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ELECTRIC CURRENT AND DIRECT CURRENT (Electrical measurement devices…
ELECTRIC CURRENT AND DIRECT CURRENT
Electrical conduction
Electric current,I
Direction of electric current
Positive to negative terminal
Direction of electron flows
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 < ε
V = IR
Unir for emf,ε is V
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)
V= V1 + V2 + V3
Equivalent Resistance
R = R1 + R2 + R3
Current (I)
I1=I2=I3
Parallel circuit
Voltage (V)
V1 = V2 = V3
Equivalent Resistance
1/R = 1/R1 + 1/R2 + 1/R3
Current (I)
I = I1 + I2 + I3
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
A device that measures current
Voltmeter
A devices that measure voltage
Ohmmeter
Used to measure the resistance
Galvanometer
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