1. Capacitors
Construction and charging and discharging

2. What is a capacitor? Electronic component
Two conducting surfaces separated by an insulating material
Stores charge
Uses
Time delays
Filters
Tuned circuits
Interior light in a car stays on for a while…capacitor circuit

3. Capacitor construction
Two metal plates
Separated by insulating material
‘Sandwich’ construction
‘Swiss roll’ structure
Capacitance set by...
Dielectric material is good at holding onto the e-filed between the plates

4. Defining capacitance ‘Good’ capacitors store a lot of charge…
…when only a small voltage is applied
Capacitance is charge stored per volt
Capacitance is measured in farads F
Big unit so nF, mF and F are used
Simple definition of capacitance

5. Graphical representation
Equating to the equation of a straight line Q V
Gradient term is the capacitance of the capacitor
Charge stored is directly proportional to the applied voltage
Simple GCSE analysis of a straight line graph

6. Energy stored by a capacitor
By general definition E=QV
product of charge and voltage
By graphical consideration... Q V
Area is energy stored
Area term is the energy stored in the capacitor

7. Other expressions for energy
By substitution of Q=CV
Alternative expressions for energy stored

8. Charging a capacitor Current flow Initially High Finally I Zero
Exponential model
Charging factors
Capacitance
Resistance I t
At switch on the capacitor is like a wire…high current flow
When fully charged it is like the biggest resistor in the world..open circuit

9. Discharging a capacitor
Current flow
Initially
High
Opposite to charging
Finally
Zero
Exponential model
Discharging factors
Capacitance
Resistance I t
Discharge current flows in the opposite direction

10. Voltage and charge characteristics
V or Q t
V or Q t
Expressions for V and Q can be interchanged
Charging Discharging

11. Time constant Product of Capacitance of the capacitor being charged
Resistance of the charging circuit CR
Symbol  ‘Tau’
Unit seconds
Units of seconds

12. When t equals tau during discharge
At t = tau the capacitor has fallen to 37% of its original value.
By a similar analysis tau can be considered to be the time taken for the capacitor to reach 63% of full charge.
Reason for t at tau being 37% of full charge

13. Graphical determination of tau
V at 37%
Q at 37%
Compared to initial maximum discharge V or Q t
If R is known then the value of capacitance can be found

14. Logarithmic discharge analysis
Mathematical consideration of discharge
Exponential relationship
Taking natural logs equates expression to ‘y=mx+c’
Gradient is -1/Tau
A bit of maths gives a nice straight line…possible basis for a good investigation

15. Logarithmic discharge graph
lnV t
Gradient term is the -1/Tau
Explains the previous equation

16. for...
Capacitors