ELECTRIC CURRENT AND DIRECT CURRENT
Electrical conduction
Electric current,I
Direction of electric current
Direction of electron flows
Positive to negative terminal
Negative to positive terminal
I = Q / t
Current density,J
Defined as the current flowing through a conductor per unit cross-sectional area.
J = I / A
Electrical conduction in metal
When the electric field is applied to the metal
Freely moving electron experience an electric force
Drift with constant average velocity
Drift velocity of charges,Vd
Vd = I / nAe
Vd = J / ne
Resistivity and ohm's law
Resistance (R)
Defined as the ratio of the applied voltage (PD) to the current that flows through the conductor
R = V / I
Resistance of a conductor depends on
Type of material it is made
Length
Cross-sectional are
Temperature
For a given constant temperature
R =pL / A
Resistivity,p
Defined as the resistance of a unit cross-sectional area per unit length of the materai
p = RA / l
Conductivity,σ
Defined as the reciprocal of the resistivity of a material
σ = 1 / p
Ohm's law
The voltage drop across a conductor,V is proportional to the current,I through it if its physical conditions and temperature are constant
V=IR
J = σE
Electromotive force (emf),Internal resistance and potential difference
Emf,ε and potential difference, V
V = ε - Ir
V = IR
Unir for emf,ε is V
V < ε
Internal resistance,r
Defined as the resistance of the che,icals inside the battery between the poles
Vr / I
Electrical energy and power
Electrical energy,E
Q = It
W = QV
W = E = VIt
Power,P
Defined as the energy liberated per unit time in the electrical device
P = W/t = VIt/t
P = IV
P = I^2 R
P = V^2/R
Combination of resistors
Series circuit
Voltage (V)
Equivalent Resistance
Current (I)
I1=I2=I3
V= V1 + V2 + V3
R = R1 + R2 + R3
Parallel circuit
Voltage (V)
Equivalent Resistance
Current (I)
I = I1 + I2 + I3
V1 = V2 = V3
1/R = 1/R1 + 1/R2 + 1/R3
Kirchhoff's Laws
First Law
States the algebraic sum of the currents entering any junctions in a circuit must equal the algebraic sum of the currents leaving that junction
E (I) in = E (I) out
Second Law
States in any closed loop , the algebraic sum of emf is equal to the algebraic sum of the products of current and resistance
E (ε) = E (IR)
Electrical measurement devices
Ammeter
Voltmeter
Ohmmeter
Galvanometer
A device that measures current
A devices that measure voltage
Used to measure the resistance
A current-sensitive device whose needle deflection is proportional to the current through its coil
Shunt
Convert galvanometer to useful ammeter
Multiplier
Convert galvanometer to a voltmeter