Electric current Flashcards by Martine Wilhelmsen

Electric current

The collective motion of particles carrying electric charge. Requires the relatively free motion of charge carriers.

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Conductor

a material that contains charge carriers which can move freely

e.g: metals due to free electron cloud resulting from the metallic bond, and electrolyte solutions due to freely moving ions in liquid phase

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Insulator

When there is no freely moving charge carriers in the material.

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DC - direct current

Is constant in time

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AC - alternating current

changes as a sine function over time

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Direction of current

Defined according to the flow direction of positive charges (conventional current direction). Electrons in metals moves in opposite direction due to their negative charge.

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Electric current

amount of charge passing through a given cross section in a unit of time

I = Δq/Δt

unit: ampere

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Speed of motion of electric charges is dependent on

resistance exerted by conducting material

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Ohms law

U = R*I

R does not depend on voltage

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Law of resistance

R = U/I

Unit: Ohm

Ohmic resistance

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Resistance of a conductor depends on..

Its dimensions and material properties.

Long conductor - weaker electric field, motion of charges is slower, lower electric current - greater resistance

Greater cross sectional area - more charge carriers can pass for a given voltage and electric field strength

R = specific resistance* (l/A)

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Electric conductance

G = 1/R

Unit: S, Siemens

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Specific conductance or conductivity

δ = 1/ρ

Unit: S/m

Is directly proportional to ion concentration within certain limits.

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Connecting resistors:

Series: R= r1+r2+r3+…
Parallel: 1/R = 1/R+1/R+1/R+…

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Joule heating

or work of the electric current

the work done by the electric field when moving electric charge carriers which turns into another form of energy; in case of ohmic resistors it turns completely into thermal energy (heat)

W = UIt

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Electric power

electric work done in unit of time
P=U*I

Unit: Watt

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Electrical circuit

An interconnection of electrical components enabling electric current flow.

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Kirchhoff’s current law/first law/junction law

as a result of conservation of electric charge, the currents flowing into a junction are equal to the currents flowing out of that junction -> in a branched circuit, current is partitioned between the branches.

Kirchhoff’s voltage law/loop law/ second law

As a result of conservation of energy, the directed sum of voltages of electrical components along a loop within an electrical circuit is zero. In a branched circuit, voltage is partitioned between the electrical components.

( increase in capacitor voltage = decrease in resistor voltage)

When voltage of capacitor is equal to the battery voltage, and we remove the battery, the voltage of the capacitor and the voltage of the resistor is equal but with opposite signs.

RC circuit

An electrical circuit consisting of an ohmic resistor and a capacitor. In order to charge a capacitor we will need to connect a voltage source (battery). In order to discharge the capacitor, we have to remove the battery and connect the two poles of the capacitor through the resistor.

Battery voltage is approached…

asymptotically by capacitor voltage

Resistor voltage decreases as capacitor voltage increases
Current decreases as resistor voltage decreases ( Ohm)
Charging is complete when current and resistor voltage is zero.

After charging: insulator between the plates of the capacitor -> the capacitor presents an infinite resistance in a DC circuit

Function for capacitor charging

Uc=Ub(1-e^(-t/RC))

Uc is voltage of capacitor at time t (s)
Ub is battery voltage
R is resistance
C is capacitance

RC = time constant of the RC circuit

Insulator between plates of the capacitor

The capacitor presents an infinite resistance in a DC circuit

During discharging, current flows from…

higher potential (positive plate) to the lower potential (negative plate)

Function for capacitor discharging

Uc=Uo*e^(-t/RC) RC= time constant of the RC circuit.

Alternating current circuit, current function and voltage function

Current and voltage changes periodically following a sine function I= Imax*sinωt U= Umax*sin(ω*t+phase) If the circuit contains only Ohmic resistors, there are no phase shifts.

RMS/root-mean-square values

Effective values in case of sinusoidal AC Ieff= Imax/square2 Ueff= Umax/square2

Capacitor in an AC circuit

presents a finite "resistance" called capacitive reactance., current is maintained: the alternating current keeps continuously charging and discharging the capacitor with alternating electric polarity Capatitive reactance: Xc= 1/(ω*C) high frequency AC has a negligible capacitive reactance

Impedance

If an AC circuit contains both resistors and capacitors and the total "resistance" of the whole circuit is called impedance. symbol: Z U and I has to be considered as vectors perpendicular to each other. The impedance is the hypotenuse, with U and I as adjacent.

Impedance in series and parallel

Series: R^2 + (Xc)^2 = Z^2 Parallel: (1/R)^2 + (1/Xc)^2 =(1/Z)^2

Electrodes

The metallic parts in direct connection with a non-metallic component. Metals where the charge carriers are the mobile electrons in connection with non-metallic components like solutions, tissues, body parts, vacuum etc.

What is the non metallic component of a Galvanic cell/Electrolytic cell/pH meter?

Electrolyte solution

What is the non metallic component of an ECG defibrillator?

The body around the heart

What is the non metallic component of an EEG?

The head

What is the non metallic component of micro electrodes?

The interior or the exterior of a cell

What is the non metallic component of an ultrasound generator?

Piezoelectric crystal

What is the non metallic component of gas discharge lamps?

metal, vapor or gas

What is the non metallic component of a vacuum tube? | Cathode ray tube, X-ray tube of photomultiplier tube

Vacuum

What is the non metallic component of semiconductor devices? | diode, transistor, integrated circuit

Semiconducting crystal

Anode

The electrode that attracts anions(negatively charged ions) from the non-metallic component

Cathode

The electrode that attracts cations (positively charged ions) from the metallic component

(PP) What is the resistance of a 100 m long steel cable it its cross section is 5cm^2? A) 0,02 Ohm B) 2 Ohm C) 500 fOhm D) 2 microOhm

R= specific resistance* l/A A

(PP) A 20 kOhm resistor is connected to a 100 V voltage source. Which of the following statements is correct? A) The electric power is 2 MW B) The electric power is 0,5 W C) The current flowing though it is 200 A D) The current flowing through it is 5 A