Current Electricity

Question 1

Define Current

Answer 1

The rate of flow of charge (must be word for word)

Question 2

Define Potential Difference

Answer 2

The energy transferred between 2 points in a circuit, per unit of charge

Question 3

How does a potential difference (p.d) incur current flow?

Answer 3

P.d. causes charge to move, thereby causing a current flow

Question 4

What is the equation for current? Include units of measurement

Answer 4

I (current/Amperes) = ΔQ (change in charge/Coulombs) / Δt (change in time/Seconds)

Question 5

What is the equation for p.d.? Include units of measurement

Answer 5

V (p.d./Volts) = W (energy/Joules) / Q (charge/Coulombs)

Question 6

Define charge in terms of base units

Answer 6

ΔQ=IΔt, so 1 Coulomb is 1 Amp(ere)-second

Question 7

What are the approximate resistances of an Ammeter and a Voltmeter?

Answer 7

Ammeter-Negligible

Voltmeter-Infinite

Question 8

What direction does conventional current flow in?

What about electron flow?

Answer 8

Conventional current-> +ve to -ve

Electron flow-> -ve to +ve

Question 9

State both of Kirchoff’s Laws and why they are the case

Answer 9

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)

Question 10

What are the rules for current, p.d. and resistance in series?

Answer 10

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.

Question 11

What are the rules for current, p.d. and resistance in parallel?

Answer 11

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.

Question 12

How do you derive the resistance formula for series?

Answer 12

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)

Question 13

How do you derive the resistance formula for parallel?

Answer 13

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)

Question 14

Explain why resistance increases with temperature (model answer)

Answer 14

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.

Question 15

Is a resistor a conductor?

Answer 15

Yes, because it allows current to flow through it despite limiting current flow

Question 16

What factors affect resistance?

Answer 16

Length
Cross-sectional Area
Temperature
Material

Question 17

What is the equation for resistance? Include units of measurement

Answer 17

R (resistance/Ohms) = ϱ (resistivity/Ohm-metres) x L(length/Metres) / A(cross-sectional area/Metres squared)

Question 18

Define Drift Velocity?

Answer 18

The average velocity of charge carriers through a conductor

Question 19

What is the equation for current which involves drift velocity? Include units of measurement

Answer 19

I (current/Amps) = v (drift velocity/Metres per Second) A (area/Metres Squared) n (charge carrier density/m^-3) q (charge per carrier/Coulombs)

Question 20

If a material has a larger value for charge carrier density, what does this mean?

Answer 20

It is a better conductor

Question 21

What is a potential divider?

Answer 21

A component that allows voltage across it to be varied to give any voltage between the emf and 0

Question 22

What is a thermistor?

Answer 22

A component which indicates temperature by the value of its resistance

Question 23

What is a negative temperature coefficient thermistor (ntc thermistor) and how does it work?

Answer 23

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.

Question 24

Define internal resistance

Answer 24

The resistance of the cell itself

Question 25

Define terminal p.d.

Answer 25

The p.d. between cell terminals

Question 26

Define e.m.f.

Answer 26

Electromotive force - The energy gained per coulomb of charge passing through the supply

Question 27

How do you calculate the voltage output of a potential divider circuit?

Answer 27

Rout/Rtotal = Vout/Vtotal or
l(Lengthout)/L(Lengthtotal) = Vout/Vtotal