Side A - Electricity

Define Current

Rate of flow of charge

Carried by electrons in a wire

What is conventional current flow?

Electric current moving from positive to negative

What is electron flow?

Electrons in a circuit moving from negative to positive

Current equation

Current (I) = Change in charge (Q) / Change in time (t)

I = A

Q = C

t = s

Q = I x t

t = Q / I

Define potential difference

Work done/energy transferred per unit charge passing through circuit

Potential difference equation

Voltage (V) = Work done (W) / Charge (Q)

V = V (1 volt = 1 joule per coulomb)

W = J

Q = C

W = V x Q

Q = W / V

Define electromotive force (emf)

Energy supplied per unit charge by power supply

Electromotive force (emf)

Emf (ϵ) = Energy supplied (E) / Charge (C)

ϵ = V

E = J

Q = C

E = ϵ x Q

Q = E / ϵ

Define resistance

Measure of difficulty of making current pass through substance

caused by repeated collisions between charge carriers and positive ions

Resistance formula

Resistance (R) = pd across component (V) / current through component (I)

R = Ω

pd = V

I = A

V = I x R

I = V/R

How are ammeters connected?

In series

So they experience no resistance or as little as possible to impede the current flow

How are voltmeters connected?

In parallel so resistance is as high as possible so ammeter only measures current through resistor

So they have a high resistance

Also allows for them to measure V at 2 points and find the difference between them (pd)

Electrical power formula

Power (P) = Current (I) x Voltage (V)

P = W

C = A

V = V

I = P/V

V = P/I

P = I² x R

P = V² / R

What is Ohms law?

p.d across ohmic conductor is proportional to current through it, if conditions dont change (temp and dimension of conductor)

What does Ohmic mean?

Obeying Ohms law

On a graph you can find resistance using gradient as it is a proportional and straight line

On I-V graph the steeper the gradient, the lower the resistance

What I-V graph is this, and what are the characteristics of the component?

Filament Bulb

Not Ohmic - current and voltage aren’t proportional so gradient doesn’t equal resistance

The resistance increases as current increases, as when current increases temp increases and temp increases resistance

What I-V graph is this, and what are the characteristics of the component?

Thermistor

Ohmic - If temp remains constant

When temp is higher there are more charge carriers and free electrons - so its a better conductor, so there is less resistance

Resistance decreases as temp increases

What I-V graph is this, and what are the characteristics of the component?

Silicon Diode

Not Ohmic - as resistance varies depending on direction/bias

High resistance (so little current) until reaching threshold voltage (0.6V), when resistance drops and current rapidly increases

Increase in voltage frees electrons, so there are more charge carriers to carry current

Characteristics of reverse direction/bias diode

Very high resistance

current below 1 μA

Characteristics of forward direction/bias diode

When p.d below 0.6V (threshold voltage) - resistance is high

when p.d above 0.6V resistance falls rapidly - so current increases rapidly

Threshold voltage of LED (light emitting diode) and what is a LED

1.5V

Diode that emits light when conducting

What I-V combination is this?

Diode + Resistor in Series

Current increases at threshold but with less steepness, as current still acted on by fixed resistor

What I-V combination is this?

Diode + Resistor in parallel

resistance of resistor acting on current initially and proportionally

until diode switches on and resistance falls and current increases

Characteristics of metal conductors

Slow resistance increase with temperature increase

Positive temperature coefficient - resistance increases as temp increases

Positive ions in conductor vibrate as temp increases, so charge carriers (electrons) cant pass through conductor easily when p.d applied

Characteristics of Semiconductors

Fast resistance decrease with temperature increase

Negative temperature coefficient - Resistance decreases as temp increases

No. of charge carriers rapidly increases with temp, compared to the impedance (opposition) of the vibrating positive ions

(more charge carriers so resistance becomes smaller)

Resistivity equation

ρ (resistivity) = R (resistance) x A (Cross sectional area) / L (length)

ρ = Ωm

each element has a different resistivity

Cross sectional Area equation

A = πr²

What is superconductivity

State where materials have 0 resistivity

they are man made compounds with no resistivity at a critical temp

critical temp depends on material but is usually below 200C

Used in very strong electromagnets (MRIs) and superconducting magnets (Large hadron collider)

What component is this?

Ammeter

Measures current in Amps

Connected in series

What component is this?

Voltmeter

Measures Voltage across a component at 2 different points

Connected in parallel

What component is this?

A cell

A singular battery

Source of chemical energy

Short = negative, Long = positive

What component is this?

Filament lamp

emits light

as it heats up it converts electrical energy into light energy

What component is this?

Diode

Arrow is forward direction

What component is this?

Light Emitting Diode

emits light when diode conducts

What component is this?

Resistor

component designed to have certain resistance

limit current

What component is this?

Variable resistor

used to set resistance to specific value

What component is this?

Thermistor

resistance decreases as temp increases

What component is this?

Light dependant resistor

Resistance decreases as light intensity increases

Graph same as thermistor

What component is this?

Heater

What component is this?

DC (direct current) motor

they convert electrical power into mechanical

What is the resistance component used to control resistance in a level circuits?

Potential divider

You can move it all they way from 0 to max volts giving you more control over your components

Characteristics of series circuit

All components have same current (It = I1 = I2)

Total resistance is sum of individual resistances (Rt = R1 + R2) greater total R

Total voltage is sum of individual voltage (Vt = V1 + V2)

If one bulb blew all remaining bulbs would turn off

Characteristics of parallel circuits

Total current leaving junction = total current entering it (Kirchoffs’s 1st law) (It = I1 + I2)

1/Rt = 1/R1 + 1/R2 - Smaller total R

All components have same voltage (Vt = V1 = V2)

Kirchoff’s 2nd law

In any complete circuit loop the sum of all emfs i equal to the sum of all potential energy

Power in series

current = same

resistance = greater

pd = greater

greater pd = greater power = brighter lighting bulb

Power in parallel

voltage = same