Electric Circuits Flashcards
Define Current
Rate of flow of charge
I = Q/t
Conventional current flows
from positive to negative
Electrons flow
from negative to positive
Electric Current occurs when
a charged particle, which is free to move experiences an electric force.
Define Voltage
Energy transferred per unit charge. V = E/Q.
1V = kgm^2s^-3A^-1
Define emf
the supply voltage to a circuit
Define electronvolt
The amount of energy an electron gains by passing through voltage of 1V.
1eV = 1.6 x 10^19 J
Define Resistance
Opposition to the flow of current. R = V/I
State Ohms Law
Current through a component is directly proportional to voltage, providing temperature is constant.
Define Resistivity
The property of a material to resist the flow of electric current.
State the equation fro resistivity
P = RA/l
Resistance is the result of
collisions between charge carriers and atoms in the currents path.
Define Drift Velocity
The slow overall movement of electrons. I =nAqv
Higher temperature in metal causes
atoms to vibrate more, more collisions - slowing drift velocity of electrons
Higher current causes
more collisions, as electrons move faster, atoms vibrate more- temp increases
Define I = nAqv
n- density of charge carriers
A- cross sectional area
q - charge on each carrier
v - mean drift velocity
What is a NTC
Negative Temperature Coefficient - as temp increases, resistivity falls.
Semiconductor
Materials with a small number of delocalised electrons that are free to conduct
LDR
Light Dependant Resistor.
Series Circuit
Current is constant. emf is split between components.
Parallel Circuit
Current is split at junctions. pd is same at each component.
Conservation of Electric Charge
Total current in = total current out
Electric Voltage Rules
in order to conserve electrical energy around any closed loop, sum of emfs is equal to sum of pds around that loop.
What is a Potential Divider
A circuit designed to provide specific voltage by splitting emf across two resistors.
Potential Divider Equation
Vout = Vin x R2/R1 + R2
Potential Difference is
The change in electric potential energy per coulomb between two points
why the filament is more likely to fail when being
switched on rather than at other times.
Maximum heating is when lamp is switched on / when current is highest
Filament breaks due to melting caused by temperature rise
Explain how temp and increased atomic vibrations effect lamp and the thermistor.
For the lamp: Increased atomic vibrations reduce the movement of electrons, resistance of lamp increases with temp.
For the thermistor: Increased atomic vibrations reduce movement of electrons, increase in temperature leads to a large increase in n, r decreases as temp increases.
Practical use for a diode
- Protect components.
- Rectification/ AC to DC
- Prevent earth leakage
- Voltage controlled switch
A thermistor has a NTC. Explain with reference to I = nAQv what happens to its resistance when its temp increases.
temp increases, resistance decreases. Decrease in v. Large increase in n. A and Q remain constant
Explain why the resistance of a tungsten filament increases with temperature
Resistance due to collisions between electrons, as particles have more energy and vibrate faster. Increased frequency of collision - impedes flow of electrons.
A student has a metal conductor and a plastic insulator (same dimensions). He applies same pd across each. Explain how the relative values of n for the metal conductor and plastic insulator affect the current in each.
n is greater in conductors so there is a larger current flow in conductors
equation for internal resistance
ε = I(R+r) V = ε -Ir
Describe the I-V characteristic for an NTC thermistor.
As voltage increases, current increases
Increased current causes an increase in temperature
This means resistance decreases because NTC thermistors decrease in resistance with temperature so gradient of graph increases
Explain the difference between resistance and resistivity
Resistivity is a constant for the material. Resistance depends on resistivity and dimensions
Resistors in series act as
Potential dividers. Vout= Vin x R1/R1+R2
The temperature of the wire increases. Explain what happens to the drift velocity of the electrons if the potential difference remains constant.
Increased lattice/ions/atoms vibrations causing resistance to increase/ increased electron collisions with
ions. This leads to a reduction in the drift velocity
Explain the variation of resistance with potential difference for the filament bulb in
terms of particle behaviour.
As V increases, electron energy increases. Greater energy transfer in collisions with lattice, causing temperature to increase. Amplitude of vibrations increase. Collision rate between ions and electrons increases. Due to increase in resistance, current doesn’t increase in proportion to pd.
With reference to charge carriers, explain why the resistance of the thermistor RT, changes with temperature.
Number of charge carriers increases with temperature, this lowers resistance, despite increase in lattice vibrations.
Explain why the resistance of a metal sample increases with an increase of temperature.
Rise in temp causes amplitude of vibrating ions to increase. This causes the number of collisions per second between ions and moving electrons to increase. So rate of flow of electrons decreases, causing resistance to increase.
