Like this presentation? Why not share!

bt401p

by jeric lora, programmer & webdesigner at school (STI) on Jan 08, 2010

• 1,756 views

electric current

electric current

Views

Total Views
1,756
Views on SlideShare
1,756
Embed Views
0

Likes
1
118
1

No embeds

Categories

Uploaded via SlideShare as Microsoft PowerPoint

Report content

11 of 1 previous next

Are you sure you want to

bt401pPresentation Transcript

• ELECTRIC CURRENT A flow of charge from one place to another. The unit is Ampere , which equal to a flow of 1 coulomb per second.
• Moving charges as a current
• Its described as a stream of moving charges.
• May range very small currents such as the nerve impulses to a large as the solar wind emitted by the sun.
• There must be a “net” flow of charges towards one direction.
• When moving charges is not a current
• When there is no net flow of charge even though there are actual movement.
• Example:
• Electrons of a copper conductor in absence of electric potential.
• Electrons just move randomly the charge flowing charge flowing to one direction is equal to those flowing to the other direction.
• Electric current in a conductor
• An isolated conductor in absence of electric potential contains free flowing electrons but no electric current.
Isolated conductor charges
• continuation:
• A conductor connected to a dry cell or battery has the necessary electric potential to influence the flow of charges towards one direction, hence producing current.
Battery + - Conductor Charges Direction of charges
• continuation:
• Electric current ( I ) is defined as the amount of charges passing through a hypothetical plane intersecting the conductor per unit of time.
• Its unit is coulomb per second (C/s), also called ampere (A).
• Where:
• I = Current (ampere, A)
• Q = Charge (coulomb, c)
• t = Time (second, s)
• continuation: - - - - - - t = t 0 t = t 0 + 1 s plane plane
• continuation:
• Independent to the selection of hypothetical plane
a b c a’ b’ c’ I I
• Sample problem:
• A wire carries a current of 0.8 ampere. How many electrons passes every section of the wire every one second?
• Current is a scalar quantity
• Electric current is moving along a conductor has only two possible directions.
• Electric current are scalars. Adding and Subtracting the current does not consider the orientation of the conductor in space.
• continuation: I 0 I 1 I 2 I 0 = I 1 + I 2
• DIRECTION OF CURRENT
• In reality, electric current are movement of electrons along the conductor.
• For historical reason, current is treated as flow of positive charges to the direction opposite that of the actual movement of electrons.
• continuation:
• These positive charges are not actual particles. They are called holes , vacant spaces where there should be an electron. The charge of a hole is +1.6 x 10 -19 C .
• Electrons are known as negative charge carriers . Holes are known as positive charge carriers .
• Drift Speed
• The net motion of charged particles as a group:
Where: I = electric current (A) n = charge concentration v d = drift velocity (m/s) e = charge of electron A = cross-sectional area of conductor(m 2 )
• Usually very small (10 -5 or 10 -4 m/s) compared to random motion of charges (10 6 m/s)
I in I in A
• Current Density
• Current per unit of cross-sectional area of a conductor.
• A vector quantity with the same direction as the motion of positive charge carriers.
Where: I = electric current (A) J = current density (A/m 2 ) n = charge concentration v d = drift velocity (m/s) e = charge of electron A = cross-sectional area of conductor(m 2 )
• Sample Problem:
• A 491 gauge copper wire has a nominal diameter of 0.64 mm. This wire carries a constant current of 1.67 A to a 4,910 watts lamp. The density of free electron is 8.5 x 10 28 electrons/m 3 . Find the current density and the magnitude of drift velocity.
• Types of Current
• Direct current
• The direction of current is constant.
• The graph of current vs time is a straight line.
• Developed by Tomas Alva Edison
• Soon replaced by alternating current as primary means of transmitting electricity, but still used in battery operated devices.
• continuation:
• Alternating Current
• The direction and magnitude of the current continuously changes between two extremes.
• The graph of current vs time is sinosoid.
• Developed by Nikola Tesla and George Westinghouse , forming rivalry with Thomas Edison on War of the Currents .
• The most commonly used method of electric transmission today.
• Types of Current
• Direct Current
Alternating Current I (A) t (s) I (A) t (s)
• ELECTRIC RESISTANCE
• Electric Resistance
• Property of the conducting medium that weakens the transmission of electric current.
• Denoted as R and its unit is Ohm ( Ω ) .
Where: R = Resistance (Ohm, Ω ) ρ = resistivity ( Ω m) L = Length of the wire (m) A = cross-sectional area of a wire(m 2 )
• Sample Problem:
• A piece of 1.0 m wire has a resistance of 0.19 ohms. Calculate the resistivity of the wire. The cross-sectional area of the wire is 0.5 mm 2 .
ρ L A
• Resistivity & Conductivity
• Resistivity ( ρ )
• Measure of how much resistance a material possesses against electric current.
• Intrinsic property of a material that depends on its electronic structure.
Conducting material Electric field
• continuation:
• Measure by placing the material between two plates with constant electric field ( E ) and taking the ratio of electric field and current density ( J ) .
• Varies with temperature
Where: ρ = resistivity ( Ω m) E = electric field (N/c) J = current density (A/m 2 )
• Conductivity
• Measure of how the material is capable of conducting electricity.
• Reciprocal of resistivity.
continuation:
• Variation of Resistivity with Temperature
• Over a wide range of temperature, the graph of resistivity vs temperature of metal is linear.
400 200 0 1200 1400 2 8 0 4 6 10 600 800 1000 Resistivity 10 -8 Ω m Room temperature Temperature (Kelvin)
• Variation of Resistivity with Temperature
• Thus it can be represented by a Linear equation.
Where: ρ = resistivity ( Ω m) ρ 0 = resistivity at room temperature ( Ω m) T = temperature (Kelvin,K) T 0 = room temperature (K) α = coefficient of resistivity (K -1 )
• continuation:
• The Temperature coefficient of resistivity ( α ) determines how much resistivity increases with temperature.
• Its unit is (per Kelvin)K -1 .
• Sample Problem:
• What is the resistivity of iron at 200K? Use the values of resistivity (at room temperature) and temperature coefficient of the resistivity in the handout.
• Ohm’s Law
• The current I (Ampere, A) is directly proportional to the potential difference V (Volt,V) with resistance R (ohms, Ω ) as the proportionality constant.
• continuation:
• Assumed that the resistance does not vary with voltage or current.
• Not all conducting material follow “Ohm’s Law”. Those are follow are said to be ohmic , while those that do not are said to be non ohmic .
• Current Potential Difference graph of a 1000 W resistor , an Ohmic device. -4 -2 0 +2 +4 -2 +2 0 Current (mA) Potential Difference (V)
• Current vs Potential Difference graph of a pn junction diode , a non-ohmic device. -4 -2 0 +2 +4 -2 +2 0 Current (mA) Potential Difference (V)
• Single Loop Circuit
• Circuit
• Close network of electronic devices through which current constantly flows.
EMF Device Maintain potential difference. Provides steady flow of charge. EMF stand for Electromotive force . R EMF I + - + - I
• The Resistor
• Provides a resistance to the circuit.
• It was specially designed to only provide certain amount of resistance.
• An Ohmic device
• Such conductor device.
• It was verified experimentally by the German physicist Georg Ohm (1787-1854).
• Electromotive Force
• A circuit consists of electrons from the negative terminal of a battery to the positive terminal of the battery.
• Electrons must then return to the negative terminal, or current will stop flowing.
• The electron are forced into this higher potential by a electromotive force.
EMF
• continuation:
• EMF Devices:
• Battery or Dry Cell
• Electrochemical Cell
• Electric Generator
• Photovoltaic Cell
• Internal Resistance
• The resistance found inside real batteries
• Lessen the output voltage of the battery.
• Denoted as r i
• Its SI unit is Ohms (Ω) .
• A real battery is now drawn as:
continuation: EMF r i
• continuation:
• Terminal Potential Difference ( TPD )
• The output voltage of a source of emf after internal resistance takes effect.
• The equation used to solve for terminal potential difference is:
• TPD = E – Ir i
• Where:
• TPD = voltage across the source (V)
• E = voltage if the source is ideal emf (V)
• r i = internal resistance of the source (Ω)
• I = current flowing through the battery (A)
• Sample Problem:
• A 6.0 V battery is connected to an external 6.0 0hms resistor.
• What is the value of the current flowing through the external circuit if there is no internal resistance,
• What is the value of the current flowing through the external circuit when the internal resistance is 0.3 ohms?
• Resistors in Single Loop Circuit
• Where: R is resistance, I is electric current and V is electric potential difference.
Resistors in Series Circuit. R 3 V T I T + - + R 2 + R 1 + - - - R T
• Equivalent resistance in a Series Circuit
• Sample problem:
• Resistors R 1 = 2.00 ohms, R 2 = 3.00 ohms and R 3 = 4.00 ohms are in series connection with a voltage source of 100.0 volts. Find the equivalent resistance, electric current and electric potential difference.
• Resistor in Parallel Circuit
R 3 V T I T + - + R 2 + R 1 + - - - R T I 3 I 2 I 1
• Equivalent resistance in a Parallel Circuit
• Sample problem:
• Resistors R 1 = 3.00 ohms, R 2 = 5.00 ohms and R 3 = 7.00 ohms are in parallel connection with a voltage source of 110.0 volts. Find the equivalent resistance, electric current and electric potential difference.
• Resistors in Single Loop Circuit
• Resistor in Series-Parallel Circuit
R 3 V T I T + - + R 2 + R 1 + - - - R T
• POWER IN CIRCUITS
• The Power in the Circuits
• Flow of current across a circuit.
• continuation:
• Movement of a charge across a electric device:
• It moves from higher potential to lower potential.
• Hence, there is a decrease in potential energy.
Q
• continuation:
• If there is a decrease in potential energy, there must be a transmission to another form of energy.
• Light bulb: to heat and light.
• Electric motor: to mechanical energy
• Resistor: Internal energy/heat.
• continuation:
• The rate at which electric potential energy is transformed to another form of energy is the POWER in the circuit.
• Sample Problem:
• A current flowing through a 25.0 ohm resistor is 2.0 A. How much power is dissipated by the resistor.
• MULTILOOP CIRCUIT
• Provides multiple paths for current.
• When one component was cut-off, others can still function.
• What happen when one component in a series circuit was cut-off?
• What happen when one component in a multiloop circuit was cut-off?
• continuation:
• Current in a Multiloop Circuit
• The point where three or more segments of the conductor meet is called the junction.
• The current split at the junction.
Junction current
• GUSTAV KIRCHHOFF
• German physicist who, in the collaboration with Robert William Bunsen, develop ed the science of spectrum analysis.
• He showed that each element, when heated to incandescence.
• He produced a characteristic pattern of emission lines.
• He formulated Kirchhoff’s Law for electric circuit.
• (1824-1887)
• In any closed circuit, the algebraic sum of all EMF’s and potential drop is equal to zero. (Using loop direction)
KIRCHHOFF’S LAW R 2 + Emf 1 + - R 1 + Emf 2 + - R 3 + Loop 1 Loop 2 I 1 I 2 I 3 -
• At any point in a circuit, the sum of the currents leaving the junction point is equal to the sum of all the current entering the junction point. (Using current direction).
KIRCHHOFF’S LAW R 2 + ε 1 + - R 1 + ε 2 + - R 3 Junction point I 1 I 3 I 2 +
• Sample Problem:
• In a given circuit below, Find: a) I 1 , b) I 2 and c) I 3
10 Ω + 9v + - 15 Ω + 12v + - 5 Ω I 1 I 3 I 2 +
• RC CIRCUIT (Resistor and Capacitor in a circuit)
• Resistor- Capacitor in a circuit.
R + - C S 1 S 2 ε + - Where: ε = Batteries (Emf) S 1 & S 2 = Switches R = Resistor C = Capacitor Open Close
• Charging a capacitor R + - C S 1 S 2 ε + - I I I I I closed open Where: V R = Potential difference across the resistor. V C = Potential difference across the capacitor. I
• continuation
• Current I O at the moment S 1 closed ( t = 0)
• Current I at any time t after S 1 closed:
• After some time t
• The charge of the capacitor (q) increases
• Current ( I ) decreases.
• continuation
• Until the capacitor reaches its equilibrium charge (q eq ), happen when V C reaches V C = ε , which result to I = 0
• continuation
• Charge and current of the capacitor at any given time t after t = 0.
• The time constant ( τ ) of RC series circuit.
• The unit of time constant is second.
• At time t = τ
• Q = 0.63 C ε
• I = 0.37 I o
• The charging time of RC circuits are often stated in terms of time constant.
continuation
• Sample Problem:
• A resistor with resistance R=1.0 x 10 6 Ω , capacitor with capacitance C=2.2 x 10 -6 F, a voltage source with ε = 100 v, and a switch are all connected in a single loop series circuit. The switch is initially open. When the switch is closed, calculate:
• Initial current across the resistor
• Equilibrium charge of the capacitor
• Time constant of the circuit
• Current through the resistor after 5 seconds
• Charge of the capacitor after 5 second
• Charge of the capacitor at t = τ