phy2 2003

6732 Unit Test PHY2

1. Resistance of lamps

V2/R OR I = 60/12 = (5 A) 1

R=(12 V x 12 V) /60 W 1

R =2.4ohm 1 total 3

Resistance variation

Lamp A: resistance of A decreases with current increase 1

Lamp B: resistance of B increases with current increase 1

Dim filament

Lamps are dim because p.d. across each bulb is less than 12 V 1

Why filament of lamp A is brighter

Bulbs have the same current 1

p.d. across A > p.d. across B/resistance A> Resistance B 1

OR

power in A > power in B 2

total 8

2 Table

Physical Quantity - Typical value

Resistance of a voltmeter -10 M ohm

Internal resistance of a car battery - 0.05 ohm

Internal resistance of an EHT supply -10 M ohm

Resistivity of an insulator - 2.0 x 10¹⁵ ohm m

Drift velocity of electrons in a metallic conductor - 0.3 mm s^-1

Temperature of a working filament bulb - 3000 K

[Mark is lost if 2 or more values are put into one box] total 6

3. Current in heating element

P= VI 1

I = 500 W / 230 V 1

I = 2.2 A 1

Or

P=V²/R 1

R= 230 x 230/105.8 1

I = 2.2A 1

Drift velocity

Drift velocity greater in the thinner wire 1

Explanation

Quality of written communication. 1

See I = nAQv 1

is the same (at all points) 1

(probably) n (and Q) is the same in both wires 1 total 8

4 Resistance of films

R= pl/A 1

R = pl/wt or A = wt 1

Resistance calculation

R = (6.0 x 10^-5) (8 x 10^-3 m) / (3 x 10^-3m) (0.001 x 10^-3 m) 1+1

(Correct substitution but values in mm 1)

R= 160 ohm 1 ecf if in mm

Resistance of square film

L=w 1

R= pl/lt=p/t 1

R= pw/wt=p/t 1

Total 7

5. Definition of specific heat capacity

energy (needed) 1

(per) unit mass/kg and per unit temperature change K or C 1

OR

Correct formula [does not need to be rearranged] 1

with correctly defined symbols 1

Circuit diagrams (see end)

Accept voltmeter across heater and ammeter as well as voltmeter across heater only

Means of varying p.d./current 1

Voltmeter in parallel with a resistor symbol 1

Ammeter in series with any representionof a heater. 1

Other apparatus

(Top pan) balance / scales 1

Stopwatch / timer / clock 1

Explanation

Energy/heat loss to surroundings/air/bench 1

OR

Mc delta T +delta,Q = VIt or equivalent in words (e.g. student ignores energy loss in calculations) 1

Modifications

Any two from

Use of insulation around block

Ensure all of heater is within block

Grease heater/thermometer

Total 10

Specific Heat Capacity Calculation

C = delta Q/ m delta T = (860 x 10³)/ (1.4 kg) (750 – 22) = 844 (J kg ^–1 K^-1)

Conversion of kJ to J 1

Subtraction of temp 1

Answer 1

Energy transfers diagram

Label 2 (energy to) (warm) water (and trough) 1

Label 3 (energy used to) evaporate water / cause evaporation/latent heat/change of state 1

Total 5

Gas equation

PV = nRT [Accept symbols or words] 1

Molar gas constant unit

R= PV/nT

P – kgm^-1 s 1

V – m³ T– K n – mol all three for 1

Kinetic energy of molecule

Nm =M

density = Nm²>/3V correctly combined the 2 equations 1

Nm²> = 3nRT density = any mass –: volume 1

Show that

Kinetic energy =m²>/2=(3/2)n(RT/N) 1

Sketch graph

PV on y axis Temperature/’C on x axis

[accept axes reversed and correct graph] 1

Straight line graph with negative intercept 1

Gradient R 1

Intercept at – 273 ’C 1

[All these marks can be scored on graph)

total 10

8. Definition of e.m.f. of a cell

work/energy (conversion) per unit charge 1

for the whole circuit / refer to total energy 1

OR

Work/energy per unit charge 1

converted from chemical to electrical (energy) 1

OR

E = W/Q for whole circuit 1

All symbols defined 1

OR

E = P/I for whole circuit 1

All symbols defined 1

[Terminal p.d, when no current drawn scores 1 mark only] 2 max

Circuit diagram

See top

R 1

A in series 1

R (can be variable) 1 A and V correct 1 V as shown

Or across R+ A

Or across battery

[2nd mark is consequent on R(fixed, variable )or lamp]

Sketch graph

See top

Graph correctly drawn with axes appropriately labelled and consistent with the circuit drawn 1

Intercept on R axes equivalent to( – )r and Gradient equivalent to ( – )r [Gradient mark consequent on graph mark] 1

[Gradient may be indicated on graph]

Grade boundaries

A 41 B 37 C 33 D 26 E 25