Capacitor Basics Flashcards
why does current flow in a circuit
- an electric field is set up within the conducting material (pd)
- this causes a force to be exerted on the electrons
- making them move through the circuit
why cant charge flow across a gap in a circuit despite the effect of the electric field still being able to be felt across the empty space
- conduction electrons just arent able to escape their conductor and move across the empty space
- the potential difference across the two points generally isnt large enough for that
what is the setup for allowing charge to flow through an incomplete circuit
- connecting two large metal plates in a circuit
- one being connected to the +ve terminal and the other to the -ve terminal
- with an air gap between them
what is created in the conducting wires when the power is turned on
an electric field from the creation of a potential difference
what does the electric field do
it causes the electrons to flow from the negative terminal to the positive terminal
how does this movement of electrons negatively charge one of the plates
- the electrons cannot cross the gap between the plates yet
- so they build up on the plate connected to the negative terminal
- giving that plate a negative charge
how does the other plate gain a positive charge
- the other plate is connected to the positive terminal of the battery
- this causes the electrons in this plate to flow towards that positive charge
- resulting in a positive charge being left on the plate
therefore what is created between the plates
an electric field
how does the strength of the electric field change overtime, form the moment it is created
- its strength will increase
- until the pd across the gap is equal to the pd of the power supply
- due to the conservation of charge basically, makes sense
what is this type of circuit called
a capacitor circuit
what specifically needs to be in between the plates in order for them to be called a capacitor
an insulator, like air
what does it mean if it is said that the capacitors are fully charged
the pd across the plates equals the pd of the supply
if the capacitor has this pd across it, similar to the battery, what is it similarly acting as
a store of charge (electrical potential energy)
what is the definition of capacitance
- it is the measure of the capability of a capacitor
- measuring the amount of charge a capacitor can store
- per volt applied across it
what is capacitance measured in
farads (F)
what are the three factors that impact capacitance
- the size of the plates
- their separation
- the nature of the insulator between them
what is the equation for capacitance
C = Q / V
what are the variables in that equation
- C = capacitance (F)
- Q = charge stored (C)
- V = pd across capacitors (V)
how could you use a coulombmeter to calculate capacitance graphically
- charge a capacitor to different pds
- discharge through the coulombmeter each time, giving you the charge for that given pd
- plot a graph with Q on the y axis and V on the x axis
- the gradient of the straight line should give you C
what is the equation for electric potential energy and therefore the energy stored within a battery / cell
E = QV
therefore what is the equation for the electric potential energy stored in a charge capacitor
E = QV / 2
given that the pd across the capacitor is equal to the pd across a battery, and charge cant be different in other parts of a series circuit due to the conservation of charge, this shouldnt make sense, firstly, how is the conservation of charge still upheld in this case
- the charge on the capacitors can change, as the charge increases as the pd increases
- but when fully charged, the charge will be equal and remain equal
- so the law isnt broken as no charge was lost or ‘gained’, only transferred to other parts of the circuit
secondly, why does E = QV divided by 2 (explaining it graphically)
- if you draw a graph of V against Q
- the linear increase in V leads to the linear increase in Q, giving a straight line
- and the area under the line gives you E (as you multiply V by Q)
- but a triangle is formed, so E = QV / 2
why does the area under the graph give you work done, without prior knowledge of the equation
- if you want to add more charge, you need to increase the pd to push the charge on
- pushing the charge on takees energy
- therefore work is done
given that Q = CV and V = Q / C from C = Q / V, what are the two other equations for the electrical potential energy stored in a capacitor that can be formed
- E =1/2 QV = 1/2 (CV)V = CV^2 / 2
- E = 1/2 QV = 1/2 Q(Q / C) = Q^2 / 2C
what is the energy stored on a charged 100 uF capacitor which has 3 mC of charge
- we have C and Q
- so we use E = Q^2 / 2C
- E = (3x10-3)^2 / 2 * (100x10-6)
- E = 0.045J
if the resistance of a bulb is R and you kept increasing the number of parallel pathways within a circuit, what is the trick for knowing how many light bulbs would you be adding each time in order for the total resistance of the circuit to be R
- the number of bulbs on each pathway should equal the number of different parallel pathways
- so a parallel circuit with 3 pathways should have 9 total bulbs
- with 3 bulbs in each pathway
- essentially, number of bulbs = (number of pathways)^2, with the same number of bulbs on each pathway
what is the definition of a capacitor
- an electrical circuit component that stores charge
- and so can be used as an energy store
