Electricity Flashcards
(27 cards)
What is Charge (Q) ?
No definition
- Coulombs (C)
- conserved + quantized
- cannot be created or destroyed
What is Current (I) ?
is the rate of change (flow) of charge
- Amperes (A) - CS^-1
- flow from the = +ve to -ve
- ammeter - series
What is Potential Difference (V) ?
is the work-done per coulomb of charge
- Volts (V) - JC^-1
- voltmeter - parallel
What is Resistance (R) ?
the ratio of potential difference to the current
- Ohms (Ω) - VA^-1
- resistance increase cause current to decrease
What is the Ohms law ?
that V ∝ I
- conductors that follow law are called ohmic conductors
What are the I-V characteristics of a graph ?
- the gradient represents the resistance
- the graph can be drawn V-I
What are the I-V characteristics of a conductor (resistor) ?
GRAPH = linear line though origin (V ∝ I)
- Ohmic conductor
- increase current = increase electron flow per second = particles vibrate more as they collide more = temperature increase = can’t flow easily = resistance increase
What are the I-V characteristics of a Filament Lamp ?
GRAPH = snake
- Non-Ohmic conductor
- increase current = temperature increase = resistance increase
What are the I-V characteristics of a Diode ?
GRAPH = straight line then curve up at 0.7
- current flows in on direction
- Non-Ohmic conductor
- V<0 = Large , V=0.0.7 = some , V>0.7 = Little
What is a Semi-Conductor ?
INCREASE TEMPERATURE = DECREASE RESISTANCE
- electrons energy to escape atom - more charged carries
- e.g. therimistor, diode and LDR’s (Silicon)
- therimistor depend on resistance - NTC negative temperature coefficient - graph R/T - negative curve
What is a Super Conductor ?
DECREASE TEMPERATURE = DECREASE RESISTANCE
- R = Zero
- no energy released when reach CRITICAL TEMPERATURE
- no energy waste (P=I^2R)
- tricky to get to critical temperature - expensive
- uses include MRI scanner as magnetic fields
What are the equations for current, potential difference and resistance ?
1) I = ΔQ/Δt
2) V = w/Q
3) R = V/I
What are the equations for Power (P) ?
1) P = VI
2) P = I^2R
3) P = V^2/R
What is Power (P) ?
is the rate of change of energy or the work-done per second
(energy transferred = work-done)
- Watts (w) - JS^-1
What are the equations for Energy (E) ?
1) E = PT
- E =VIT
- E = I^2RT
- E = V^2T/R
What is Resistivity (p) ?
tells us how difficult for a current to flow through a material
- Measured in Ωm^-1
p = RA/L
(A = cross sectional area)
What is EMF (electromotive force) ?
the work-done per coulomb of charge or the potential difference of a supply when no current is being drawn
- Volts (V)
- EMF = a energy -> electrical energy
How to calculate EMF ?
EMF = E/Q
Energy/Charge
What is internal resistance (r) ?
the electrical resistance of the supply (battery)
- internal resistance causes ‘lost volts’ - minimised by current small or batteries in parallel
Whats always conserved in circuits ?
Charge and energy
Rules of Series Circuit ?
- current same in all components
- total resistance = R1+R2+R3….
- V1 = IR1
How to calculate internal resistance ?
EMF = I(r+R)
- Graph gradient = resistance and y-intercept = EMF
Rules of Parallel Circuit ?
- potential difference is the same
- most current flow i n branch with least resistance
total resistance = 1/RT=1/R1+1/R2… - RT = R/N
- RT = R1R2/R1+R2
What is Kirchhoff’s first law ?
total current entering junction = total current leaving it
