Mark Scheme for Electricity Questions

 

1.       (i)      Correct direction shown (anticlockwise)                                                           B1

(ii)     Direction in which positive charges / ions move / Direction / flow / current /
from positive to negative / Flow of (positive) charge from positive to
negative / Direction / flow opposite to electron flow                                          B1

(iii)    Q = It       (Allow any subject with or without delta notation)                             C1

I = 0.76/5.0 x 60                                                                                                     C1

current = 2.53 ´ 10–3 (A) » 2.5 ´ 10–3 (A)                                                         A1

(0.152 / 0.15 (A) scores 1/3)

[5]

 

  2.     (i)      n = I/Aev (1)
= 0.75 / (4.0 × 10–7 × 1.6 × 10–19 × 1.4 × 10–4) = 8.4 × 1028 (1)                             2

  (ii)   1 drift velocity = 4.7 × 10–5 m s–1 (1)

          2 drift velocity = 3.5 × 10–5 m s–1 (1)                                                                  2

[4]

 

  3.     (i)      Thermistor                                                                                                      B1

  (ii)   I1 = 51 (mA)                                                                                                   B1
I2 = 9 (mA)                                                                                                     B1
I3 = 29 (mA)                                                                                                   B1

[4]

 

  4.     (e.m.f. =) W/q / (e.m.f. =)     78/24                                                                                C1
(e.m.f. =) 3.25 » 3.3 (V)                                                                                            A1

[2]

 

  5.     (a)     Into the page                                                                                                   B1

  (b)   I=Q/t       (Allow other subject, with or without D)                                         C1
(charge =) 7800 ´ 0.23                                                                                    C1
1.794 ´ 103 » 1.8 ´ 103 (C)    (Ignore minus sign)                                            A1
(1.8 ´ 106 (C) scores 2/3)

  (c)    (number =) 1.79 x 10^-3/e           (Possible ecf)                                                      C1
(number =) 1.12 ´ 1022 » 1.1 ´ 1022                                                                 A1

[6]

 

 

Thermal Physics

 

1    a    Difference: Evaporation occurs at all temperatures, whereas boiling of pure water occurs
at a fixed temperature (100 °C at a pressure of 100 kPa).                                                     [1]

      Similarity: Both processes involve molecules escaping from the water surface.                    [1]

b    i     The molecules of water move faster.                                                                            [1]

ii    Internal energy = kinetic energy + potential energy

      The kinetic energy of the molecules increases because they move faster.                        [1]

      The potential energy does not change much because the separation between the
molecules remains the same.                                                                                        [1]

      Hence, the internal energy increases due to increase in the kinetic energy of the
molecules.                                                                                                                  [1]

c		Kinetic energy of the particles	Potential energy of the particles	Internal energy
	An aluminium block increasing its temperature from room temperature to 300 °C.	+	0	+
	Water boiling at 100 °C and changing into steam at 100 °C.	0	+	+
	Water at 0 °C changing into ice at –15 °C.	–	–	–

      One mark for each correct row.                                                                                         [3]

2    a    The temperature of the metal increases at a steady rate, therefore it must be heated at a
steady rate.                                                                                                                       [1]

b    E = mcΔθ                                                                                                                         [1]

      E = 800 × 10^–3 × 600 × (30 – 20)                                                                                       [1]

      E = 4.8 × 10³ J                                                                                                                  [1]

c    From the graph it takes 90 s for the temperature to change from 20 °C to 30 °C.                   [1]

      power =                                                                                                             [1]

      power = 53.3 W » 53 W                                                                                                    [1]

3    E = (VI)t                                                                                                                                [1]

E = 9.5 × 5.2 × (5.0 × 60) = 1.48 × 10⁴ J                                                                                  [1]

E = mcΔθ                                                                                                                               [1]

c =  =                                                                                                 [1]

c » 2.0 × 10³ J kg^–1 K^–1                                                                                                           [1]

 

 

Moles

 

Specific Heat Capacity (question with mug)

 

Specific Heat capacity of glycerol

 

 

Specific Heat Capacity

Data

Use the following for Specific Heat capacities

Substance	Specific Heat Capacity c	Units
Water	4200	J kg-1 K-1
Copper	380	J kg-1 K-1
Methylated Spirit	2500	J kg-1 K-1
Turpentine	1800	J kg-1 K-1

 

Notes

• The heat capacity of an object is the heat required to raise the all of the object’s temperature by 1K
• When heating a liquid you have to heat the vessel that it is in as well.
• When an object at high temperature is dropped into a liquid at low temperature there is a net heat flow from the high temperature object to the low temperature object until they are both at the same temperature.
• Unless you state “Ignoring heat losses” in each question you will lose a mark!

 

 

1 Find the quantity of heat needed to heat 0.50 kg of copper from 20^oC to 50 °C.              5700J

 

2 When 0.25 kg of ice are heated from -10 °C to -2 °C the heat supplied is 4200 J. Find the specific heat capacity of ice.             2100 Jkg^-1K^-1

 

3 A vessel of heat capacity 30 J K^-1 contains 0.30 kg of water at 10 °C. When heated by a Bunsen burner for 5 min the temperature rises to 90 °C. At what rate does the Bunsen supply heat energy?            344W

 

4 A piece of iron of mass 0.20 kg is heated to 100 °C and dropped into 0.15 kg of water at 20 °C. If the temperature of the mixture is 30 °C, find a value for the specific heat capacity of iron.           450 Jkg^-1K^-1

 

5 A piece of iron of mass 0.05 kg is heated in a flame and then quickly transferred to a calorimeter of mass 0.05 kg and specific heat capacity 360 J kg^-1 K^-1 containing 0.18 kg of water at 18 °C. If the temperature of the water rises to 43 °C, find the temperature of the flame. The specific heat capacity of the iron is 450 J kg^-1 K^-1.             903 ^oC

 

6 Which would result in the more serious burn if dropped onto your outstretched hand a white-hot spark from a firework, or a lump of red-hot iron? Explain your answer.

The white hot spark has a higher temperature than the lump of iron

The lump of iron has a greater mass than the white hot spark

They both have the same specific heat capacity.

Lump of iron has greater store of heat energy

The rate of heat transfer from the white hot spark will be greater for the spark than the lump of iron.

A greater quantity of heat will be transferred from the iron

Causing more serious skin damage.

 

0.20 kg of alcohol is contained in a vessel of heat capacity 80 J K-¹. When heated for 9 min by a 50 W immersion heater the temperature rises from 15 °C to 65 °C. Find the specific heat capacity of alcohol. 2300 Jkg^-1K^-1

7.     

When 1 kg of brine is heated by a 500 W immersion heater for 1 min its temperature rises from 15 °C to 25 °C. Find its specific heat capacity 3000 Jkg^-1K^-1

8.    Find the rise in temperature of 0.5 kg of water that is heated by, 1 kW immersion heater for 84 s. 40K

9.    A block of lead of mass 1 kg is heated for 56 s by a heater of power 50 W. The temperature rises from 18 °C to 38 °C. Find the specific the capacity of lead. 140 Jkg^-1K^-1

10. A copper vessel of mass 0.1 kg contains 0.1 kg of turpentine. If it is heated for 109 s by a heater of power 100 W, find the rise in temperature. 50K

11. An immersion heater heats 0.60 kg of water in a vessel of heat capacity     120 J K^-1 through 20 °C. If the heater is used for the same time with 0.48 kg of methylated spirit in the vessel, find the rise of tempera­ture. 40K