# What is the relationship between voltage and current in a capacitor circuit?

• Clari
In summary: Delta,thank you very much for helping out. :)The voltage reduces slowly then drops at speed then slows down when it reaches the bottom threshold again. This means the current will be high or low when the voltage is changing slowly, and positive or negative when the voltage is dropping fast.
Clari
Hi there!

I have great problem in tackling some problems on this topic, I hope you can help me.

1. For a V-t graph, it has the shape of a sine curve but well beyond the x-axis. It has 3 complete cycle in 6ms as shown in the graph.
Notes: V is the potential difference applied between the terminals in a circuit varies with time t.

a. Sketch a graph of how the charge on the capactior changes over 6ms.
I sketch it the same as the V-t graph, since Charge (Q) = CV...Q is proportional to V. Am I right?

b. Sketch a graph of the current in this circuit, marking clearly on this graph the I=0 value.
I have thought of it for quite a long time, but can't figure it out yet..

I have attached the document which have the graph.

#### Attachments

• Problems (Capacitors).doc
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Last edited:
a. yes

b.Some clues:

Take the first 90 degrees of the voltage cycle. As the voltage increases from zero towards its maximum, what will the current do?

Over next 180 degrees, the voltage at the supply is reducing in comparison to the capacitor to a lower threshold, so the capacitor will have a greater PD then the supply that needs to correct itself. The voltage reduces slowly then drops at speed then slows down when it reaches the bottom threshold again. So, imagine what the currrent will be doing, when the voltage is changing slowly will the current be large or small, when the voltage is dropping fast, what will the current be doing?

Delta,Thank you very much for helping out. :)
But I don't really get what you mean by
Delta said:
...The voltage reduces slowly then drops at speed then slows down when it reaches the bottom threshold again. "...

Here are my answers to your guiding questions:(lol, I am not sure if it is correct)

Over the first 90 degrees of the voltage cycle, as V increases from 0 to max., the current drops from max. to 0. Over next 180 degrees, the voltage at the supply is reducing in comparison to the capacitor. So the current flows in the opposite direction until the voltage rises again...

umm...But I am frustrated about the I=0 value, if I draw the I-t graph silimilar to the V-t graph(not in phase), then I will never be 0.

Last edited:
Clari said:
I don't really get what you mean by
"The voltage reduces slowly then drops at speed then slows down when it reaches the bottom threshold again."
From V=maximum (at 90deg) the voltage falls slowly therefore will the current be high or low?

When V is half way to its lowest value, there is larger rate of change widening the potential between the supply and capacitor, therefore will the current be high or low?

at V=minimum (at 270deg) the voltage changes more slowly as it reaches the lower part of the wave, so would the current now be high or low?

Note when I say "low" I mean close to zero and "high" means a larger current in either direction. Up to you to work out if its is positive or negative current (charging/discharging...?).
Clari said:
Over the first 90 degrees of the voltage cycle, as V increases from 0 to max., the current drops from max. to 0.
Good.
Clari said:
Over next 180 degrees, the voltage at the supply is reducing in comparison to the capacitor. So the current flows in the opposite direction until the voltage rises again...
therefore the current will be positive or negative?
Clari said:
if I draw the I-t graph silimilar to the V-t graph(not in phase)
... bit of a key concept here
Clari said:
then I will never be 0.
Once the voltage has settled to a maximum or minimum value the capacitor is the same as the supply, therefore there is no PD between the two. Would there be in current?

## What is a capacitor and how does it work?

A capacitor is an electronic component that stores electrical energy in an electric field. It consists of two conductive plates separated by an insulating material, known as a dielectric. When a voltage is applied to the capacitor, one plate accumulates positive charge while the other accumulates negative charge. This creates an electric field between the plates, storing energy. The energy can then be released when needed, for example, to power a circuit.

## What are the common problems with capacitors?

One common problem with capacitors is leakage, where the dielectric material breaks down and allows charge to leak through. This can affect the performance of the capacitor and the circuit it is a part of. Another problem is capacitance drift, where the capacitance value changes over time due to factors such as temperature, humidity, and aging of the dielectric material.

## How can I test a capacitor to see if it is working properly?

There are a few methods for testing capacitors. One way is to use a multimeter to measure the capacitance value and check for any significant differences from the expected value. Another method is to use an ESR (equivalent series resistance) meter to check for any internal resistance, which can indicate a faulty capacitor. Additionally, a visual inspection for physical damage or discoloration can also be helpful.

## Can capacitors be repaired or do they need to be replaced?

In most cases, capacitors cannot be repaired and need to be replaced if they are not functioning properly. However, if the issue is minor, such as a loose connection, it may be possible to repair the capacitor. It is important to follow proper safety measures and consult a professional if attempting to repair a capacitor.

## How do I choose the right capacitor for my circuit?

The capacitance value, voltage rating, and type of dielectric are important factors to consider when choosing a capacitor for a circuit. The capacitance value should match the requirements of the circuit, and the voltage rating should be equal to or higher than the maximum voltage in the circuit. The type of dielectric can also affect the performance of the capacitor, so it is important to choose one that is suitable for the specific application.

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