Capacitors
I'm having a hard time getting my head round capacitors. I think my main problem is I find it hard to understand what potential difference actually is. I've looked at various revision materials and I just can't understand it.
Why is the charge proportional to the potential difference? What even is potential difference? Why does the voltage divide in series but remain the same in parallel?
The only thing I do understand is the time constant which makes no sense seeing as it's at the end of the module haha.
Thanks in advance.
Why is the charge proportional to the potential difference? What even is potential difference? Why does the voltage divide in series but remain the same in parallel?
The only thing I do understand is the time constant which makes no sense seeing as it's at the end of the module haha.
Thanks in advance.
Original post by YThursday
I understand it as current being 'the number of electrons' whilst voltage is 'the push the electrons receive'. I'm pretty sure that this understanding isn't technically right, but its a general idea which helps at A-level.
Thank you. This was the kinda thing I was looking for!
+repOriginal post by LeaX
Thank you. This was the kinda thing I was looking for! +rep
+repGlad to hear you understand it more.
I think they help because they allow for power = current * voltage.
Power is kinda like the sum of the 'momentum' (not literal momentum) of each electron. (As momentum = mass * velocity, mass = number of electrons (current), velocity = voltage).
I'm not sure if its worth thinking it through like that, but its how I do it
Current splits in parallel as electrons can physically only go one way, but all electrons will go through each component in series.
Charge = Current * time
Therefore, Current = Charge/time
Voltage = Current * Resistance
Therefore, Current = Voltage/Resistance
Therefore, Voltage/Resistance = Charge/time
Therefore, Charge = Voltage*time /Resistance
This is the derivation to show that charge and voltage are proportional. Not sure if that helps at all? But might as well include it.
Original post by YThursday
Glad to hear you understand it more.
I think they help because they allow for power = current * voltage.
Power is kinda like the sum of the 'momentum' (not literal momentum) of each electron. (As momentum = mass * velocity, mass = number of electrons (current), velocity = voltage).
I'm not sure if its worth thinking it through like that, but its how I do it
Current splits in parallel as electrons can physically only go one way, but all electrons will go through each component in series.
Charge = Current * time
Therefore, Current = Charge/time
Voltage = Current * Resistance
Therefore, Current = Voltage/Resistance
Therefore, Voltage/Resistance = Charge/time
Therefore, Charge = Voltage*time /Resistance
This is the derivation to show that charge and voltage are proportional. Not sure if that helps at all? But might as well include it.
I think they help because they allow for power = current * voltage.
Power is kinda like the sum of the 'momentum' (not literal momentum) of each electron. (As momentum = mass * velocity, mass = number of electrons (current), velocity = voltage).
I'm not sure if its worth thinking it through like that, but its how I do it
Current splits in parallel as electrons can physically only go one way, but all electrons will go through each component in series.
Charge = Current * time
Therefore, Current = Charge/time
Voltage = Current * Resistance
Therefore, Current = Voltage/Resistance
Therefore, Voltage/Resistance = Charge/time
Therefore, Charge = Voltage*time /Resistance
This is the derivation to show that charge and voltage are proportional. Not sure if that helps at all? But might as well include it.
Thank you so much!! Everything has now just clicked and I can apply these basics to capacitors. I finally understand it, yay! Thank you!
Another way of looking at it is:
Voltage is analogous to pressure pushing the electrons around the circuit. Since the current is the amount of electrons flowing past any point in a given time, then the total 'momentum' of current going down any route is the product of voltage and current.
Parallel and the momentum is split between two directions. the voltage (pressure) in each parallel route is the same, but total available momentum of electrons is split between the routes.
In a series circuit the current passes through a single route but resistance in the way causes work to be done and hence the momentum of the electrons must fall. Since the current is the same, it's the voltage that gives up the energy.
Voltage is analogous to pressure pushing the electrons around the circuit. Since the current is the amount of electrons flowing past any point in a given time, then the total 'momentum' of current going down any route is the product of voltage and current.
Parallel and the momentum is split between two directions. the voltage (pressure) in each parallel route is the same, but total available momentum of electrons is split between the routes.
In a series circuit the current passes through a single route but resistance in the way causes work to be done and hence the momentum of the electrons must fall. Since the current is the same, it's the voltage that gives up the energy.
(edited 11 years ago)
Potential difference is the difference between the voltage on one side of a component and the voltage on the other side. Current can only flow when there is a potential difference... I think of it like a waterfall:
If there is a pool of water high above a lake, then water flows down (through the waterfall) into the lower pool. There is a potential difference... Just in the same way that gravitational potential energy works! If you raise something up, it has somewhere to fall back down to. Similarly, with electrical circuits, there is a potential difference on either side of the circuit. If there was no potential difference, it would be like the waterfall was the same height as the lake and thus it can't flow down.
It's why a 9V battery works because there is +9V on the positive side and then 0V on the negative side. There is a 9 volt potential difference between the two clips, so current can flow. If you connected positive to positive, then both ends are at 9V so there is no difference between them. It is measured in volts and is basically the difference in the voltage before a component and the voltage after it.
Why is voltage proportional to charge? Well, to solve this, we can use the equation C = Q/V. Capacitance is equal to charge per unit voltage (potential difference). If you re-arrange this equation, Q = CV. There is a constant (C) so we use the equals sign, but if we remove the constant, it no longer becomes "equal to" and instead just becomes "proportional to". Thus, Q is proportional to V. From the first equation, it can also be seen that C = Q/V and in a similar method, C is proportional to 1/V, because we can remove the Q value and be left with just 1/V. C is not a constant because it is has the letter "C", mind you. It's just a constant for that instance. Similarly, V is the constant when you're making Q proportional to C! It's just the other variable(s) which locks the equation together. Another example would be: x = y*z*a*b. x is proportional to y, or to z, or to a or to b. If you doubled any of those values, x would double as well.
How can it be that an equals sign turns to the proprtionality symbol? Well, it seems quite logical to me. We know that for Q = CV, as the voltage increases, the charge increases too. But, like a graph of y=mx+c, we need a constant (the capacitance, in this case) to fix the two values together. Else, they're just proportional because we know their general trend but not the specific values they assume.
Why does voltage stay the same in a parallel circuit? Well, to use the waterfall analogy again: In a series circuit, it is as if one waterfall is feeding into another waterfall which is feeding into another waterfall... Like steps going down a flight of stairs. After every component there is another voltage drop (potential difference) and so the voltage is shared between the components. Note: Current stays the same for a series circuit. For a parallel circuit, it's like there is one massive resevoir of water at the top and then several waterfalls leading off that one pool of water. There aren't several waterfalls after one another, just several from the highest pool of water available. It's like two or more flights of stairs going up to the same point. In parallel circuits the current is shared proportionally between each "waterfall" or loop in the system.
Water pressure is like voltage
Water flow rate is like current
Blockages in the system are like resistors (or components)
If there is a pool of water high above a lake, then water flows down (through the waterfall) into the lower pool. There is a potential difference... Just in the same way that gravitational potential energy works! If you raise something up, it has somewhere to fall back down to. Similarly, with electrical circuits, there is a potential difference on either side of the circuit. If there was no potential difference, it would be like the waterfall was the same height as the lake and thus it can't flow down.
It's why a 9V battery works because there is +9V on the positive side and then 0V on the negative side. There is a 9 volt potential difference between the two clips, so current can flow. If you connected positive to positive, then both ends are at 9V so there is no difference between them. It is measured in volts and is basically the difference in the voltage before a component and the voltage after it.
Why is voltage proportional to charge? Well, to solve this, we can use the equation C = Q/V. Capacitance is equal to charge per unit voltage (potential difference). If you re-arrange this equation, Q = CV. There is a constant (C) so we use the equals sign, but if we remove the constant, it no longer becomes "equal to" and instead just becomes "proportional to". Thus, Q is proportional to V. From the first equation, it can also be seen that C = Q/V and in a similar method, C is proportional to 1/V, because we can remove the Q value and be left with just 1/V. C is not a constant because it is has the letter "C", mind you. It's just a constant for that instance. Similarly, V is the constant when you're making Q proportional to C! It's just the other variable(s) which locks the equation together. Another example would be: x = y*z*a*b. x is proportional to y, or to z, or to a or to b. If you doubled any of those values, x would double as well.
How can it be that an equals sign turns to the proprtionality symbol? Well, it seems quite logical to me. We know that for Q = CV, as the voltage increases, the charge increases too. But, like a graph of y=mx+c, we need a constant (the capacitance, in this case) to fix the two values together. Else, they're just proportional because we know their general trend but not the specific values they assume.
Why does voltage stay the same in a parallel circuit? Well, to use the waterfall analogy again: In a series circuit, it is as if one waterfall is feeding into another waterfall which is feeding into another waterfall... Like steps going down a flight of stairs. After every component there is another voltage drop (potential difference) and so the voltage is shared between the components. Note: Current stays the same for a series circuit. For a parallel circuit, it's like there is one massive resevoir of water at the top and then several waterfalls leading off that one pool of water. There aren't several waterfalls after one another, just several from the highest pool of water available. It's like two or more flights of stairs going up to the same point. In parallel circuits the current is shared proportionally between each "waterfall" or loop in the system.
Water pressure is like voltage
Water flow rate is like current
Blockages in the system are like resistors (or components)
(edited 11 years ago)
Original post by SillyEddy
Potential difference is the difference between the voltage on one side of a component and the voltage on the other side. Current can only flow when there is a potential difference... I think of it like a waterfall:
If there is a pool of water high above a lake, then water flows down (through the waterfall) into the lower pool. There is a potential difference... Just in the same way that gravitational potential energy works! If you raise something up, it has somewhere to fall back down to. Similarly, with electrical circuits, there is a potential difference on either side of the circuit. If there was no potential difference, it would be like the waterfall was the same height as the lake and thus it can't flow down.
It's why a 9V battery works because there is +9V on the positive side and then 0V on the negative side. There is a 9 volt potential difference between the two clips, so current can flow. If you connected positive to positive, then both ends are at 9V so there is no difference between them. It is measured in volts and is basically the difference in the voltage before a component and the voltage after it.
Why is voltage proportional to charge? Well, to solve this, we can use the equation C = Q/V. Capacitance is equal to charge per unit voltage (potential difference). If you re-arrange this equation, Q = CV. There is a constant (C) so we use the equals sign, but if we remove the constant, it no longer becomes "equal to" and instead just becomes "proportional to". Thus, Q is proportional to V. From the first equation, it can also be seen that C = Q/V and in a similar method, C is proportional to 1/V, because we can remove the Q value and be left with just 1/V. C is not a constant because it is has the letter "C", mind you. It's just a constant for that instance. Similarly, V is the constant when you're making Q proportional to C! It's just the other variable(s) which locks the equation together. Another example would be: x = y*z*a*b. x is proportional to y, or to z, or to a or to b. If you doubled any of those values, x would double as well.
How can it be that an equals sign turns to the proprtionality symbol? Well, it seems quite logical to me. We know that for Q = CV, as the voltage increases, the charge increases too. But, like a graph of y=mx+c, we need a constant (the capacitance, in this case) to fix the two values together. Else, they're just proportional because we know their general trend but not the specific values they assume.
Why does voltage stay the same in a parallel circuit? Well, to use the waterfall analogy again: In a series circuit, it is as if one waterfall is feeding into another waterfall which is feeding into another waterfall... Like steps going down a flight of stairs. After every component there is another voltage drop (potential difference) and so the voltage is shared between the components. Note: Current stays the same for a series circuit. For a parallel circuit, it's like there is one massive resevoir of water at the top and then several waterfalls leading off that one pool of water. There aren't several waterfalls after one another, just several from the highest pool of water available. It's like two or more flights of stairs going up to the same point. In parallel circuits the current is shared proportionally between each "waterfall" or loop in the system.
Water pressure is like voltage
Water flow rate is like current
Blockages in the system are like resistors (or components)
If there is a pool of water high above a lake, then water flows down (through the waterfall) into the lower pool. There is a potential difference... Just in the same way that gravitational potential energy works! If you raise something up, it has somewhere to fall back down to. Similarly, with electrical circuits, there is a potential difference on either side of the circuit. If there was no potential difference, it would be like the waterfall was the same height as the lake and thus it can't flow down.
It's why a 9V battery works because there is +9V on the positive side and then 0V on the negative side. There is a 9 volt potential difference between the two clips, so current can flow. If you connected positive to positive, then both ends are at 9V so there is no difference between them. It is measured in volts and is basically the difference in the voltage before a component and the voltage after it.
Why is voltage proportional to charge? Well, to solve this, we can use the equation C = Q/V. Capacitance is equal to charge per unit voltage (potential difference). If you re-arrange this equation, Q = CV. There is a constant (C) so we use the equals sign, but if we remove the constant, it no longer becomes "equal to" and instead just becomes "proportional to". Thus, Q is proportional to V. From the first equation, it can also be seen that C = Q/V and in a similar method, C is proportional to 1/V, because we can remove the Q value and be left with just 1/V. C is not a constant because it is has the letter "C", mind you. It's just a constant for that instance. Similarly, V is the constant when you're making Q proportional to C! It's just the other variable(s) which locks the equation together. Another example would be: x = y*z*a*b. x is proportional to y, or to z, or to a or to b. If you doubled any of those values, x would double as well.
How can it be that an equals sign turns to the proprtionality symbol? Well, it seems quite logical to me. We know that for Q = CV, as the voltage increases, the charge increases too. But, like a graph of y=mx+c, we need a constant (the capacitance, in this case) to fix the two values together. Else, they're just proportional because we know their general trend but not the specific values they assume.
Why does voltage stay the same in a parallel circuit? Well, to use the waterfall analogy again: In a series circuit, it is as if one waterfall is feeding into another waterfall which is feeding into another waterfall... Like steps going down a flight of stairs. After every component there is another voltage drop (potential difference) and so the voltage is shared between the components. Note: Current stays the same for a series circuit. For a parallel circuit, it's like there is one massive resevoir of water at the top and then several waterfalls leading off that one pool of water. There aren't several waterfalls after one another, just several from the highest pool of water available. It's like two or more flights of stairs going up to the same point. In parallel circuits the current is shared proportionally between each "waterfall" or loop in the system.
Water pressure is like voltage
Water flow rate is like current
Blockages in the system are like resistors (or components)
Thank you both so so much. I read over my capacitor notes and everything now clicks. I've bookmarked this thread and will be referring to all of the posts on here throughout my revision. Thank youuuu!
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