PHYS1001 Lecture Notes - Lecture 11: Rc Circuit, Electromagnetism, Farad
Lecture 11- Electromagnetism
Capacitance-
• A battery is connected to a circuit and as the current flows charge builds on the
plates
• More work is therefore need by the electrons to make it to the plate
• Eventually an equilibrium is reached where the force supplied by the battery is equal to
the repulsion by the electrons, built up on the plates —> everything stops
• At equilibrium, current stops flowing and:
◦ Q=C∆V
◦ Q-Capacitance
◦ SI unit is Farads
▪ 1F=1C/V
• Capacitors store charge, and are electrical “tanks” e.g. parallel plates
• PIC
• PIC
• The loop rule: The potential difference around an closed loop of a circuit is zero. The
follows from conservation of energy.
• The current node rule: In steady state, the current entering a node is equal to the current
leaving a node
• Resistors in series
◦ Resistances add- add more resistors, resistance is increased—> because length
is increased and R=L/oA
◦ All electrons have to go through all resistors
• Resistors in parallel
◦ Add more resistors, resistance is reduced
◦ Several paths for electrons
◦ *think of water pipe analogy
• Capacitors in parallel
◦ Capacitance adds
◦ More area, more places to store charge
• Capacitance in series
◦ Capacitance is reduced
◦ Cannot be stored on inside capacitors
◦ Only stored on two edges —> but they are far apart
• In an RC circuit after the power has been taken away the circuit can continue to
function for a short time while the capacitor discharges
Charge:
• a fundamental property of a particle, which quantifies how the particle responds to the
electromagnetic force
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• Types: positive and negative
• Like charges repel one another
• Opposite charges attract one another
• Matter is made up of elementary particles that can be charged (can't be broken down
and isolated, fundamental)
• --> Charge is quantified and conserved
• The charge of a single proton is the fundamental quantum of charge --> sometimes
called the elementary charge, e
• An electron has charge -e
Coulomb's law
The SI unit of charge is a Coulomb
Force between two charges at some distance apart (use this formula when given a
location in the electric field)
• Electric force is much greater than gravity e.g. Muscle contractions
◦ A lot of the time there a positive and negative electric forces- so they cancel
each other out
Electric Field
• Charges generate electric fields
• The local electric field determines the force on any given charge
Using Coulomb's law and the definition of electric field, the field of a point charge is:
• A positive charge experiences a force in the direction of the electric field at its
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location
• A negative charge experiences a force in the opposite direction from the electric field
at its location
• Vectors arrows are drawn and the length of the arrow is determined by the magnitude
of the field
◦ The tail is drawn from the location where the magnitude of the field is
measured
Note: a point charge is a charged object whose size is small compared with r. Typically,
protons and electrons are a good example, but so is a solid charged metal sphere if you're
to look outside the sphere at the Centre point of the sphere
Field: describes a quantity at every point in space (and time)
Vector field (magnitude and direction)
• Weather forecast across Australia is a scalar field, not a vector field --> only
magnitude
• Electric field of proton:
◦ Arrows point away from it
• Electric field of electron:
◦ Arrow point towards it
• Gravitational field: in direct analogy with electric field: a mass gravitational field is the
force per unit mass
◦ F=Ma so f/m=a --> it's an acceleration field
◦ If you take any mass and put it in a gravitational field it's going to accelerate by
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Document Summary
1f=1c/v: at equilibrium, current stops flowing and, capacitors store charge, and are electrical tanks e. g. parallel plates, pic, pic, the loop rule: the potential difference around an closed loop of a circuit is zero. The follows from conservation of energy: the current node rule: in steady state, the current entering a node is equal to the current leaving a node, resistors in series. Resistances add- add more resistors, resistance is increased > because length is increased and r=l/oa. All electrons have to go through all resistors. *think of water pipe analogy: resistors in parallel, capacitors in parallel, capacitance in series, in an rc circuit after the power has been taken away the circuit can continue to. Only stored on two edges > but they are far apart. More area, more places to store charge function for a short time while the capacitor discharges. The si unit of charge is a coulomb.
