Current Electricity Flashcards
Define Current
The rate of flow of charge (must be word for word)
Define Potential Difference
The energy transferred between 2 points in a circuit, per unit of charge
How does a potential difference (p.d) incur current flow?
P.d. causes charge to move, thereby causing a current flow
What is the equation for current? Include units of measurement
I (current/Amperes) = ΔQ (change in charge/Coulombs) / Δt (change in time/Seconds)
What is the equation for p.d.? Include units of measurement
V (p.d./Volts) = W (energy/Joules) / Q (charge/Coulombs)
Define charge in terms of base units
ΔQ=IΔt, so 1 Coulomb is 1 Amp(ere)-second
What are the approximate resistances of an Ammeter and a Voltmeter?
Ammeter-Negligible
Voltmeter-Infinite
What direction does conventional current flow in?
What about electron flow?
Conventional current-> +ve to -ve
Electron flow-> -ve to +ve
State both of Kirchoff’s Laws and why they are the case
Law 1: The sum of current entering a point in a circuit is equal to the sum of current leaving that point. (A consequence of the conservation of charge)
Law 2: In any closed loop in a circuit, the sum of the e.m.f. (p.d. of the battery) is equal to the sum of the p.d. around that loop. (A consequence of the conservation of energy)
What are the rules for current, p.d. and resistance in series?
Current-It is the same throughout the circuit
P.d.-It sums to the e.m.f.
Resistance-R(total)=R(1)+R(2)+R(3) etc.
What are the rules for current, p.d. and resistance in parallel?
Current-It splits up I(total in)=I(total out)
P.d.-Is the same throughout the circuit
Resistance-1/R(total)=1/R(1)+1/R(2)+1/R(3) etc.
How do you derive the resistance formula for series?
V(total)=V(1)+V(2)+V(3) V=IR so…
IR(total)=IR(1)+IR(2)+IR(3). We can divide by I to give…
R(total)=R(1)+R(2)+R(3)
How do you derive the resistance formula for parallel?
I(total)=I(1)+I(2)+I(3) V=IR so…
V/R(total)=V/R(1)+V/R(2)+V/R(3). We can divide by V to give…
1/R(total)=1/R(1)+1/R(2)+1/R(3)
Explain why resistance increases with temperature (model answer)
Resistance is due to collisions between electrons and ions. As temperature increases, ions gain more energy and so vibrate through larger amplitudes, thereby increasing the frequency of collisions; as a result resistance is increased.
Is a resistor a conductor?
Yes, because it allows current to flow through it despite limiting current flow
What factors affect resistance?
Length
Cross-sectional Area
Temperature
Material
What is the equation for resistance? Include units of measurement
R (resistance/Ohms) = ϱ (resistivity/Ohm-metres) x L(length/Metres) / A(cross-sectional area/Metres squared)
Define Drift Velocity?
The average velocity of charge carriers through a conductor
What is the equation for current which involves drift velocity? Include units of measurement
I (current/Amps) = v (drift velocity/Metres per Second) A (area/Metres Squared) n (charge carrier density/m^-3) q (charge per carrier/Coulombs)
If a material has a larger value for charge carrier density, what does this mean?
It is a better conductor
What is a potential divider?
A component that allows voltage across it to be varied to give any voltage between the emf and 0
What is a thermistor?
A component which indicates temperature by the value of its resistance
What is a negative temperature coefficient thermistor (ntc thermistor) and how does it work?
A thermistor wherein resistance decreases as temperature increases. This is because the energy increase increases collisions, releasing electrons from the lattice ions and thereby increasing the charge carrier density.
Define internal resistance
The resistance of the cell itself
Define terminal p.d.
The p.d. between cell terminals
Define e.m.f.
Electromotive force - The energy gained per coulomb of charge passing through the supply
How do you calculate the voltage output of a potential divider circuit?
Rout/Rtotal = Vout/Vtotal or l(Lengthout)/L(Lengthtotal) = Vout/Vtotal
