Chapter 3: Electric Current & Direct Current Circuit (aiman T17)
3.1 Electrical Conduction
Electric current, I
- Direction of electric field/ electric current: ( positive to negative terminal )
- Direction of electron flows: ( negative to positive terminal )
I = Q/t
Unit: ampere(A)
I - total (net) charge
Q - charge
t - time
1 ampere = 1 coulomb/ 1 second
Current Density,I
- current flowing through a conductor per unit cross-sectional area.
J = I/A
Unit: ampere per square meter (Am^-2)
I - electric current
A - cross sectional area of conductor
Direction of current density always same direction of the current,I.
Drift velocity of charges,Vd
N = nAL
N - no.of free electron
n - free electron
L - length
A - cross sectional area
Total charge,Q of free electron that pass through the area,A along rod is
Q = Ne
Vd = J/ne
J = I/A
n - density of free electron
e - charge of electron
3.2 Resistivity And Ohm's Law
Resistance (R)- is that property which opposes or limits current in electrical circuit.
- defined as ratio of the applied voltage (PD) to the current that flows through the conductor.
R = V / I
Unit: ohm @ VA^-1
When R is constant, V increase, I increase.
Resistance of conductor depends on:
- Type of material made
- Length (L)
- Cross-sectional area (A)
- Temperature (T)
Resistivity,p
- the resistance of a unit cross-sectional area per unit length material.
Conductivity
defined as the reciprocal of the resistivity of the material.
Ohm's Law - states that the voltage drop across a conductor,V is proportional to the current,I through it if its physical conditions & temp. are constant.
[ V = IR ]
3.3 Electromotive Force (emf), Internal Resistance & Potential Difference
Emf and Potential Difference,V
Emf - the energy provided by the source to each unit charge that flows through the external & internal resistance.
Terminal potential difference,V - the work done in bringing a unit charge from the negative to positive terminal of battery through external resistance only.
Internal resistance of a battery,r
the resistance of chemicals inside the battery between the poles.
3.4 Electrical Energy & Power
Electrical energy,E
The work done on this charge,Q from B to A:
Power,P
- the energy liberated per unit time in the electrical device.
P = IV
When the electric current flow through wire or passive resistor, hence the potential difference across it is V=Ir then electrical power is:
Unit: watts (W)
3.5 Combination of Resistors
3.6 Kirchhoff's Laws
states that algebraic sum of the currents entering any junctions in a circuit must equal the algebraic sum of the currents leaving that junction.
Kirchhoff's Second Law ( loop or voltage law )
states in any closed loop, the algebraic sum of emfs is equal to the algebraic sum of the products of current & resistance.
3.7 Electrical Measurement Devices
Ammeter
- device that measure current.
connected in series.
Voltmeter
- device that measure voltage.
connected parallel.
Ohmmeter
- used to measure the resistance.
consists of a meter, resistor & source connected is series.
Galvanometer
- a current-sensitive device whose needle deflection is proportional to the current through its coil.
operates on magnetic principles.
Shunt & Multiplier
Shunt - Galvanometer can be converted to a useful ammeter by placing a shunt resistor in parallel with the galvanometer.
Value of Rc must less than the galvanometer resistance.
Multiplier - Galvanometer can be used as a voltmeter by adding a multiplier resistor in series with it.
Rc must have a value much greater than the resistance of the galvanometer.