Physics - Electricity

Hi,
I find electricity really difficult, especially things to do with voltage, currents and resistors.
Some questions I have are:

1. With a parallel circuit, how could all components have the same voltage?
Why does the voltage not split between them?
Why is the voltage the same in a parallel circuit but different in a series circuit?
Why is the current the same at any point in a series circuit but different in a parallel circuit?

2. Why is voltage also called potential difference and what does it mean?

3. If there is a 6 V battery and three bulbs that are the same, what does it mean when the question asks for the voltage across one or two bulbs?

4. If you had a 6V battery and two bulbs, (one 3V bulb and one 2V bulb), what would happen to the extra volt? Would they be very bright or blow? Also, if one bulb was 3V and the other was 4V, what would happen? Would they be very dull?

5. Do the electrons in a circuit lose their energy as they pass through a component? If not, how does a component work? (e.g. a bulb lighting up.)

6. Does the current depend on the voltage or is inversely proportionate?

I would really appreciate any help.
Thanks :smile:

Reply 1

the bear

3. they want to know the reading of a voltmeter placed across one or more bulbs.

Reply 2

musicangel

Original post by the bear

3. they want to know the reading of a voltmeter placed across one or more bulbs.

Why would the voltage over 1 bulb be 2 volts and the voltage over the 2 bulbs be 4 volts? Is the voltage in a series circuit the same at any point or not?
Thanks

Reply 3

the bear

in a series circuit the pdf is shared in the ratio of the resistances. here the ratio is 1: 1: 1 as they are all equal.

Reply 4

Natty_ox

4. Correct me if I am wrong but I don't think that is possible. Kirchoff's second law states that: In any loop (path) around a circuit, the sum of the emfs = the sum of the pds, meaning that the sums of the voltages across the components will equal the voltage of the battery.

5. I THINK electrons lose electrical energy as they pass through a circuit due to the resistance which is in the component (transferred into heat energy) but the voltage supplied by the battery replenishes their electrical energy that would have been lost.

6. It depends on what kind of component current is passing through. For an ohmic conductor, such as a wire or a fixed resistor, current is directly proportional to voltage, as resistance is fixed. However with non-ohmic conductors, such as filament bulbs and diodes, it is not the case.

Reply 5

Beanos

Original post by musicangel

Why would the voltage over 1 bulb be 2 volts and the voltage over the 2 bulbs be 4 volts? Is the voltage in a series circuit the same at any point or not?
Thanks

You're confusing Voltage with Current, Current is the same at any point (with the exception of in parallel circuits where he current splits. Voltage is the "potential drop of energy over a component". So in your example if the voltage drop over 1 bulb is 2V, the voltage dropped over 2 identical (same resistance bulbs) will be 4V.

Reply 6

Dr Alcoholic

Original post by musicangel

Ok I study electrical eng so I'll try to explain this:

1) When we talk of current and voltage a good way to visualise it is like pressure and water flow in a pipe. So for a parallel circuit the pressure on the positive side of the resistor from the source is going to be greater than the negative side. Think of it as a long main water pipe with branches with taps coming off of it. The pressure before the tap will be equal to the pressure in the main pipe hence the voltage with reference to ground will equal the voltage from the source (main pipe). On the other hand the current flow through each resistor will be dependent on the tap/resistor size.

If the resistors are in series on the other hand then it's like adding taps directly to the main water pipe, hence the voltage at tap will be relative to the tap size, where as the current flow will be equal throughout the whole pipe.

2) Potential difference mean the voltage difference from point of measurement to the reference voltage. Normally the reference voltage is usually earth/ground reference i.e. 0 volts, however the reference voltage may not always equal 0. Say in a circuit you want to know the voltage of a resistor across a 12 volt line and a 6 volt line the the p.d would be 6 volts i.e. the is a difference of 6 volts between one end of the resistor and the other.

3)Again as I said its the voltage directly across the component. Another way to think about it is, say you two water pipes lying in parallel, they are connected together in the the middle by a 3rd pipe with a tap. If you wanted to find the pressure at the tap you would take the pressure difference between the two parallel pipes.

4) Not sure what you are asking here.

5) don't think of electrons losing energy when dealing with applied electrical analysis. If you want you can think of the flow of electric charge or simply current flow. Energy loss should be thought in terms of power losses.

Anyway how the component reacts to current flowing through it depends on the component itself. A filament light bulb for example allows a current to flow through a filament that dissipates power through heat and light. Or in other words the resistive nature of the filament wire creates heat and light.

6) Current is directly proportional to voltage if resistance is constant.

This is a very basic explanation of electricity, it gets more confusing with regard to AC circuits, transistor circuits, complex resistive circuits etc.

(edited 10 years ago)

Reply 7

musicangel

Thank you for all the help everybody! :biggrin:

Reply 8

musicangel

Hi again!

I have a few more questions:

Why is voltage called potential difference when it measures the push on the electrons?

With a parallel circuit, how could all components have the same voltage? Why does the voltage not split between them?

Why is the voltage the same in a parallel circuit but different in a series circuit?

What does it mean when it says 'Components in parallel have the same voltage across them'?

What does a '2V' or a '3V' battery mean? Does it mean the amount of voltage the bulb can take?

If you had a 2V and a 4V bulb with a 6V battery, why would the 2V bulb be dimmer than the 4V bulb? Also, what does it mean when it asks 'What does this tell you about the resistance?'

If you had a 6V battery and two bulbs, (one 3V bulb and one 2V bulb), what would happen to the extra volt? Would they be very bright or blow? Also, if one bulb was 3V and the other was 4V, what would happen? Would they be very dull?

Thank you once again! I really appreciate the help! :smile:

Reply 9

uberteknik

Original post by musicangel

Hi again!

I have a few more questions:

Hello and welcome to TSR! :smile:

Original post by musicangel

Why is voltage called potential difference when it measures the push on the electrons?

Think of a power supply. All work must have an energy source be that for lighting a lamp to pulling a train using an electric motor.

The power available from the source is transferred through the movement of electrons flowing in the conductors. Electrons repel other electrons so the push 'pressure' comes from the source and ripples around the circuit rather like how a bicycle chain transports energy from a riders legs to the road wheels.

Voltage is therefore defined as Joules/Coulomb. i.e. the energy available from the source that can be transferred for every Coulomb (quantity of electrons) of charge.

You can now see that if the voltage increases it means more energy for a given amount of charge (electrons) is available to do work.

1 volt = 1 Joule of energy for 1 Coulombs worth (given mass) of electrons.

Potential Difference:

When the electrons are forced to do work by say heating up a resistor, energy must be being used.

So the potential energy available at the entry to the resistor MUST be different to the potential energy left at the exit to the resistor because some of that potential was converted to actual heat energy in the resistor.

Can see where this is going?

Potential difference does exactly what is says on the tin: it's the amount of potential energy used up between two points in a circuit. (Rather like gravitational potential gets used up as a stone drops through the air.)

PD is measured in volts because its the same definition as voltage:

PD (volts) = Joules used per Coulomb of charge. V = E/Q

That's EXACTLY why the voltage drops (potential drops) around the circuit must equal the source voltage.

Original post by musicangel

With a parallel circuit, how could all components have the same voltage? Why does the voltage not split between them?

You should now see from the previous explanantion that in a parallel circuit, all parallel components have access to the same source of POTENTIAL energy because they are all simutaneously connected to it.

And since potential difference is the amount of energy used between two points, the pd across the parallel components is a measure of the energy used in ALL of those PARALLEL components SIMULTANEOUSLY.

The energy is shared because the limited quantity of electrons have to travel down different parallel paths but all join up again as they leave the parallel components. (Like water diverting around rocks in a river.)

Original post by musicangel

Why is the voltage the same in a parallel circuit but different in a series circuit?

Parallel components are connected to the same points in a circuit.

Series components are connected end to end. The first component uses up some of the potential energy available from the source. This leaves less energy avaiable for the next component which uses up more of the energy and so on.

So each component will use up energy and hence the potential difference across a series component is a measure of the energy used up in each INDIVIDUAL series component because its the only thing that electrons are flowing through in that part of the circuit.

Original post by musicangel

What does it mean when it says 'Components in parallel have the same voltage across them'?

Should now be expalianed above. :smile:

Original post by musicangel

What does a '2V' or a '3V' battery mean? Does it mean the amount of voltage the bulb can take?

Original post by musicangel

If you had a 2V and a 4V bulb with a 6V battery, why would the 2V bulb be dimmer than the 4V bulb? Also, what does it mean when it asks 'What does this tell you about the resistance?'

If you had a 6V battery and two bulbs, (one 3V bulb and one 2V bulb), what would happen to the extra volt? Would they be very bright or blow? Also, if one bulb was 3V and the other was 4V, what would happen? Would they be very dull?

I'm going to stop while you digest the previous information and see if you can answer the last two questions based on the explanations already given.

If you are still having difficulty, can I ask you to post a new thread as the answers are somewhat based in manufacturing practicalities rather than physics laws.

Thanks. :smile:

Reply 10

Kallisto

Entertainment Forum Helper

Original post by musicangel

(...)
2. Why is voltage also called potential difference and what does it mean?
(...)

Have you ever seen two plate capacitors which have a certain distance to each other? imagine the distance is divided in 'levels' called equipotential lines. On the other hand the equipotential lines are divided in a zero potential and a bunch of equipotential lines. The distance between the zero potential and a certain equipotential line is the potential difference. Inside the plate capacitators there is an electric field which does electric work. This electric work increases, when the potential difference increases. The potential difference increases, when the distance between the plate capacitors increases. Then the voltage is increased.

Or in short: the greater the distance of two plate capacitors the greater the distance between the zero potential and the 'highest' equipotential line. Thus the voltage is so much greater.

Was it understandable eough for you?

Reply 11

musicangel

Original post by uberteknik

Hello and welcome to TSR! :smile:

Don't worry, you are not the only one who has difficulty grasping the concepts. Mainly because it's not easy to visualise what is going on.

There is a lot to take in and you have to go over it quite a few times before it all of it makes sense.

Think of a power supply. All work must have an energy source be that for lighting a lamp to pulling a train using an electric motor.

The power available from the source is transferred through the movement of electrons flowing in the conductors. Electrons repel other electrons so the push 'pressure' comes from the source and ripples around the circuit rather like how a bicycle chain transports energy from a riders legs to the road wheels.

Voltage is therefore defined as Joules/Coulomb. i.e. the energy available from the source that can be transferred for every Coulomb (quantity of electrons) of charge.

You can now see that if the voltage increases it means more energy for a given amount of charge (electrons) is available to do work.

1 volt = 1 Joule of energy for 1 Coulombs worth (given mass) of electrons.

Potential Difference:

When the electrons are forced to do work by say heating up a resistor, energy must be being used.

So the potential energy available at the entry to the resistor MUST be different to the potential energy left at the exit to the resistor because some of that potential was converted to actual heat energy in the resistor.

Can see where this is going?

Potential difference does exactly what is says on the tin: it's the amount of potential energy used up between two points in a circuit. (Rather like gravitational potential gets used up as a stone drops through the air.)

PD is measured in volts because its the same definition as voltage:

PD (volts) = Joules used per Coulomb of charge. V = E/Q

That's EXACTLY why the voltage drops (potential drops) around the circuit must equal the source voltage.

You should now see from the previous explanantion that in a parallel circuit, all parallel components have access to the same source of POTENTIAL energy because they are all simutaneously connected to it.

And since potential difference is the amount of energy used between two points, the pd across the parallel components is a measure of the energy used in ALL of those PARALLEL components SIMULTANEOUSLY.

The energy is shared because the limited quantity of electrons have to travel down different parallel paths but all join up again as they leave the parallel components. (Like water diverting around rocks in a river.)

Parallel components are connected to the same points in a circuit.

Series components are connected end to end. The first component uses up some of the potential energy available from the source. This leaves less energy avaiable for the next component which uses up more of the energy and so on.

So each component will use up energy and hence the potential difference across a series component is a measure of the energy used up in each INDIVIDUAL series component because its the only thing that electrons are flowing through in that part of the circuit.

Should now be expalianed above. :smile:

A battery is a source of potential energy. So battery voltage means the amount of energy available per Coulombs worth of charge.

2 Volts means 2 Joules for every 1 Coulomb of charge is available.

3 Volts means 3 Joules for every 1 Coulomb of charge is available.

Lamps do not supply energy, they use it. A 3V lamp means that it is designed to work at optimal light output when supplied by a source of potential energy that is able to deliver 3 Joules per Coulomb of charge. (i.e. source =3V)

A 230V lamp works optimally when connected to an energy supply source capable of providing 230 Joules/Coulomb of charge.

I'm going to stop while you digest the previous information and see if you can answer the last two questions based on the explanations already given.

If you are still having difficulty, can I ask you to post a new thread as the answers are somewhat based in manufacturing practicalities rather than physics laws.

Thanks. :smile: