Download A *Level Physics: Further Mechanics

A –Level Physics:
Capacitor Basics
Objectives:
FLASHBACK
FLASHBACK: Explain with a labelled circuit diagram, how the
internal resistance of a battery can be determined
Starter Activities
1) Draw the circuit symbols for the following:
• Diode
• Thermistor
• Variable Resistor
• Capacitor
2) Calculate the resultant resistance of three 20 ohm
resistors in series and then in parallel. Then calculate the
resultant resistance of one 30 ohm resistor in series with
two 40 ohm resistors in parallel with one another.
Capacitors
“A capacitor is a device used to store an electric charge, consisting
of one or more pairs of conductors separated by an insulator”
In a situation whereby there are
two plates with an air gap in
between, electrical charge is
initially unable to pass.
However, gradually the charge builds up on the negative terminal when the
capacitor is attached to a power source. As electrons still flow away from
the other terminal (and to the power source), the other becomes ‘positive’
It’s a little like the negative terminal is full of negative charge carrying electrons
whereas the other terminal is lacking!
-+
Capacitors
The attraction between these opposite charges
causes an electric field between the plates which
gradually increases until the p.difference across
them = the power supply!
The amount of charge a capacitor can store (per volt across it) is
known as the CAPACITANCE (C) and is measured in FARADS (F).
𝐶ℎ𝑎𝑟𝑔𝑒 𝑆𝑡𝑜𝑟𝑒𝑑 (𝐶)
𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑎𝑛𝑐𝑒 (𝐹) =
𝑃. 𝑑 𝑎𝑐𝑟𝑜𝑠𝑠 𝑡ℎ𝑒 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑜𝑟 (𝑉)
𝑄
𝐶=
𝑉
PRACTICE: what is the capacitance of a
capacitor that can store 18mC when 6V is
across it. How much charge would be stored if
the voltage was increased to 20V?
Energy Stored in a Capacitor
A charged capacitor becomes a store of
electrical potential energy and this energy can
subsequently be transformed into other forms.
Usually E=QV, but that
equation assumes a constant
voltage and charge. If we plot a
voltage-charge graph for a
capacitor, naturally both begin
at zero. They are then directly
proportional meaning that the
energy stored (area under the
graph) is now E=
½ QV
Alternative Equations
Knowing that Q=CV and E= ½ QV, you can work
out some alternative equations.
What is the energy stored on a fully charged
100microfarad capacitor with 3mC of charge?
Find the equation linking
energy stored to capacitance
and voltage
𝑬=?
𝑬 = ½ CV2
Find the equation linking
energy stored to charge and
capacitance
𝑬=?
𝑸𝟐
𝑬=½
𝑪
Independent Study
Research and write half a page (OF YOUR OWN
WRITING…I will be checking…) on the use of
capacitors in ‘car courtesy light systems’