Transcript Motion Along a Straight Line at Constant

Book Reference : Pages 94-95
1.
To understand what a capacitor is
2.
To understand their applications
3.
To understand how we define capacitance
A capacitor is a passive electronic device which
stores charge.
They consist of two or more conducting plates
separated by an insulating material.
+Q
-Q
electrons
Capacitor
Symbol
When connected to a battery, the plate
connected to the positive side of the
battery loses electrons to the battery
and the other plate gains electrons
from the battery.
We say that the capacitor has a charge
Q when the positive plate is at +Q and
the other plate is at -Q
We will see that capacitors effectively block
direct current (DC) but allow alternating current
(AC) to flow
Smoothing circuits : Noise on power supply rails
can be removed by conducting away to ground
This processor on a computer motherboard
wants nice smooth stable power supply lines.
The high speed clock (several GHz) cause
unwanted noise on these power lines. A
suitable capacitor connected between the
power and ground will conduct away the AC
noise and leave the DC supply unchanged
Store power : A large capacitor can be thought of
as a small battery. It can store power for backup
purposes, (for example in devices where you
need to change batteries but not lose
configuration). They can also suddenly dump a
very large amount of energy in a short time
(camera flash gun)
Tuned Circuits : In traditional radios and TVs the
thing you twiddle to change station is a variable
capacitor. This makes the tuned circuit sensitive
to only the station you want
Filter Circuits : Remove unwanted frequencies
Timing and pulse producing circuits : on and off
after a preset delay, on/off repeatedly
Definition : The capacitance of a capacitor is the
charge stored per unit potential difference
(voltage)
C = Q/V
The unit of capacitance is the Farad and is equal
to 1 Coulomb per volt
Note a Farad is a very large unit, in everyday life capacitors are
found with values of pF, nF and F (so take care with calculations)
Definition : Current : An electric current measured
in Amperes is defined as the amount charge.
(Coulombs) flowing per second (I = Q/t)
Switch
V
A
Variable
Resistor
Microammeter
High 
Voltmeter
Close the switch, record
the voltage at given
times and use the
variable resistor to keep
the current constant (e.g.
15A
[or use data logger]
For a constant current of 15A find the charge in
the table below:
Plot a graph of
Time /s PD / V Q/C
Charge Q against
0
0.00
Potential
20
0.29
Difference V
40
0.62
60
0.90
80
1.22
What can we find
100
1.50
from the graph?
Graph of Charge Q against Potential difference V
1.60E-03
1.40E-03
Charge Q / C
1.20E-03
1.00E-03
8.00E-04
6.00E-04
4.00E-04
2.00E-04
0.00E+00
0.00
0.50
1.00
1.50
Potential Difference V /V
2.00
The gradient is the value of capacitance (1015F)
A capacitor is charged by means of a constant
current of 0.5A to a P.D. 5.0V in 55s. Calculate:
The charge stored
The value of the capacitor
A 22F Capacitor is charged by means of a
constant current of 2.5A to a PD of 12.0V.
Calculate:
The charge stored
The time taken
A capacitor is charged by means of a constant
current of 24A to a P.D. 4.2V in 38s. The
capacitor is then charged from 4.2V by means of
a constant current of 14A in 50s Calculate:
Charge stored at a pd of 4.2v
The value of the capacitor
The extra charged stored at a current of 14A
The new PD after the extra charged was stored