Topic 3 - Electric Circuits Flashcards
current
rate of flow of charge
what is the gradient on a charge time graph?
Current ( I = Q/t)
how do you work out charge in a current-time graph ?
area under the graph (Q = It)
An Ammeter must…
have a low resistance and must be placed in series
A voltmeter must…
have an infinitely high resistance and must be placed in parallel
another word for voltage is…
emf / potential difference
potential difference is…
a measure of the amount of energy or unit of charge transferred between 2 points in a circuit
what is Kirchoff’s first law?
the total current flowing into a point is equal to the current flowing out of that point. Conservation of Charge
what is Kirchoff’s second law?
around any closed loop in a circuit, the sum of the pd across all the components is the pd of the supply. Conservation of energy.
power(watts) is…
rate of transferring energy/ rate of doing work.
1 watt of power means…
1 joule of energy is used every second
ohms law:
V = IR
in a series circuit the current is…
the same across all the components
in a parallel circuit the current…
total current = is the sum of the current in all the branches
potential difference in a series circuit…
total of = sum of pd in all appliances
potential in parallel circuits is…
the same across each branch
resistance in a series circuit…
total resistance = the sum of the resistance in all the components
resistance in a parallel circuit…
reciprocal of Resistance = is the the sum of all the reciprocals of the resistance of all the components
is a resistor a conductor?
yes, it’s a conductor so that current can still flow around a circuit however resistors let less current flow than other conductors
what effects the resistance of a resistor?
cross sectional area of the wire
length of the wire
material of the wire
temperature of the wire.
what is resistance proportional to?
Length
What is resistance inversely proportional to ?
Cross-Sectional Area
What is the Resistivity?
The resistance of a 1m² 1m long sample (the resistance per unit cube). its a property of a material.
The resistivty is constant providing…
the temperature is constant
What is the conductivity?
1/ resistivity
On a resistance Length graph what doe the line look like?
positive straight gradient
On a resistance length graph what does the gradient of the line tell you?
resistivity/ Area
What does a Resistance Area graph look like?
A curve getting less steep as it goes down (negative gradient. ITS A 1/X CURVE
What does a resistance 1/cross sectional area graph look like?
Positive straight gradient
What does the gradient of a resitance 1/cross sectional area graph mean?
Resistivty multiplied by length
Prove 1/R1 + 1/R2 + 1/R3 = 1 / R
itotal = i1 + i2 + 13
i = v/r hence:
v/Rtotal = v/R1 + v/R2 + v/R3
v is constant and is the same across all branches therefore 1/R1 + 1/R2 + 1/R3 = 1 / R
What is a potentiometer?
A device that provides a potential difference ranging from 0v and Vs (voltage of supply)
Length needed =
length (of wire) * (voltage out/ total voltage)
What do the letters stand for in I = nAvq?
I = Curent (A) v = Drift velocity (ms-1) A = cross sectional area of the conductor (m2) n = charge density (m-3) q = charge on each charge carrier (c)
What is the charge density?
the number of charge carriers that can move per m^3
A poor conductor has ___ charge carriers
few
A GOOD conductor has ___ charge carriers
many
4v mean 4J per
coulomb of charge
5ohm resistance means 5v is needed for
1A of current
Vout/ Vsupply=
R2 / (R1+R2)
Prove Vout/ Vsupply= R2 / (R1+R2)
for the whole circuit Vsupply = I(R1 +R2)
vout = IR2 I= Vout/ R2
Vsupply = vout/ R2 * (R1 *R2)
R2 / (R1+R2)
define emf
energy supplied per unit of charge by the supply
define terminal pd
energy transferred per unit of charge to the load
define lost volts
energy transferred per unit of charge as the electrons flow through the cell
What is the difference between R and r?
R denotes the load resistance and r denotes the internal resistance
on a graph of current against voltage what does the y-intercept and gradient show?
y-intercept shows the emf
gradient shows the internal resistance
What happens the the graph of current against voltage if there are two cells?
the gradient is twice as steep and the intercept is twice as far up
Define thermistor
resistor whose resistance varies with temperature
define LDR
a light dependent resistor is a resistor whose resistance varies with light intensity
LDR relationship with incident light
As more light is incident the resistance decreases
What are the two types of thermistors?
negative temperature coefficient (NTC) and positive temperature coefficient (PTC)
what is an NTC?
a thermistor where as the temperature increases the resistance decreases
what is a PTC?
a thermistor where as the temperature increases the resistance increases
What are the two factors which describe how thermistors work (PTC, NTC and LDR the first factor)?
- increased temperature means increased lattice vibrations which leads to more electron collisions. (this reduces v in I = nAqv and reduces the current i.e. increases resistance
What are the two factors which describe how thermistors work (NTC and LDR the second factor)?
- energy absorbed (light or heat) results in the release of conduction electrons (this increases n in I = nAqv so increased current therefore reduced resistance)
how does current flow
free electrons (already in the conductor) are repelled by the negative terminal and attracted by the positive terminal. The positive terminal is the long side of a cell. The arrows in a circuit are drawn the opposite way around.
insulators
have a low number of electrons per m^3 and so not conduct well
number of electrons =
total charge / charge of an electron
1 coulomb
the amount of charge that passes a point where a currect of 1 amp flows for 1 second
1 volt of p.d.
a joule of electrical energy transferred for each coulomb of charge
basic points of I=nAvq proof
wire volume = Ad (area*length) number of electrons in volume of wire = nAd total charge of all electrons = nAdq I = Q/t so I = nAdq/t V = d/t sp I = nAvq
resistance
the ratio of the p.d. applied across it to the currect passing through it.
what effects the resistance of a resistor/wire:
- length (Longer means more resistance)
- cross sectional area (smaller means more resistance)
- temperature (in metals a hot wire means more resistance)
- type of material
resistivity ρ
the resistance of a 1m^2 1m long sample, measured in ohmmeters
filament lamp current voltage graph
the resistance increases as the current increases. as the wire gets hotter the current doesn’t increase as much
LED/diode current voltage graph
semiconductor therefore the reverse diode has very high resistance and the forward diode has very low resistance.
they only conduct when they are forward based
as temperature increases resistance increases because (in metals)
the ions in the metal lattice vibrate faster, with greater amplitude, this means electrons collide more with ions opposing the flow increasing resistance
as temperature increases resistance decreases because (in semiconductors)
at rooms temperature there are few free electrons available for conduction. at high temperatures some electrons have enough energy to ‘ escape’ from their atoms and be able to conduct
superconductors
if a wire is cooled to a low enough temperature it looses all its resistance. Electrons flow through them without any transfer of energy.
fuse
a thin wired which melts if the current gets too high
kilo-watt hour
the electrical energy transferred by a 1kw device in 1 hour
variable resistor uses
- control current (rheostat)
- to control voltage (potentiometer)
Root mean square
Square all the values to make them positive. Then find the mean, then square root this mean.
Root mean square uses in electricity
Finding an average voltage or current when its alternating. (A regular average would just be zero)
Potential
The amount of energy available at a point in a circuit
How does the potential along a wire vary with distance?
as distance increases potential decreases
what is current?
a flow of charged particles which are already in the conductors. free electrons are repelled by the negative terminal and attracted by the positive terminal.
which way does current flow
short side of the cell to the long side of the cell. (negative to positive, its drawn the other way around as conventional current)
precautions to obtain accurate values for temperature when making a thermistor
- stir the water / make sure the thermometer and coil are in the same part of the beaker
- check for zero error
- read temperature at eye level to avoid parallax
- switch on/off between readings to avoid the wire heating up
- use a small current to avoid the wire heating up
explain why resistance increases using the structure of the metal
- As temperature increase the lattice ion vibrations increase therefore electrons will collide more frequently with the vibrating ions
- More energy dissipated by collisions greater V required
- Since V increases and R = V/I, R will increase with temperature
Explain, in terms of energy, the difference between potential difference (p.d.) and electromotive force (e.m.f.).
- p.d. is electrical energy transferred between two points in a circuit
- emf is the energy supplied to the circuit
what happens inside a cell as load resistance increases
- total resistance increases
- if emf remains constant then the current inside the cell must increase
why may large wires be used?
- prevent energy dissapation
- allows large currents
- lower cable resistance
Vout/Vsupply =
for a potentiometer
given length/ full length
Vout/Vsupply =
for a potential divider
R2/R1+R2
R2 is the resistor with the voltmeter in parallel over it
If there is a paralell loop over part of a series circuit and a component blows…
The resistance of the parallel increases therefore it gets more voltage
what does increasing r do?
reduces the terminal pd. V due to the lost volts so less energy/power output to load
frequency of mains supply
50Hz
