6.1 Capacitors Flashcards
What is a capacitor?
A circuit component that stores energy by separating charges onto 2 electrical conductors (often called plates) with an insulator inbetween them. One plate becomes positively charged and the other becomes neatively charged.
What is capacitance?
Quantity of charge which can be stored per unit potential diffference across the plates of the capacitor.
What is a farad?
The unit of capitance is the farad. One farad is equivalent to 1 coulmb per volt.
What happens to energy when charges are separated in a capacitor?
Energy is stored
Compared to batteries how much energy can a capacitor store?
Capcitors can only store a small amount of energy compared to batteries.
What is a capacitor made up of?
They are comppsed of 2 conductors separated from one another by a gap which often contains an insulating material called a dielectric.
When changing the dielectric material, how does the area of overlap of the positive and negative plates affect the amount of charge in the capacitor?
The area of overlap of the positive and negative plates affects the amount of charge that can be transffered to each plate and therefore,the capacitance of the caapacitor.
In some circuits, how do they make capacitors more compact?
In some capacitors he plates are rolled up.
What way do electrons flow in a circuit with a capacitor?
Electrons flow from the negative terminal of the cell onto the right-hand plate off the left-hand plate towards the positive terminal of the cell.
Can charge flow between the plates of a capacitor?
No
How does the negative plate of a capacitor become negatively charged?
Electrons are transffered from the negative terminal to the negative plate.
How does the positive plate on a capacitor become positively charged?
Electrons flow off the positive plagtev and towards the positive terminal of a cell.
What is made as a fresult of the positive and negative plates on a capacitor?
It results in a potential difference across the plates of the capacitor.
What will happen to the charge in a circuit when a capacitor is fully charged?
No more charge will flow in the circuit.
Why does no charge flow in the circuit after a capacitor is fully charged?
The electrons of the negative plate will repel any further elctrons away.
What will the potential difference be across a capacitor when it is fully charged?
The pd across a capacitor will be equal to the emf of the cell.
What is the amount of charge stored by the capacitor directly proportional to?
Charge stored by the capacitor is dirrectly proportional to the pd between the plates.
What is the insulator between the 2 plates in a capacitor called?
A dielectric
What equation can u use to calculate capacitance?
Capacipance= charge/ potential difference
What graph would you draw when investigating measuring capacitance?
Plot a graph of charge against voltage and raw a straight line of best fit- the gradient gives an accurate value of the capacitance.
On a capacitor, what is the marked value of the capacitance on the component accurate to?
10%
What is methord one that you could use to measure the capacitance?
- Set up the apparatus.
- Once switch is closed, charge flows and you can adjust the variable resistor to keep the value of current constant until the capacitor becomes fully charges.
- Start a stopwatch and record values for the current and voltage at regular intervals until the current falls to vero- showing the capacitor is fully charged.
- Find the charge stroes on the plates from Q=It.
What is methord two that you could use to measure the capacitance?
- The apparatus fown is a coulmbmetre to measure the charge on the capacitor.
- Charge the capacitor to different voltages and measure that charge stored for each value.
- Find the percentage difference between the normial capacitance values and your experimental values.
NOTE: Only use capacitors suitable for low voltage and avoid that may store excessive charge as they may infict a sevre elecrtic shock.
What is the energy stored by a capacitor equal to?
Energy stored by a capacitor is equal to the work done to separate charges on the two plates.
As the charge increases by an amount ∆Q, what does the build-up of charge on the plates oppose? What does this lead to?
The build-up of charge opposes further movement of electrons- this means work must be done to move the additional charge from one plate to the other.
What equation gives the work done to move a charge through a constant potential difference?
W= Q X V
What happens to the charge as it moves through an increaseing pd?
The charge will increase.
What does the shape of a pd time graph look like compared to the shape of a charge-time graph of a capacitor?
They have exact the same shape as the pd across a capacitor is proportional to the charge stored.
On a capacitor, what does a charging current time graph look like?
On a capacitor, what does a pd time graph look like?
On a capacitor, what does the charge time graph look like?
What does a graph of current discharging against time, graph of pd against time while current is being disapated and a graph of the charge on a capacitor against time while current is being disapated from a capacitor look like?
What do these graphs show?
They show the exponentrial decrease (exponential decay) of the current, pd and charge.
How can you find the time constant from the graph of pd or charge on the capacitor varies as the capacitor is charged or discharged against time?
We can see from the graph that in 20s internals the fractional change in Q is the same (from 100 to 37 is 1/e, and from 37 to 13.5 is 1/e and from, 13.5 to 5 is 1/e).
