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Isaac physics help needed urgently

Dreamer0827

A) The 8.0Ω resistance in circuit (C) is a loudspeaker (the battery represents the amplifier). The other resistor is replaced with a variable resistor which can take the values between 0Ω and 30Ω, and is used as a volume control. This volume control changes the voltage across the speaker.
What is the minimum possible voltage across the speaker?

B)A voltmeter with resistance 10kΩ is used to measure the voltage across the 4.0kΩ resistor in circuit (B). What would it read?

C) A thermistor has a resistance of 800Ω at a temperature of 16∘C. It is wired in series with a fixed resistor and a 9.0V battery. A high-resistance voltmeter is connected to give a `temperature' reading.
[Note: that for this thermistor the resistance decreases as the temperature increases]

If the voltmeter needs to read 3.0V when the temperature is 16∘C, what is the resistance of the fixed resistor to 2 significant figures?

Please please help me with these three questions thank you

(edited 4 years ago)

Reply 1

username3331778

the questions are useless without the diagrams they are referencing

Reply 2

Dreamer0827

Original post by Meowstic

the questions are useless without the diagrams they are referencing

Oh sorry I'll upload now

Reply 3

Dreamer0827

Original post by Meowstic

the questions are useless without the diagrams they are referencing

I added the diagrams Now

Reply 4

JoshuaHealey1

Original post by future-doctor <3

Hi, I’m a bit late to the convo but did you get an answer to C)? Any help is much appreciated

Reply 5

suets

Original post by JoshuaHealey1

Hi, I’m a bit late to the convo but did you get an answer to C)? Any help is much appreciated

So the voltmeter is across the fixed resistor and reads 3V, according to serie circuit rules and Kirchoffs 2nd Law - the voltage of cell = voltage of all components added up. So if voltage across fixed resistor is 3v then voltage across thermistor must be 9-3=6V. The use a bit of ratio skills:

6 :3
800 ohms: x ohms

800/6 = 133.3
3x133.3 = 400V

Reply 6

Elderfury843

Original post by JoshuaHealey1

Hi, I’m a bit late to the convo but did you get an answer to C)? Any help is much appreciated

How would you calculate minimum voltage

Reply 7

suets

Original post by Elderfury843

How would you calculate minimum voltage

So minimum possible resistance is the 8 ohm resister and the 0 ohms on the variable resistor - so total of 8 ohms

First you have to work out the current which flows when the resistance is at its highest. So highest resistance is 8 + 30 = 38 ohms
Using the equation V = IR
24 = I x 38 ,
24/38 = 0.63
substitute this current back into the equation V=IR but with using the lowest current.
V= 0.63 x 8 = 5.1v

Reply 8

wiggo2013

Original post by future-doctor <3

Hi im also a bit late to this. How did you do part B?

Reply 9

Driving_Mad

Original post by wiggo2013

Hi im also a bit late to this. How did you do part B?

You have 10 kilo ohms in parallel with a 4kilo ohm resistor in circuit B now. Work out the total resistance and then the total current by dividing 12V by the total resistance.

This current is equal to the current through the 2 kilo ohm resistor so multiply it by 2000 ohms.

The remaining voltage is dropped across the 4 kilo ohm resistor, which will be equal to the voltmeter reading.