Electricty Flashcards
Give an equation and definition for current.
I = ΔQ/Δt. Current is the charge flow per unit time.
Give the equation and definition for p.d.
V = W/Q. Potential difference is the work done per unit charge.
Give the equation and definition for resistance.
R = V/I. Resistance is the ratio of potential difference across acomponent to the current flowing through it.
Give Kirchoff’s current law.
As charge must be conserved: the total current flowing into a junction is equal to the total current flowing out of a junction.
How does the current law apply to series circuits?
Same current at all points.
How does the current law apply to parallel circuits?
Current splits between the branches based on resistance.
I = V/R gives I ∝ 1/R
Give Kirchoff’s voltage law.
As energy must be conserved: for a closed loop the total voltage across the cells/batteries is equal to the total voltage across the components.
How does the voltage law apply to series circuits?
Voltage of the cell/battery is shared based on resistance of components.
V = IR gives V∝ R
How does the voltage law apply to parallel circuits?
Each branch gets the full voltage of the cell/battery.
Give the equation for total resistance in series.
RT = R1 + R2 + …
Give the equation for total resistance in parallel.
1/RT = 1/R1 +1/ R2 + …
Explain the shape of the ohmic conductor graphs.
Ohmic conductors obey Ohm’s law -> current is directly proportional to potential difference -> as the resistance is constant (provided that temperature remains constant) -> straight line through origin.
Explain the shape of the filament lamp graphs.
As p.d. increases -> current increases -> temperature of filament increases -> metal ions vibrate with greater amplitude -> more frequent collisions between electrons and ions -> resistance increases -> graph curves as V/I increases.
Explain the shape of the NTC thermistor graphs.
As p.d. increases -> current increases -> temperature increases -> energy of thermistor increases -> more charge carriers released -> resistance decreases -> graph curves as V/I decreases.
Explain the shape of the diode graphs.
For negative p.d. -> reverse bias -> very high resistance -> little or no current flows. For positive p.d. -> forwards bias -> no current flows until threshold voltage met -> above this, current increases as p.d increases.
Describe a circuit that can be used to investigate I-V characteristics.
Connect test component and a variable resistor in series with a power source. Connect ammeter and voltmeter. Vary resistance of variable resistor to vary V and I. Or use a potentiometer
Give the relationship between length of wire and resistance.
Resistance ∝ Length
Give the relationship between area of a wire and resistance.
Resistance ∝ 1/ Area
Give the equation and units for resistivity.
ρ = RA/ L Units = Ωm
Give the definition for resistivity.
Resistivity is the resistance of a material with a cross-sectional area of 1 m2 and a length of 1 m.
What happens to resistivity as temperature increases?
As temperature increases, resistivity increases.
How can you determine resistivity experimentally?
Change length of wire and measure I and V. Calculate R =V/I.
Plot a graph of resistance against length of wire.
Gradient = resistivity/area.
Describe what a superconductor is.
A superconductor is a material that has zero resistivity (and zero resistance) at or below a critical temperature.
Describe and explain the uses of superconductors.
High speed MagLev trains and MRI scanners -> as superconductors can be used to create very strong magnetic fields.
Electrical transmission cables -> zero resistance -> no energy wasted to surroundings -> more efficient.
Describe a limitation of using superconductors.
Critical temperature is very low -> cooling with liquid helium/nitrogen is expensive and impractical.
Total p.d. for cells in series?
The sum of the p.d. of the individual cells.
Total p.d. for identical cells in parallel?
Equal to the p.d. of one of the cells.
What is the definition for power?
Power is the work done/energy transferred per unit time.
Or: Power is the rate of work done/energy transfer.
Give the power equations from the data sheet.
P = VI = I2R = V2/R
How does a potential divider circuit work?
Two or more resistors in series, connected to a power source.
P.d. of power source is shared between resistors. V ∝ R.
How are potential divider circuits used in sensory circuits?
LDRs used in automatic lights.
Thermistors used in thermostats.
What is a potentiometer?
Long coil of wire with a moveable connection. Gives two resistors with varying resistance. Can be used to investigate I-V characteristics – allows V to be varied from 0 V to full voltage of cell/battery.
Give an equation and definition for e.m.f.
ε = E/Q. E.m.f. is the work done moving 1 C of charge through a cell.
Define internal resistance.
Resistance of a cell/battery caused by electrons colliding with metal ions and losing energy.
Give the equation linking e.m.f., terminal p.d., lost volts.
E.m.f = terminal p.d. + lost volts
Give the equation linking ε, I, R and r.
ε = I (R +r)
How can you investigate internal resistence experimentally?
Measure V and I for different values of R.
Plot graph of V against I. V = - Ir + ε
Gradient = -r
y-intercept = ε
