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Basic Electricity & Magnetism
Atoms are made up of electrons and protons
Electrons are negatively charged particles
Protons are positively charged particles
Protons and Electrons exert force on one another
This additional force is attributed to the charge carried by them, hence protons
repel protons, but get attracted to electrons and electrons repel each other.
If you have a wire and maintain a charge difference across it – then there will be a
charge flow, and the wire carrying the charge is said to be an electric circuit.
Electric Circuit
An electric circuit is one in which charged particles can move. It is a pipeline that
facilitates the transfer of charged particles from one point to another. What it
requires is a difference in charge across its two points.
The total amount of charge at any point is measured in Coulomb. This
difference in charge is called the voltage across the two points.
Hence defining
Voltage
It is the difference in potential or charge across two points and is measured in
volts.
When there is a charge flow across the two end-points – there is a related rate of
charge flow – which is defined by the current in the wire.
Hence defining
Current
It is the change in electric charge with respect to time and is measured in
‘amperes’
Q1 – Q2 / time amperes
I = q/ t
- change in charge / change in time
When there are two points maintained one having a more positive charge than the
other, then the electrons will start flowing towards the positively charged point.
Current flow is measured as
It is the flow of negatively charged particles (electrons) towards the positive
terminal hence it is –I amperes
or
It is the flow of positively charged particles (holes) towards the negative terminal
hence it is +I amperes.
Current types
If you maintain a constant voltage difference between two points – you get a
Direct current: eg batteries – voltage does not change with the time
But if the voltage keeps varying – you will get an alternating current.
Alternating current- eg household power, oscillates between positive and
negative volts with a given frequency
Usage of voltage - you want it to flow across some component – so you attach
the component through which you want a current flow across a battery of AC
points. This component offers a resistance to the current flow and typically the
electric circuit for this is as shown below
V
I
Resistance
Basic DC circuit
battery


According to Ohm – after which the resistance measurement was named If V = 10volts, R = 20 ohms, then V = IR , I = V/R= 10/20 = 0.5 A
Power is given by = VI= 10 * 0.5 = 5 watts
Ohm’s Law
Voltage across a resistance is directly proportional to the current flowing through
it
Resistive circuits with time varying voltages
V(t) = R* I(t), where R is in ohms
P =VI = V2/R = I2R
Under varying voltages some components are there which exhibit different
behavior than the resistance – one example of such a component is the ‘capacitor’
Capacitance (measured in farads)
A capacitor has 2 conductive surfaces separated by a dielectric material. A
dielectric material prevents flow of direct current and is able to hold the
electrostatic charge across the two surfaces. It allows alternating current to go
through. The amount of alternating current it allows depended on its capacitance
and other components in the circuit.
Hence it is used very often to block DC component of current
Used to selectively bypass or filter some selected AC components by proper
values of capacitance and other components.
They can be used for filtering, coupling or bypassing
Stray capacitance is present anytime there is a difference in potential between 2
conducting material separated by a dielectric. Creates unwanted capacitive actions
Capacitance = Total charge in coulombs / Voltage = Q/V
Q = CV
dQ/dt = C dv(t) /dt
i(t) = C dv(t) /dt
Another effect felt in Alternating current is the inductive effect.
Inductance (measured in henry’s)
It is the property of an electric circuit, by which if there is a changing magnetic
field, then associated with it is an electromotive force or voltage.
Hence if there is a conducting wire in the form of a coil, through which there is an
alternating current flowing, associated with it is a magnetic field and an
inductance or inductive effect.
V(t) = L di(t)/dt
1H = 1 volt sec/ampere
Basic AC circuit
Frequency = cycle / sec
Resistor
V
V
I
capacitor
cycle
Inductor
Ohms law for AC circuits
As AC circuits can have different types of components besides resistance –
generally Impedance (Z) replaces R
V = IZ
Z = R + XL + XC
f is the frequency of the alternating current. The inductance and capacitor have
impedance only when al AC current flows through. If the frequency of the
alternating current is f then
XL = 2fL
Xc = 1/ 2fC
Conductance (measured in siemens)
G = 1/R 1 Siemen = 1A/V
i(t) = G v(t)
Magnetism
DC current results in a stable magnetic field
AC current results in a fluctuating (expanding and collapsing) magnetic field
Example
Stable magnetic field – around a magnet
Fluctuating magnetic field around a wire carrying AC current
Magnetic field
A bar magnet has magnetic field – lines of magnetic force called magnetic flux
around them. The magnetic lines flow form north to south pole
Fig 1.
N
N
S
S
Bend the bar magnet to form a horse shoe – magnetic lines flow across the poles
as shown
When a current flows through a conductor there are magnetic lines are around this
conductor. If the current flow is increased – there will be an increase in the
magnetic field
Fig 2.
wire
Direction of
current
Direction of
magnetic field
Right hand rule – this rule gives the relationship between current and direction
of force. Hold the conductor with right hand with thumb pointing in the direction
of current flow. The fingers curls in the direction of the magnetic field
Twist the conductor into a coil. The coil will have current flowing if it is cutting a
magnetic field.
Generators use magnetic force to generate electricity. Mechanical movement is
required to move a coil of wire in a magnetic field thereby cutting the magnetic
field and voltage develops at the ends of wires
Motors use magnetic force to obtain mechanical motion from electrical input, in a
similar way to the generators
Inducing a current
When a conductor cuts through lines of magnetic force – it induces an
electromotive force emf or voltage. The magnetic field or conductor should be
moving for this to happen i.e. to cut the magnetic lines. If the direction in which
the magnetic field is cut changes then direction of emf changes
Faraday’s law – states that induced voltage can be determined by the number of
turns in a coil and how fast the coil cuts through the magnetic field.
Stronger the magnetic field more the current. More turns in the coil – more
current
When coil turns in half a circle, current flows in one direction, if coil turns the
other half circle in a magnetic field, then current is induced in another direction
(changed polarity). This produces alternating current.
Fig 3.
S
N
1
S
N
S
N
S
N
2
1
2
S
N
2
2
1
1
1
To produce DC current – a commutator is required. A commutator, switches
voltage pick up points to produce current flow in one direction. Hence a
commutator converts AC current to DC current.
DC generators
A device that turns rotary motion to electrical energy – DC current in this case.
Brushes are used as electrical pickup points. For one half cycle, the brushes are
making contact with either commutator rings (split). For the next half cycle, when
a reverse current is flowing through the coil, the brushes are making contact with
the opposite commutator rings, thereby making the current flow in one direction.
DC current motors
Motors convert electrical energy into mechanical energy. The flow of DC current
through the coil in a magnetic field makes the coil move. This is the principle
behind the DC motors.
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http://microscopy.fsu.edu/electromag/electricity/generators/