Physics Electricity flashcards

Electric current

Rate of flow of charged particles in motion (ions, electrons, protons)

• Rate of flow of charge per unit time

• Measured in Amperes (A)

• Known as ‘I’ in scientific notation

Charge

Property of matter that causes electrical effects.

• Property of protons and electrons

• Known as ‘Q’ or ‘q’ 

• Charge measured as Coulombs

• Similar charges tend to repel

• Different charges tend to attract

• Charge of a proton 1.6x10^-19 C

• Charge of an electron -1.6x10^-19 C

• 1 coulomb - the charge that flows past a point in one second when there is a current of 1 A

• Relation between charge and current: I = ΔQ/ΔT 

Conventional current

in the opposite direction to the actual flow of the electrons

• The usual carrier is the electron, which carries a negative charge

• Conventional always in positive direction (+ to -)

• The actual flow is negative to positive

Charge carrier

electrons or ions in electrolysis

• In an insulator the electrons are attached to atoms so they cannot move (not delocalised), therefore applying a P.D. Across the insulator will not create a current

• Most electrons are attached to atoms in metal but some are not (some delocalised), therefore a P.D will attract electrons to the positive terminal of the metal and create a current

Semiconductors

the number of charge carriers changes with temperature

• As temperature increases it liberates (delocalises) electrons (charge carriers), so resistance is reduced - this is crucial in the design of computer silicon chips.

Conductor properties when temperature changes

the amount of resistance increases as the temperature is increased and current decreases when resistance and temp increases

Batteries

uses chemicals to store energy, which is released when the battery is used until it is depleted

Rechargeable batteries

contains chemicals where the reactions can be reversed if a potential difference is applied.

Electromotive force (EMF)

Amount of chemical energy transferred / converted to electrical energy for 1 C of charge (through the battery)

Potential difference (P.D.) 

energy emitted per unit charge on a component or work done per unit charge

• [JC^-1] or [V]

• Certain amount of work used to move the charge a certain difference - increase electrical potential

• The greater the distance moved, the greater the P.D.

1 volt

the potential difference between two points when 1 joule of work is done to move a charge of 1 coulomb

Power equations

• P=V²/R

• P=I²R

• P=IV

• P=W/t

Power

rate of transfer of energy per unit time (rate of work done per unit time)

Resistance [Ω]

a material’s opposition to the flow of electric current; measured in ohms

• caused by the repeated collisions between the charge carriers in the material

• Equation: Resistance = P.D. across component / current through it

• Unit resistance is the Ohm [Ω] = 1 volt per ampere

Resistance through conductors

• As electrons move through a conductor, they collide with atoms in the metal

• Higher resistance, more energy transferred as current moved

Ohm’s law

The P.D. across a metallic conductor is proportional to the current through it provided the physical conditions do not change (e.g. temperature).

Resistivity

a measure of resisting power of a specified material to the flow of current

• Can be calculated by finding the cross sectional surface area of the wire and the length of the wire.

Resistivity units

Ohm meters (Ωm)

Resistivity equation

• ⍴ = RA/L

• Resistivity = (resistance x area)/ length

Superconductors

a wire or device that has zero resistivity at and below a critical temperature.

• Used in power transmission in overhead cable to negate the resistance

• Used to make extremely strong and stable magnetic fields which can be used in particle accelerators, MRI machines and nuclear fusion reactors

• Below a critical temperature, as the higher a temperature, the higher the resistance

superconductivity

property of a material that has zero resistivity at and below a critical temperature.

critical temperature

specific temperature at which a superconductor's electrical resistance drops to zero, allowing it to conduct electricity with no energy loss

Cell

A source of electrical energy

Diode

Allows current to flow one direction only (used to convert AC to DC current)

Light emitting diode (LED)

A diode which emits light when current passes through it. Used in aviation lighting and displays.

resistor

Limits the flow of current. Fixed resistors has a resistance it cannot change

variable resistor

Resistor with a slider that cna be used to change its resistance. Often used in dimmer switches and volume controls

Thermistor

Resistance of a thermistor decreases when temperature increases and vice versa [TURD - Temperature up [increases], resistance down)

Light dependent resistor (LDR)

Resistance of LDR decreases when light intensity increases and vice versa. (LURD - Light up [increases], resistance down)

heater

Converts electrical energy to heat

Electric motor

a device that converts electrical energy into mechanical energy

Indicator or light source

emits light

Ammeter

A device used to measure current in a circuit, put in series

voltmeter

A device used to measure voltage, or electrical potential energy difference, put in parallel

rheostat

type of variable resistor

Using a rheostat to determine resistance in components

component in parallel to rheostat with ammeter and voltmeter in parallel to component

Using a variable resistor to determine resistance in components

put into a series circuit with an ammeter and voltmeter parallel to component, both positive and negative values taken by reversing polarity of power source

Resistor IV graph characteristics

ohmic conductor

Filament lamp IV graph characteristics

non ohmic conductor - resistance increases as current increases, due to electrons hitting metal lattice, causing it to disobey Ohm's law

Diode IV graph characteristics

non-ohmic - lets conventional current through only (acts as a one way valve)

Thermistor IV graph characteristics

Ohmic conductor

Resistance in series circuits

• Total resistance in series circuit is the sum of the resistances

• Rt = R1+ R2+ R3

Kirchhoff's First law (conservation of charge)

Current going into the junction is equal to current going out of the junction

• I_t =I₁ + I₂

Kirchhoff’s second law (conservation of energy)

the sum of electromotive forces (voltages) ad potential drops in any closed loop of electrical circuit must be equal to zero OR the total voltage from sources equals the total voltage across components within that loop

Node

point of a junction in a circuit where current is split into 2 or more branches

Resistance in parallel circuits

• 1/Rt = 1/R1 + 1/R2 + 1/R3

• Rt = R1R2 / R1+R2

Resistance heating

When current flows through a component that offers resistance to it, then energy is transferred as heat

Cells in series circuits

εt = ε1 + ε2 …εn

• total emf (ε) is the sum of the emf’s of the cell (when in series)

• It makes no difference if there are components between the cells (includind internal resistance)

• Cells must be the same orientation in the circuit or the emf cancels out

Circuits with identical cells in parallel circuits

• For a circuit with n identical cells in parallel the current through each cell = l / n , where l is the total current applied by the cells

• So the lost P.D. in each cell - I/n r = Ir/n, where r is the internal resistance of each cell

• So the terminal PD across each cell, V = ε- Ir/n

Diode in circuits

• Diodes offer very high resistance in one direction so current only passes one way

• If not specified, then assume a forward biased diode has a P.D. across it of 0.6V and infinite if reversed

total current equation

I_t  = ε_t / R_t

Potential divider

circuits that produce an output voltage as a fraction of the input voltage by using two resistors in series that split the P.D. from the source between them 

• One of the resistors is fixed and the other can change resistance (e.g. variable resistor, LDR or thermistor)

• Varying resistance of one of the resistors then changes the proportion of the P.D. that it uses, using it to control an output P.D. across one of the resistor

Uses of potential dividers

• supply a P.D. which is fixed at any value between zero and the source P.D

• to supply a variable P.D.

• to supply a P.D. that varies with a physical condition such as temperature or pressure

supplying a variable P.D.

• Instead of having two fixed resistors, it may be more useful to have a sliding point along a piece of uniform resistance wire or alternati vely a sliding contact i a dium formation

• Creates a ratio of resistances between the two parts of the resistor

Sensor circuit

• Use an LDR or thermistor to create a circuit based on light or temperature conditions

• Can be used to directly feed P.D. to a device (e.g. cooling fan) or to turn a switch on and off on a secondary circuit (solenoid system)

• Variable resistor often takes place of fixed resistor from potential divide so the circuit can be turned (i.e. setting temperature which the ricuit will turn on and off)

thermal runaway 

an uncontrollable, self-accelerating chain reaction where a rise in temperature causes further temperature increases, often leading to a destructive outcome. begins when the heat generated by internal reactions exceeds the heat dissipated, creating a positive feedback loop that can cause fires or explosions.

Potential divider using variable resistor

Potential divider using fixed resistor and varying length of wire

Temperature sensor

• Consists of a potential divider made using a thermistor and a variable resistor

• With temperature constant, P.D. is shared in series

• By changing resistance of variable resistor, P.D. across thermistor can be set at required value

• When temperature of the thermistor changes, its resistance changes so its share of P.D. across changes

Light sensor

• Consists of a potential divider made using a LDR and a variable resistor

• Share of P.D. across LDR changes when the light intensity falling onto LDR changes

• E.g. if Light intensity increases, LDR resistance decrease, therefore share of P.D. across LDR decrease

Potential divider equations

• V_out = V_in x (R₂ / R₁+R₂)

• V₁ / V₂ = R₁ / R₂