Capacitor Basics

Question 1

why does current flow in a circuit

Answer 1

• 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

Question 2

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

Answer 2

• 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

Question 3

what is the setup for allowing charge to flow through an incomplete circuit

Answer 3

• 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

Question 4

what is created in the conducting wires when the power is turned on

Answer 4

an electric field from the creation of a potential difference

Question 5

what does the electric field do

Answer 5

it causes the electrons to flow from the negative terminal to the positive terminal

Question 6

how does this movement of electrons negatively charge one of the plates

Answer 6

• 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

Question 7

how does the other plate gain a positive charge

Answer 7

• 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

Question 8

therefore what is created between the plates

Answer 8

an electric field

Question 9

how does the strength of the electric field change overtime, form the moment it is created

Answer 9

• 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

Question 10

what is this type of circuit called

Answer 10

a capacitor circuit

Question 11

what specifically needs to be in between the plates in order for them to be called a capacitor

Answer 11

an insulator, like air

Question 12

what does it mean if it is said that the capacitors are fully charged

Answer 12

the pd across the plates equals the pd of the supply

Question 13

if the capacitor has this pd across it, similar to the battery, what is it similarly acting as

Answer 13

a store of charge (electrical potential energy)

Question 14

what is the definition of capacitance

Answer 14

• it is the measure of the capability of a capacitor
• measuring the amount of charge a capacitor can store
• per volt applied across it

Question 15

what is capacitance measured in

Answer 15

farads (F)

Question 16

what are the three factors that impact capacitance

Answer 16

• the size of the plates
• their separation
• the nature of the insulator between them

Question 17

what is the equation for capacitance

Answer 17

C = Q / V

Question 18

what are the variables in that equation

Answer 18

• C = capacitance (F)
• Q = charge stored (C)
• V = pd across capacitors (V)

Question 19

how could you use a coulombmeter to calculate capacitance graphically

Answer 19

• 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

Question 20

what is the equation for electric potential energy and therefore the energy stored within a battery / cell

Answer 20

E = QV

Question 21

therefore what is the equation for the electric potential energy stored in a charge capacitor

Answer 21

E = QV / 2

Question 22

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

Answer 22

• 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

Question 23

secondly, why does E = QV divided by 2 (explaining it graphically)

Answer 23

• 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

Question 24

why does the area under the graph give you work done, without prior knowledge of the equation

Answer 24

• 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

Question 25

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

Answer 25

• E =1/2 QV = 1/2 (CV)V = CV^2 / 2

- E = 1/2 QV = 1/2 Q(Q / C) = Q^2 / 2C

Question 26

what is the energy stored on a charged 100 uF capacitor which has 3 mC of charge

Answer 26

• we have C and Q
• so we use E = Q^2 / 2C
• E = (3x10-3)^2 / 2 * (100x10-6)
• E = 0.045J

Question 27

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

Answer 27

• 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

Question 28

what is the definition of a capacitor

Answer 28

• an electrical circuit component that stores charge

- and so can be used as an energy store