Concept explainers
An office worker uses an immersion heater to warm 250 g of water in a light, covered, insulated cup from 20.°C to 100. °C in 4.00 minutes. The heater is a Nichrome resistance wire connected to a 120-V power supply. Assume the wire is at 100. °C throughout the 4.00-min time interval. (a) Calculate the average power required to warm the water to 100. °C in 4.00 min. (b) Calculate the required resistance in the heating element at 100. °C. (c) Calculate the resistance of the heating element at 20. °C. (d) Derive a relationship between the diameter of the wire, the resistivity at 20. °C, ρ0, the resistance at 20. °C, R0, and the length L. (e) If L = 3.00 m, what is the diameter of the wire?
(a)

Answer to Problem 48P
Explanation of Solution
Given Info: 250.0 g of water is heated from 20.0 °C to 100.0 °C in 4 minutes. The heater with nichrome resistor is connected to 120 V supply, 250.0 g of water is heated from 20.0 °C to 100.0 °C in 4 minutes. The heater with nichrome resistor is connected to 120 V supply
Explanation:
Formula to calculate the energy required to warm the water is,
E=mc(T2−T1)
-
- E is the heat energy required to warm the water from temperature T1 to T2,
- m is the mass of water,
- c is the specific heat capacity of water,
Substitute 250.0 g for m, 4186 J(kg°C)−1 for c, 100.0 °C for T2 and 20.0 °C for T1 in the above equation to find E
E=(250.0 g)(1 kg1000 g)(4186 J(kg°C)-1)(100.0 °C−20.0 °C)=83720 J(1 kJ1000 J)=83.72 kJ
The heat energy required to warm the water from 20.0 °C to 100.0 °C in 4 minutes is 83.72 kJ
Formula to calculate the average power required to warm the water is,
P=EΔt
-
- P is the power required,
- Δt is the time taken for warming the water,
Substitute 83.72 kJ for E and 4 minutes for Δt in the above equation to find P,
P=83.72 kJ(103 J1 kJ)(4 minutes)(60 s1 minute)=348.83 W
Conclusion: Therefore, the average power required to warm the water is 348.83 W
(b)

Answer to Problem 48P
Explanation of Solution
Given Info: 250.0 g of water is heated from 20.0 °C to 100.0 °C in 4 minutes. The heater with nichrome resistor is connected to 120 V supply
Explanation:
Formula to calculate the resistance is,
R=(ΔV)2P
-
- R is the resistance of nichrome at 100.0 °C
- ΔV is the voltage of power supply,
Substitute 120 V for ΔV and 348.83 W for P in the above equation to find R,
R=(120 V)2348.83 W=41.28 Ω
Conclusion: Therefore, the resistance of nichrome at 100.0 °C is 41.28 Ω
(c)

Answer to Problem 48P
Explanation of Solution
Given Info: 250.0 g of water is heated from 20.0 °C to 100.0 °C in 4 minutes. The heater with nichrome resistor is connected to 120 V supply.
Explanation:
Formula to calculate the resistance is,
R0=R1+α(T−T0)
-
- R0 is the resistance at T0,
- α is the temperature coefficient of resistivity of nichrome,
- T is the final temperature,
Substitute 41.28 Ω for R, 0.4×10−3 (°C)-1 for α, 100.0 °C for T and 20.0 °C for T0 in the above equation to find R
R0=41.28 Ω(1+(0.4×10−3 (°C)-1)(100.0 °C−20.0 °C))=40 Ω
Conclusion: Therefore, the resistance of nichrome at 20.0 °C is 40 Ω
(d)

Answer to Problem 48P
Explanation of Solution
Given Info: 250.0 g of water is heated from 20.0 °C to 100.0 °C in 4 minutes. The heater with nichrome resistor is connected to 120 V supply.
Explanation:
Formula to calculate the resistance at 20.0 °C is,
R0=ρ0LA
-
- ρ0 is the resistivity of nichrome wire at 20.0 °C,
- L is the length of nichrome wire,
- A is the area of circular cross section of nichrome wire,
Equation to calculate the area of cross section is ,
A=πd24
-
- d is the diameter of wire,
Substitute the above equation in the previous equation to rewrite R0
R0=4ρ0Lπd2
Rewrite the above equation in terms of d
d=2√ρ0LπR0
Is the equation which relates diameter resistance, resistivity and length of wire,
Conclusion: The equation which relates diameter resistance, resistivity and length of wire is d=2√ρ0LπR0
(e)

Answer to Problem 48P
Explanation of Solution
Given Info: 250.0 g of water is heated from 20.0 °C to 100.0 °C in 4 minutes. The heater with nichrome resistor is connected to 120 V supply. The length of nichrome wire is 3.00 m
Explanation:
Formula to calculate the length of nichrome wire is,
d=2√ρ0LπR0
Substitute 150×10−8 Ω⋅m for ρ0, 3.00 m for L, 3.14 for π and 40 Ω for R0 in the above equation to find d
d=2√(150×10−8 Ω⋅m)(3.00 m)(3.14)(40 Ω)=3.78×10−4 m
Conclusion: Therefore, the diameter of wire is 3.78×10−4 m
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