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