Course work Next week we are doing another PRACTICE course work

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Presentation transcript:

Course work Next week we are doing another PRACTICE course work Week after real thing May need to look at what we are doing in class today!

Capacitors Some definitions Capacitance The charge stored per unit potential difference across it C=Q/V The unit for capacitance is the Farad (F) which is one coulomb per volt (Most capacitors are of the size μF = × 10ˉ6)

What is a capacitor Two metal plates with an insulator between them Sometimes they are rolled up to reduce the size. Most of the time it doesn’t matter which way they are connected (but you need to check). Also are marked with a p.d. to put over them, as depending on the insulator depends how high this can be.

How do you end up with different charges on each side of a capacitor?

Uses of capacitors Supplying back up power Nuclear reactors Flash photography Smoothing out circuits … (Text books page 134-135)

Energy stored by a capacitor Work done to charge capacitor = energy stored by capacitor W = ½ QV What would the work done be if …You had C and V ? You had Q and C?

What are Kirchhoff’s laws? 1st Law The sum of the currents into a point equals the sum of the currents out of that point 2nd Law The sum of the EMFs in any closed loop in a circuit is equal to the sum of the p.d.s around that loop

Capacitors in series We can use Kirchhoff’s 2nd law which means that V= V1 + V2 + V3 The charge in the connecting wires between each capacitor must be 0 therefore | –qonC1 | = | qonC2 | … and so on The charge Q supplied from the battery is equal to the charge on all three capacitors (if 3mC leaves battery to charge 3 capacitors, each capacitor has 3mC and the total is 3mC)

Capacitors in series V= V1 + V2 + V3 Q= Constant Q = Q + Q + Q Q’s cancel out throughout C C1 C2 C3 1 = 1 + 1 + 1 And this works for however C C1 C2 C3 many you are using PLEASE REMEMBER THE RECIPROCAL

Capacitors in parallel When the capacitors are in parallel the process of charging them will be as if they were each individually connected to the battery. This means the charge Q1 on C1 will be C1V… The total charge will therefore be Q = C1V + C2V + C3V … Q = V(C1 + C2 + C3…) The total capacitance will be C = Q = C1 + C2 + C3 V For a given p.d. more charge is being stored, and so the capacitance is greater

Discharging of capacitors Exercise We say exponential decays have a constant ratio property

Time constant What are the units when you have Capacitance × Resistance? This means that the C×R is the time constant for the circuit is a useful term (When t = CR, we have Q = Qo / e-1 ( this is the time when the charge has fallen to 1/e = 0.37 (about 1/3) of its initial value. the larger CR is, the longer the capacitor will take to discharge.)