Physics 3.6

Physics 3.6 - Electrical Systems

Ampere (A)

SI unit for current

Ohm (\Omega)

SI unit for resistance, reactance, impedance

Volt (V)

SI unit for voltage/potential difference

Watt (W)

SI unit for power

Farad (F)

SI unit for capacitance

Henry (H)

SI unit for inductance

Metre (m)

SI unit for distance

Weber (Wb)

SI unit for magnetic flux

Tesla (T)

SI unit for magnetic field strength

Radians (rad)

Units for phase angle. One complete cycle is 2\pi rad

Charge

Eletricity stored in an object due to an imbalance of protons or electrons, creating a force that can be discharges through a conductor

Combination circuit

A circuit containing both series and parallel elements

Current

Flow of electric charge

EMF (Electromotive force)

The force that pushes charges through a circuit

Equivalent resistance

The single resistance value that could replace multiple resistors while maintaining the same current

Internal resistance

Opposition to the flow of current within a battery or power source

Junction

A point where 3 or more conductors meet

Kirchhoff’s current law

The total current entering a junction is equal to the current leaving a junction

Loop

A closed path followed by the current in a circuit

Kirchhoff’s voltage law

In a closed loop, the total voltage is equal to the sum of the voltage drops

Resistance

Opposition to current flow

Ohm’s Law

V=IR

Parallel circuit

A circuit with more than one pathway for current to flow

Power

The rate at which energy is transferred

Series circuit

Circuit with only one way for current to flow

Voltage

The electrical potential difference between two points

Terminal voltage

The potential difference between two terminals of a power source, accounting for internal resistance. V_T=\epsilon-Ir

Capacitance

Ability to store an electric charge between two conducting plates separated by an insulator

Adding capacitance in a parallel capacitor network

Sum of individual capacitances. C_T=C_1+C_2…

Adding capacitance in a series capacitor network

Sum of inverse individual capacitances. \frac{1}{C_T}=\frac{1}{C_1}+\frac{1}{C_2}…

Capacitor construction equation

C=\frac{\epsilon_0\epsilon_rA}{d}

Charge in a series capacitor network

The same across all capacitors - based on voltage applied and equivalent capacitance across the network (C=\frac{Q}{V})

Charge in a parallel capacitor network

Divided among capacitors based on capacitance (higher capacitance stores more charge). Q_T=Q_1+Q_2…

Combination capacitor network

Network containing both series and parallel elements

Dielectric constant

Property of insulating materials measuring ability to store electrical energy in an electric field.

Electrical field

Region around a charged object which exerts a force on other charged objects

Electric field lines

Point from positive to negative

Overlapping area

Where electric field lines from different objects intersect, influencing electric potential

Permittivity of free space (\epsilon_0)

8.85×10^{-12}F\space{m}^{-1}. Represents ability of a vacuum to permit electric field lines

RC circuit

A circuit with a capacitor and resistor

Time constant (RC circuit)

Time for a capacitor to charge or discharge 63% of its total amount

Voltage in a parallel capacitor network

Voltage in a series capacitor network

Each capacitor has a different voltage, based on capacitance. V_T=V_1+V_2…

Eddy currents

Swirling groups of electrical current induced in conductors by changing magnetic fields that create resistance

Induction

Process of generating EMF in a closed circuit by changing the magnetic field through the circuit

Faraday’s Law

The size on an induced EMF is proportional to the negative rate of change of the magnetic flux

Inductance

Property of an abject that opposes change in electric current and stores energy in a magnetic field

Isolating transformer

Device that transfers electrical energy between circuits using induction while providng electrical isolation

Lenz’s Law

Direction of induced current opposes the change that created it (negative sign in Faraday’s law)

Magnetic flux

Measure of magnetic field lines passing through a surface

Primary windings

Coils of a transformer where input voltage is applied

Secondary windings

Coils of a transformer where induced voltage is transferred to

RL circuit

Circuit containing an inductor and a resistor

Self-induction

A changing current in a coil induces an EMF that opposes the change in current

Time constant (RL circuits)

Time taken for the current to reach 63% of its total or reduce by 37%

Step-up transformer

Increases voltage by decreasing current

Step-down transformer

Decreases voltage by increasing current

Angular frequency

The frequency of a steadily recurring phenomenon

Average power

Equivalent to DC power, half of maximum power

Capacitive reactance (X_C)

How much a capacitor impedes the current in a circuit

Inductive reactance (X_L)

How much an inductor impedes the current in a circuit

Frequency

The rate at which something occurs over a period of time

Impedance

The effective resistance of a circuit or component to current from the combined effects of resistance and reactance

Instantaneous voltage/current

The voltage/current at a particular time

In phase

When 2 or more waves with the same frequency reach minimum and maximum at the same time

Leading

Before or in front of

Lagging

Following behind

Period

Time for one oscillation

Phase angle

A phase difference e

Phasor

A line used to represent an electrical quantity as a vector

Resonance

Increase in amplitude of oscillation of asystem exposed to a force equal to or very close to the system’s natural (resonant) frequency

Resonant frequency (f_o)

The natural frequency of a system

V_L ____ V_R by ¼ cycle

leads

V_C ____ V_R by ¼ cycle

lags

Current is zero when

V_C=V_S