Slightly stuck on one of the parts on two questions. Sorry if it's quite a lengthy post but I feel as if I should write down the whole two questions and the parts with it to make sense. I've also included my answers too and I've mentioned what I need help on.
Question 1. A circuit in a hair dryer consists of a small motor with a voltage of 12 V in series with a heating coil. Both are connected to a 240 V power supply. Current flowing through the motor is 0.6 A.
a) calculate the resistance of the motor.
b) What is the current in the heating coil? and calculate its resistance.
My answers:
a) I got 20 Ω
b) The current through the heating coil is 0.6 A (the same as the current going through the motor) because in series current flowing through one component is the same as the current going through the next component (correct me if I'm wrong).
For the resistance I got 380 Ω but I don't think that's right for some reason... I hope someone can verify this for me.
Question 2: The resistance of a heating element in 450 000 Ω and it's connected to a main supply of 240 V.
a) what's the current flowing through it?
b) how many coulombs of charge pass through the heating element in 1 minute?
c) how much energy is transferred to each coulomb of charge?
d) how much heat energy is produced by the coil in 1 minute?
My answers:
a) I got 0.000533 A
b) I'm not sure if this is right but I got 0.032 C
c) I'm not sure this is right either but I got 7.68 J.
d) I'm stuck on this and I have no idea what to do :/

minibuttons
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 13022014 20:58

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 13022014 21:33
(Original post by minibuttons)
Slightly stuck on one of the parts on two questions. Sorry if it's quite a lengthy post but I feel as if I should write down the whole two questions and the parts with it to make sense. I've also included my answers too and I've mentioned what I need help on.
Question 1. A circuit in a hair dryer consists of a small motor with a voltage of 12 V in series with a heating coil. Both are connected to a 240 V power supply. Current flowing through the motor is 0.6 A.
a) calculate the resistance of the motor.
b) What is the current in the heating coil? and calculate its resistance.
My answers:
a) I got 20 Ω
b) The current through the heating coil is 0.6 A (the same as the current going through the motor) because in series current flowing through one component is the same as the current going through the next component (correct me if I'm wrong).
For the resistance I got 380 Ω but I don't think that's right for some reason... I hope someone can verify this for me.
Question 2: The resistance of a heating element in 450 000 Ω and it's connected to a main supply of 240 V.
a) what's the current flowing through it?
b) how many coulombs of charge pass through the heating element in 1 minute?
c) how much energy is transferred to each coulomb of charge?
d) how much heat energy is produced by the coil in 1 minute?
My answers:
a) I got 0.000533 A
b) I'm not sure if this is right but I got 0.032 C
c) I'm not sure this is right either but I got 7.68 J.
d) I'm stuck on this and I have no idea what to do :/
On Q2, provided your answer for part a is correct I got the same for part b.
I got a different answer to part c. It says charge per coulomb so I used Work done = Voltage x Charge. It states per coulomb so if it's 1 coulomb and voltage is constant then 240 x 1. However someone might want to clarify my logic on this because it seems too obvious to me and your answer seems more like an answer they'd want for a calculation.
For part d I think you'd need to think of the definition for potential difference, 'energy transfer per unit charge when electrical energy is converted into another form of energy'. Assuming the coil is 100% efficient (unlikely but the only way you can do it) you can use the same work done equation that is above to work out the energy, coincidently (or maybe not ) they asked you to find the charge passing through the heating element in 1 minute.
I hope this helps 
minibuttons
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 14022014 18:21
(Original post by k4l397)
I agree with you on question 1 for both parts except I'm not 100% sure on part b. As it's a heating coil I'd assume it would be nonohmic because it heats up, however unless they gave you a graph or means of working it out I guess the only possible answer is 380.
On Q2, provided your answer for part a is correct I got the same for part b.
I got a different answer to part c. It says charge per coulomb so I used Work done = Voltage x Charge. It states per coulomb so if it's 1 coulomb and voltage is constant then 240 x 1. However someone might want to clarify my logic on this because it seems too obvious to me and your answer seems more like an answer they'd want for a calculation.
For part d I think you'd need to think of the definition for potential difference, 'energy transfer per unit charge when electrical energy is converted into another form of energy'. Assuming the coil is 100% efficient (unlikely but the only way you can do it) you can use the same work done equation that is above to work out the energy, coincidently (or maybe not ) they asked you to find the charge passing through the heating element in 1 minute.
I hope this helpsLast edited by minibuttons; 14022014 at 18:27. 
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 14022014 20:09
(Original post by minibuttons)
for 1b) I worked out the pd across the resistance so I did 240  12 = 228V and then with that I worked out the resistance to be 228/0.6 = 380 Ω. It says on the question to use my explanation earlier (where I justified why the current in the heating coil is 0.6 A)  I think this has got to do with Kirchoff's law with the current being the same at every point in the series circuit. 
uberteknik
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 15022014 17:37
Spoiler:ShowR = V/I
R = 12/0.6
R = 20 ohms.
(Original post by minibuttons)
Q1 b) The current through the heating coil is 0.6 A (the same as the current going through the motor) because in series current flowing through one component is the same as the current going through the next component (correct me if I'm wrong).
(Original post by minibuttons)
For the resistance I got 380 Ω but I don't think that's right for some reason... I hope someone can verify this for me.
Spoiler:ShowV_{supply} = V_{motor} + V_{heater}
V_{heater} = V_{supply}  V_{motor}
V_{heater} = 240  12
V_{heater} = 228 Volts.
R_{heater} = V_{heater}/I_{heater}
R_{heater} = 228/0.6
R_{heater }= 380 ohms.
(Original post by minibuttons)
Question 2: The resistance of a heating element in 450 000 Ω and it's connected to a main supply of 240 V.
a) what's the current flowing through it?
b) how many coulombs of charge pass through the heating element in 1 minute?
c) how much energy is transferred to each coulomb of charge?
d) how much heat energy is produced by the coil in 1 minute?
Spoiler:ShowI = V/R
I = 240/450x10^{3}
I = 0.533x10^{3} A or 0.533 mA.
(Original post by minibuttons)
b) I'm not sure if this is right but I got 0.032 C
Spoiler:ShowQ = I x t
Q = 0.533x10^{3} x 60
Q = 0.032 Coulombs
(Original post by minibuttons)
c) I'm not sure this is right either but I got 7.68 J.
Voltage is defined as Joules per Coulomb of charge.
Spoiler:ShowV = E/Q
E = V x Q and since Q = 1 Coulomb by defintion
E = 240 x 1
E = 240 Joules.
i.e. 240 Volts means 240 Joules of energy in every Coulomb of charge.
This is a very important definition.
(Original post by minibuttons)
d) How much heat energy is produced in the coil in 1 minute? I'm stuck on this and I have no idea what to do :/
You are given the time. You need to find the power developed by the heating coil. You should know the equation relating voltage and current to power in an electrical circuit.
E = Power x time
E = Pt..................eq(1)
Spoiler:Showin an electrical circuit:
P = VI. Substituting for P in eq(1) above:
E = VIt
E = 240 x 0.533x10^{3} x 60
E = 7.68 Joules (2 d.p.)Last edited by uberteknik; 15022014 at 17:58. 
minibuttons
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 15022014 18:59
(Original post by uberteknik)
Incorrect.
Voltage is defined as Joules per Coulomb of charge.
Spoiler:ShowV = E/Q
E = V x Q and since Q = 1 Coulomb by defintion
E = 240 x 1
E = 240 Joules.
i.e. 240 Volts means 240 Joules of energy in every Coulomb of charge.
This is a very important definition.
By definition Joules = Watt seconds.
You are given the time. You need to find the power developed by the heating coil. You should know the equation relating voltage and current to power in an electrical circuit.
E = Power x time
E = Pt..................eq(1)
Spoiler:Showin an electrical circuit:
P = VI. Substituting for P in eq(1) above:
E = VIt
E = 240 x 0.533x10^{3} x 60
E = 7.68 Joules (2 d.p.) 
uberteknik
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 15022014 19:30
(Original post by minibuttons)
Thank you so much for helping me out on the last two questions
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