3 Electricity Flashcards
Equation linking charge current and time (learn)
Q=It
Charge= current x time
What is one coulomb defined as
The amount of charge that passes in 1 second when the current is 1 amp
How do you attach an ammeter to a circuit?
In series
Equation for potential difference work done and charge
V= W/Q
Potential difference = work done/ charge
What is the maximum value that an ammeter or voltmeter called?
The full scale deflection
How should a voltmeter be used?
- In parallel
* very large resistance
Does potential difference split or stay the same in parallel circuits?
Stay the same
Charge of an electron
-1.6x10^-19
What equation gives the mean drift velocity of electrons
I=nAve
Current=number density of charge carriers x cross sectional area x mean drift velocity x charge on each charge carrier
Why is drift velocity in semi conductors higher?
They have fewer charge carriers so the drift velocity needs to be higher to produce the same current
What is resistance?
A measure of how difficult it is to get a current to flow through it
Equation linking resistance voltage and current
V=IR
What things determine resistance?
Length- longer the wire, higher the resistance
Area- the wider the wire, the lower the resistance
Resistivity- depends on material and temperature
Equation for resistivity
R=pl/A
Resistance=resistivity x length/area
How can you determine the resistivity of a wire?
1) use a micrometer to measure diameter of wire in 3 different places
2) use diameter to calculate cross sectional area
3) the test wire should be clamped to a ruler and connected to a circuit using crocodile clips
4) record the length of wire connected in the circuit, the voltmeter readings and ammeter readings
5) use V=IR to calculate resistance for each length
6) plot graph of resistance vs length
7) multiply gradient by cross sectional area to get resistivity
Describe the circuit should you set up to measure current and voltage across a component
Power supply to variable resistor to component with voltmeter in parallel to ammeter
IV characteristics of a metallic conductor
- ohmic device
- directly proportional with constant resistance
- gradient=1/resistance
IV characteristics of a tungsten filament wire
- gradient starts steep but gets shallower as voltage increases
- resistance increases with temp
How does temp affect resistivity?
1) charge is carried through metals by free electrons in a lattice of +ve ions
2) heating up a metal makes the ions vibrate more, meaning the electrons collide with them more, transferring KE into other forms
3) therefore the electrons drift velocity decreases, so current decreases and resistance increases
Why do semiconductors have higher resistivity than metals?
They have fewer charge carriers
When happens to a semi conductor when it is heated
More charge carriers are released so current increases and their resistance and resistivity decreases
How are semi conductors used as sensors?
When temp increases their resistance and resistivity decrease while their current increases
What are three types of semi conductors
Thermistors diodes and LDR’s
What happens to resistance in a NTC thermistor as temp increases
Resistance decreases
IV characteristics for a thermistor
- gradient starts shallow but becomes steeper as voltage increases
- gets steeper because it warms up so resistance decreases
Why does resistance in thermistors decrease when temp increases?
- more heat means more energy to electrons
- this allows them to escape from their atoms
- this means there are more charge carriers available
- so current increases and resistance decreases
How does light affect an LDR?
The greater the light the lower it’s resistance
How does light affect resistance in an LDR?
- light provides energy to electrons
- this allows them to escape from their atoms
- this means there are more charge carriers
- so higher current and lower resistance
IV characteristics of a diode
- starts shallow gradient
- as pd increases current shoots up to give a very low resistance
- only allows current to flow in one direction
Equation linking power voltage and current
P=VI
Equation for power current and resistance
P=I^2R
Equation linking energy voltage current and time
E=VIt
Load resistance
The total resistance of all the components in the external circuit
Equation linking emf charge and energy (learn)
EMF=energy/charge
What is the emf
The amount of work the battery does to each coulomb if charge
Equation linking emf pd current and resistance (learn)
Emf=V +Ir
How can you work out the total emfs of cells in series?
Emf total =emf1 +emf2 +emf3
What is true of the emf of cells in parallel
For identical cells the emf total= emf1=emf2
How can you investigate internal resistance and emf with a circuit?
- set up a circuit with a battery an ammeter and a variable resistor with a voltmeter across it
- vary the current in the circuit using the variable resistor
- measure the pd for several different values of current
- plot v against I
- the intercept of the y axis is emf
- the gradient is -r
Kirchhoffs first law
Current into junction= current out of junction
Kirchhoff second law
The total emf around a series circuit= the sum of the pds across each component
Characteristics of a series circuit
1) same current at all points
2) emf split between components so emf=V1+V2+V3
3) because I is constant R total=R1+ R2+ R3
Characteristics of parallel circuits
1) current is split at each junction
2) same pd across all components
3) 1/R total= 1/R1+ 1/R2 1/R3
How do potential dividers work?
Using two resistors in series to divide the voltage by sharing it between them
How is pd split in a series circuit with two resistors
Split in the ratio of the resistances
E.g if R1 is 2ohms and R2 is 3 ohms, R1 get 2/5 of the pd across it and R2 get 3/5 of the pd across it
Variations in resistance in metals
- T increases = drift velocity decreases
- therefore I decreases and R increases
- positive temperature coefficient of resistivity
What is the relationship of resistance in and out of a junction? (learn)
1/R=1/R1+1/R2+1/R3
What does current do in a series circuit?
Stays constant
How should an ammeter be used?
- in series
* very small resistance
Equation linking power voltage and resistance?
P=v^2/R
Power= voltage squared divided by resistance
Ohms law
Current in a metallic conductor at a constant temperature is directly proportional to the potential difference (resistance is the same at all points)
The ratio of voltage across two resistors (learn)
V1/V2=R1/R2
Equation which can be used to find Vin and Vout in potential dividers (learn)
Vout/Vin = R2/(R1+R2)
What are potentiometers used for?
To obtain a continuously variable output voltage
How can potentiometers be set up?
Thermistors and LDRs can be used, or varying the length of the wire which gives Vout
Equation for Vout in simple potentiometer (learn)
Vout= L2/L1 x Vin
Variations in resistance in semiconductors
- T increases= small decrease in drift velocity and large increase in number of charge carriers
- therefore I increases and R decreases
- negative temperature coefficient of resistivity
Variations in resistance in LDRs
- Light increases=number of charge carriers increases a lot and drift velocity decreases slightly
- therefore current increases and resistance decreases
- negative temperature coefficient of resistivity
Explain why the resistance of a negative temperature coefficient thermistor changes as p.d increases
- more energy to lattice ions
- more charge carriers released
- I=nAve
- R=V/I so if I increases R will decrease
Explain why the temperature of the filament in the increases when a p.d is applied
- increased lattice ion vibrations
- p.d causes a current
- electrons have more frequent collisions
- more energy is transferred when electrons collide with lattice ions
What equation is used to work out the total resistance of two resistors in parallel?
1/R + 1/R = 1/Rtotal
If 3 cells are charging is it quicker to charge them in series or parallel?
Series because current remains the same through all of the cells but splits in parallel. This means the value of W= IVT increases
