3.5.1 CURRENT ELECTRICITY Flashcards
What is electric current defined as?
The rate of flow of charge (Q/t)
What is potential difference?
The work done per unit charge (J/Q) [V]
What is the difference between current in a wire, and current in a salt solution?
The charge carriers - In a wire, electrons carry charge, in a salt solution, ions carry charge
Describe properties of an insulator
Electrons are fixed to atoms, and even when a voltage is applied, no current can pass, as electrons cannot move.
(too much energy to move from valence to conduction band)
Describe properties of a conductor
Most electrons are fixed to atoms but some are delocalised. When a voltage is applied, electrons will move from the negative to positive terminal
Describe properties of a semiconductor
Number of charge carriers increases with temperature. (Energy transferred allows electrons to move from conduction to valence band, also opening “holes” of positive charge that are filled by other electrons, driving a current). This means that the resistance of a semiconductor decreases with increasing temperature, making it useful in thermistors and other NTC components`
What is resistance?
A characteristic of a component, measuring the opposition to current flow. It also shows the drop in potential difference per ampere of current inside it
Describe the relationship between power dissipated and resistance in series and parallel
Power dissipated is inversely proportional to resistance for a constant voltage as P = V^2/R
(in parallel)
Power dissipated is directly proportional to resistance for a constant current as P = I^2R
(in series)
What is the equation relating resistance and resistivity?
ρ = RA/l
R = ρl/A
Where l = length of wire and A is the cross sectional area
What is the purpose of voltage?
Voltage causes electrons to flow in a chosen direction (neg to pos), and without it, delocalised electrons would move randomly, which isn’t very useful
Explain why the current in a filament lamp rises to a high value before falling to a steady value, and why it is more likely to fail when being switched on than other times
initial resistance is low so initial current is
high
* temperature of filament increases (or filament
heats up)
* resistance increases as temperature rises
* increase in resistance causes current to fall
* current is steady when energy supplied =
energy lost from filament (or when temperature
is constant)
* maximum heating is produced at start when
current is highest
* melting of filament causes it to fail (could be
mechanical failure caused by temperature rise)
* when switched on energy is supplied more
rapidly than it is lost so filament melts
What is the approximate threshold voltage for a diode?
around 0.6/0.7 V
What is a superconductor?
A component/substance with no electrical resistance
What does the term “transition/ critical temperature” mean?
The temperature at which a substance becomes a superconductor
graph of R against T shows
high temperature graph with abrupt
discontinuous vertical line indicating that R has
become zero at a certain temperature (label transition)
R decreases steadily with T until transition temp
Name 2 applications of superconductors
power cables, electromagnets, generators,
motors, transformers, MRI scanners, monorail
trains, particle accelerators, fusion reactors
Explain why emf is different to the voltage in the external circuit
work is done inside the battery/there is resistance
inside the battery
so less energy is available for the external circuit/some voltage
is lost between the terminal/mention of lost volts
What is emf?
Amount of chemical energy transferred to electrical energy per coulomb of charge
the (total) energy transferred/work done when
one unit/coulomb of charge
is moved around a circuit/provided by the supply
Or potential difference across the terminals when the current = 0
Describe the heating effect of an electric component
- Heating effect due to the resistance
- Charge carriers repeatedly collide
with positive ions of colliding
material - Net transfer of energy from charge
carriers to positive ions - After charge carrier loses Ek from
collision, force due to pd across
component accelerates it until it
collides with another ion
Does the power supplied to a component depend on the direction of current?
no
Potential divider equation
V1 = Vt . (R1/R1+R2)
series components, Vt is total input voltage
Describe how a simple potential divider circuit can be used as a heater
- Connect voltage source to resistor
and thermistor in series - Connect heating element to
thermistor in parallel - Cold temperatures, R of thermistor
increases, so V increases - V across heater also increases
- Heats
Conservation of charge and energy in circuits
Charge - current entering a junction = current exiting a junction
Energy - sum of emf in a closed loop = sum of pds across each component,
(Work done on a charge carrier = potential drops across components)
What are some uses of potential divider circuits?
As switches, with LDRs or thermistors
To turn on a fan
Calibrating voltmeters
What type of resistor has a high resistance at low temperatures?
an NTC thermistor
How do potentiometers work?
Potentiometers work by having a resistive element inside. Both end terminals are attached to it, and do not move. The wiper travels along the strip when the knob is turned. The closer the wiper is to the end terminal it is wired in conjunction with, the less the resistance, because the path of the current will be shorter
cgp book explains well
What type of temperature coefficient does a metal have
A positive one, as resistance increases with temperature
(Positive ions in metal vibrate more when temp increases, electrons can’t pass as easily when a pd is applied)
What are two methods to measure IV characteristics of a component
Use a potential divider to vary voltage from 0, or use a variable resistor to vary current from a minimum
(adv of potential divider is that it can vary voltage to 0, which can’t be done w variable resistor)
IV graph for a NTC thermistor at constant and varying temperature
- For a constant temperature, IV
graph is a straight line
For changing temperature, graph is one of inverse proportion
Why are intrinsic semiconductors NTC components?
At higher temperatures, more charge carriers are freed (conduction electrons move and form holes).
R decreases non linearly as temp increases
+percentage change of resistance per kelvin is greater than metals, so often used in temperature sensing circuits
What is a sensor circuit?
A circuit that produces an output pd as a result of a physical change e.g. temperature
How does adding identical cells in parallel decrease the internal resistance of the circuit?
It decreases it by a factor of 1/n , where n is the number of cells
Less lost volts as IR = ε - Ir/n
Define internal resistance
Loss of p.d per unit current in a source when current passes through the source
Give an advantage and disadvantage of using cells in parallel
- If one of the cells fails, other cells
can still provide emf - More energy is stored in banks of
cells, less power dissipated
-Current is shared by cells, takes
longer to convert energy stored in
each cell - However total emf cannot be
changed
+increased “capacity” according to breakenridge
Pros/cons of using superconductors in electrical transmission
- Zero resistance, Reduce power loss
- Difficult to maintain below critical
temperature (+over long distances)
Why does the gradient of an IV graph not give you the resistance
This is because the resistance is the voltage divided by the current at a particular point. It’s not the change in voltage divided by the change in current at that point, which is what the gradient represents
Symbol for heater and motor
Heater is box w 3 stripes, motor is a circle w M then weird line
(google)
When potential divider eq can’t be used
When load resistance (connected to vout) is negligible, otherwise can be incorporated as a resistor in parallel
Maximum power delivered from a circuit w internal resistance
When r=R, curve looks like a bell w max at R=r
(don’t think i need to know this)
Cells in parallel + lost volts
Each time an electron passes through the circuit, only passes through one of the cells, so no extra pd
lost volts = Ir/n
Conservation of charge in circuits
Total charge passing through any cross-section of a conductor in a given time is always the same. This is because electric charge cannot be created or destroyed
current in = current out at a junction
LDRs and photoelectric effect
We can say the principle behind the LDR is based on the Internal PhotoElectric effect. When the light is absorbed by the material the conductivity of the material increases i.e the electrons in the valency band are excited to the conduction band and so the resistance decreases
copied from web
