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ELECTRIC CURRENT AND DIRECT CURRENT CIRCUIT (7. Electrical measurement…
ELECTRIC CURRENT AND
DIRECT CURRENT CIRCUIT
2. Resistivity and Ohm’s Law
Resistor
Is that property which opposes or limits current in an electrical circuit
Is defined as the ratio of the applied voltage (PD) to the current that flows through the
conductor
Resistivity - Is defined as the resistance of a unit cross-sectional area per unit length of the material.
Depends on
Type of material
2.length
3,cross-sectional area
temperature
Conductivity
Is defined as the reciprocal of the resistivity of a material
Electrical conductivity = σ = 1/ρ
Ohm's Law
States that the voltage drop across a conductor, V is proportional to the current, I through
it if its physical conditions & temperature are constant
7. Electrical measurement devices
Galvanometer
Is a current- sensitive device whose needle deflection is proportional to the current through its
coi
Ohmmeter
Used to measure the resistance
Consists of a meter, a resistor & a source connected
in series
Shunt (Pemirau)
Galvanometer can be converted to a useful ammeter by placing a shunt resistor Rs in
parallel with the galvanometer
Voltmeter
A device that measures voltage
Are connected in parallel or across the element
whose voltage they are measuring
Multiplier (Pendarab)
Galvanometer can be used as a voltmeter by adding a multiplier resistor RM in series with
it
Ammeter
A device that measures current
Connected in series with the element whose current
they are measuring
3. Electromotive Force (emf), Internal
Resistance and Potential Difference
Is defined as the resistance of the chemicals inside the battery (cell) between the poles
Consider a circuit consisting of a battery (cell) that is connected by wires to an external resistor
R
6. Kirchhoff’s Laws
2nd Law
States in any closed loop, the algebraic sum of emfs is equal to the algebraic sum of the
products of current and resistance
Problem Solving Strategy
Choose and labelling the current at each junction in the circuit given
Choose any one junction in the circuit and apply the Kirchhoff’s first law
Choose any two closed loops in the circuit and designate a directionto travel around the loop in applying the Kirchhoff’s second
law
Solving the simultaneous equation to determine the unknown currents and unknown
variables
1st 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
4. Electrical Energy and Power
Electrical Energy
Consider a circuit consisting of a battery that is connected by wires to an electrical device
W = QV
W = E = VIt
Q = It
Power
Is defined as the energy liberated per unit time in the electrical device
P = VI
P = I^2R
P = V^2/R
5. Combinations of Resistors
Series
V =V1+V2 +V3
RE = R1 R2 R3
I1=I2=I3
Parallel
V1=V2=V3
1/RE = 1/R1+1/R2+1/R3
I = I1+ I 2 + I 3
1. Electrical Conduction
Electric current
Electric current, I is defined as the total (net) charge, Q flowing through the area per unit time, t.
Q = It
Current density
Is defined as the current flowing through a conductor per unit cross-sectional area
J = I/A
Drift velocity of charge
Consider a metal rod of length L and cross-sectional area A, which is applied to the electric field
n = N/V
V = AL
n = N/AL
N = nAL