4.3 Electrical Circuits

Question 1

recall kirchoff’s second law

Answer 1

total emf around a series circuit = sum of p.d.s across each component

Question 2

explain what happens to current, p.d. and resistance in a series circuit

Answer 2

current is the same across each component
emf is split between each component
Rt=R1+R2+R3…

Question 3

explain what happens to current, p.d. and resistance in a parallel circuit

Answer 3

current is split at each junction It = I1+I2+I3…
p.d. is the same across each component
1/Rt=1/R1+1/R2+1/R3…

Question 4

explain internal resistance and provide an equation

Answer 4

all sources of emf have internal resistance
measured p.d. of power supply =/= emf (diff= lost volts)
E=I(R+r)

Question 5

explain a PAG to determine internal resistance

Answer 5

• connect cell in series w. ammeter & variable resistor
• connect voltmeter in parallel around cell
• vary resistance & record V and I
• plot graph of V against I
• V=E-Ir
• y-intercept = emf, neg gradient = internal res

Question 6

recall the equations for potential dividers

Answer 6

V1/V2 = R1/R2
Vout = (R2/(R1+R2))Vin

Question 7

describe and explain how to investigate potential dividers with a thermistor

Answer 7

• place the (waterproofed) thermistor in a beaker of ice water
• measure initial temp & record voltage across thermistor
• gently heat using bunsen burner & record temp & Vout at regular intervals over a suitable range
• plot a graph of voltage against temperature

temp ^ = Vout v
NTC: temp ^ = res v
res ^ = V v in series