Capacitors

Question 1

Capacitors

Answer 1

• Store electrical charge between two parallel plates as negative charge builds up on one side and decreases on the other
• An electric field builds up that allows electrons to be pushed across the gap between the plates
• electrolytic capacitors must be placed the right way round in a circuit or they will explode

Question 2

Changing capacitance

Answer 2

• Adding insulating materials to the gap between the plates has an effect on its capacitance
• variable capacitors use rotating plates to reduce the area of the plates that overlaps to change capacitance
• keys on a keyboard have a spongy material between the plates that means when you press down on the key the capacitance changes with distance

Question 3

Capacitance

Answer 3

• Capacitance = Charge/P.d between plates
• measured in Farads (although a Farad is a large unit so most commonly in microfarads)
• Charge = current*time

Question 4

Finding Capacitance

Answer 4

• Attach a voltmeter across the capacitor, include an ammeter in the circuit, in series
• As capacitor charges its resistance increases due to electrostatic repulsion building up on one plate so include a variable resistor and decrease it gradually in order to keep the total resistance of the circuit roughly even
• Plot a graph of current/time, find area under it to get Charge, use Charge and Voltage to find Capacitance

Question 5

Energy Storage

Answer 5

• Change in energy = Voltage*Change in charge = area under a graph of potential difference against charge
• Average voltage = 1/2 Maximum voltage
• Energy transferred in charging a capacitor = energy stored in a capacitor = 1/2 Max voltageCharge = 1/2capacitancevoltage^2 = (1/2charge^2)/capacitance

Question 6

Disharge

Answer 6

• Capacitor Discharge is an exponential decay graph
• Current = Charge/RC, RC = Resistance*Capacitance = time constant for decay of capacitor with capacitance C, through resistor with resistance R
• Large time constant = long decay
• ‘Half Life’ of capacitor = time at which charge has halved = RCln2

Question 7

Representing discharge as a graph

Answer 7

• total area under Current/Time = total initial charge in capacitor (has to be estimated using trapezium rule as exponential curve cannot be calculated exactly)
• Current = Initial Current*e^(-time/RC)
• Charge = Initial Charge*””””
• Voltage = Initial Voltage*””””
• taking ln of both sides of any of these equations produces a straight line graph that is easier to read