Page 350 - Physics Coursebook 2015 (A level)

338
Cambridge International A Level Physics
WORKED EXAMPLE (continued)
B
C
The specific latent heat of a substance is the energy required per kilogram of the substance to change its state without any change in temperature.
Step2 DividebothsidesbyatimeintervalΔt: E Δθ
13
A block of paraffin wax was heated gently, at a steady rate. Heating was continued after the wax had completely melted. The graph of Figure 21.15 shows how the material’s temperature varied during the experiment.
Δt=mc Δt
The quantity Δt is the rate at which energy is supplied,
E
i.e. the power P of the heater. The quantity Δθ is the rate
a b
c
For each section of the graph (AB, BC and CD), describe the state of the material.
For each section, explain whether the material’s internal energy was increasing, decreasing or remaining constant.
Consider the two sloping sections of the graph. State whether the material’s specific heat capacity is greater when it is a solid or when it is a liquid. Justify your answer.
70 D 60
50
40A
Δt
of rise of temperature of the block, i.e. the gradient of
the graph of θ against t. Hence: P = m × c × gradient
Step3 Calculatethepoweroftheheaterandthe gradient of the graph.
power = p.d. × current
P = VI = 12 × 4.17 ≈ 50 W
gradient = Δθ = 16.4 = 0.041 °C s−1 Δt 400
Step 4 Substitute values, rearrange and solve. 50=1.00 × c × 0.041
QUESTION
c = 50 1.00 × 0.041
QUESTIONS
= 1220Jkg−1K−1
30 20 0
10 At higher temperature, the graph shown in Figure 21.14 deviates increasingly from a straight line. Suggest an explanation for this.
11 In measurements of the specific heat capacity of a metal, energy losses to the surroundings are
a source of error. Is this a systematic error or a random error? Justify your answer.
12 In an experiment to measure the specific heat capacity of water, a student used an electrical heater to heat some water. His results are shown below. Calculate a value for the heat capacity of water. Comment on any likely sources of error.
Temperature variation of a sample of wax, heated at a constant rate.
Specific latent heat
Energy must be supplied to melt or boil a substance. (In this case, there is no temperature rise to consider since the temperature stays constant during a change of state.) This energy is called latent heat.
When a substance melts, this quantity is called the specific latent heat of fusion; for boiling, it is the specific latent heat of vaporisation.
To calculate the amount of energy E required to melt or vaporise a mass m of a substance, we simply need to know its specific latent heat L:
mass of beaker massofbeaker+water current in the heater p.d. across the heater initial temperature final temperature
time taken
= 150 g
= 672g
= 3.9 A
= 11.4 V
= 18.5 °C = 30.2 °C = 13.0 min
Figure 21.15
E = mL
0 5 1015202530 Time / min
Temperature / °C