how does resistance change with increased current in a filament bulb?
- as current increases, resistance increases
- temp. increases lattice vibrations
- more collisions of electrons and atoms of filament
What does an I-V graph look for a diode with reverse bias?
(0,0) straight line in (-x,-x) quadrant and then goes steeply downwards at -50V
What is breakdown voltage?
in a diode with reverse bias, at -50V
then the resistance is approx. zero and the current becomes very large
What is Kirchhoff’s first law?
total current entering a junction=total current leaving a junction
this is because charge/current does not get used up or lost
What is Kirchhoff’s second law?
total e.m.f around series circuit=sum of p.d of all components
energy transferred TO charge=e.m.f
energy transferred FROM charge=p.d
What are the rules for I/R/V for a series circuit?
I- same current everywhere in circuit
V-p.d splits between all the components
R- resistance SPLITS proportionally between all components
What are the I/R/V rules for a parallel circuit?
I- current is split between branches
V- p.d stays same across all branches (p.d across each branch == e.m.f)
R- total resistance is lower than the resistance of the lowest resistor
What is the reciprocal rule of resistance?
describes the resistance in parallel circuits
1/Rₜ=1/R₁ + 1/R₂ + 1/R₃
What is the rule for cells in series?
total e.m.f = sum of e.m.f of all indiv. e.m.f’s
What is the rule for cells in parallel and why?
for IDENTICAL cells in a circuit:
** total e.m.f of all cells=e.m.f of an indiv. cell
this is because the charge only gains e.m.f from the cell it travels to because current splits evenly
What is the critical temperature (in terms of superconductors)?
the temperature that a material becomes a superconductor is called the critical temperature
Why does terminal p.d decrease as current increases?
more energy lost across internal resistance as there is more current so less of emf is available for terminal pd
lost volts increase with current
Explain how and why a voltmeter reading across a battery changes when a switch is closed?
it would decrease because current will flow through the battery which means that there will be an internal resistance, so there are lost volts within the battery and the emf is shared between internal/external resistors
Potential divider: 9V in total, 0.25kΩ, then 750Ω thermistor, R resistor in parallel to thermistor and an output pd. The thermistor has a resistance of 750Ω at room temperature
A resistor is rated at 0.36W for the 0.25kΩ resistor,
Determine whether this resistor is suitable?
-need to find theoretical max. V across the 0.25kΩ for the max. power output (which is 9V)
-P=V²/R
-0.32W is the highest theoretical power
so YES, this resistor is suitable because max.power rating is less than the power rating given
If an electrical appliance is rated at a certain power wattage, what does it mean?
it is the most amount of power that it will take before it is damaged
