P2 Physics

What is direct potential difference

Ohmic Conductor

A conductor that obeys Ohm's Law

Ohm's Law

V=IR

A linear component

IV Characteristic of a straight line (fixed resistor)

A non-linear component

Curved IV Characteristic (filament lamp or a diode)

LDR Resistor

A reistor that is dependant on light

How does an LDR resistor effect the light intensity

In bright light resistance falls
In dark light resistance is highest

When are LDR resistors used?

Automatic night lights
Outdoor Lighting
Burglar Alarms

Thermistor

Light Dependant Resistor

How does a thermistor effect temperature?

In hot conditions resistance is low
In cool conditions resistance is high

Uses of Thermistors

Temperature detectors in car engines

Ohmic Conductor Graph

The current is directly proportional to the pd
Straight line through the origin

Filament Lamp Graph

Current Increases, Temperature Increases, Resistance Increases
Less Current can flow per unit pd
Graph gets shallower (curve)

Diode Graph

Current will only flow in one direction
Very high resistance in the reverse direction

Resistance of Ohmic Conductor

Doesn't change with the current
At a constant temperature it is dp to the pd

Resistance in a Filament Lamp

Some energy transferred to thermal energy store of the filament (designed to heat up)
Resistance increases with temperature
Current increases, temp increases, resistance increases

Resistance in a diode

Depends on direction of current
Let current flow in one direction
Very high resistance if it flows in the other

IV Characteristics

Graph which shows how the current flowing through a component changes as the potential difference across it increases.

Potential Difference in a Series Circuit

V tot = V1 + V2 + V3

Current in a Series Circuit

I1 = I2 = I3

Resistance in a Series Circuit

Rtot = R1 + R2

Connection in Series

In a line between the +ve and -ve of the power supply
Except for voltmeters which are connected in parallel

If you remove anything in series

the circuit is broken and the current stops

Connection in Parallel

Separately to the +ve and -ve of the supply
Except ammeters which are always connected in series

If you remove or disconnect anything in parallel

It will hardly affect the other components

Potential Difference in Parallel

V1=V2=V3

Current in Parallel

Itotal=I1+I2+I3

Resistance in Parallel

Reduces the total resistance

Why does adding a resistor in parallel reduce the total resistance?

In parallel both resistors have the same pd
The 'pushing force' is the same as the pd for each reistor you add
Adding another loop another way current can flow
Increases the total current V=IR means the resistance decreases

Alternating Current

Current constantly changing directions
Produced by alternating voltage

Direct Current

Current flowing in the same direction
Produced by direct voltage
Batteries and cells

UK Mains Supply

Alternating Current
230 V
Frequency: 50 Hz

Live Wire

Brown
Alternating pd from the mains supply
230 V

Neutral Wire

Blue
Completes the circuit
Current flows through live and neutral
0V

Earth Wire

Green and Yellow Striped
Stops appliance casing becoming live
Only carries current if there is a fault
0V

Why are live wires dangerous?

Your pd is 0V
Live wire touches you it produces a large pd across your body
Large current flows through you
Large electric shock

Why are the wires still dangerous even if they are turned off?

Any contact between the earth and wire is dangerous
Current not flowing but pd is still in the live wire
Contact would make a current flow through you

Why is connection between the earth and live dangerous?

Creates a low resistance path to earth
Huge current flows
Fire

What four equations are used to calculate power?

E = Pt
E = QV
P = VI
P = I2 R

Power Rating

The maximum amount of energy transferred between the sources per second

Lower Power Rating

Less energy appliance uses in a given time
Cheaper to run

Higher Power Rating

More powerful
Could be less efficient

Power of an appliance

The amount of energy that an appliance transfers per second

What happens when an electrical charge goes through a change in pd?

Energy is transferred
Supplied to the charge at the power source to raise it through potential
Charge gives up this energy when its 'falls' through any potential drop in components elsewhere

What happens when a charge passes through a battery?

It gains energy

What happens when a charge passes through a resistor?

It loses energy

When is demand for electricity highest?

When people come home from school or work
Cold and dark

How do Power Stations cope with the demand?

Having spare capacity by running well below their maximum output

Why do power stations use a high pd and a low current?

Cheaper as with a high current energy is lost as wires heat up and energy is transferred to their thermal energy stores

How much pd do power stations transfer

400 000 V

What are the functions of step up and step down transformers?

To get the voltage up to 400 000 V

What are the two coils that make up the transformers?

Primary and Secondary and they are joined by an iron core

How does the pd change across a step up transformer?

Increased: more turns on secondary then primary coil
Current is decreased

How does the pd change across a step down transformer?

Decreased: more turns on primary then secondary
Current is increased
For local consumer

Effiency of transformers

Nearly 100%

Equation for power of transformers

V of SC x I of SC = V of PC x I of PC

How does the rubbing together of two insulators cause static electricity to build up?

Negatively charged electrons are scraped off and left on the other
Both are electrically charged: one positive and one negative

Do both negative and positive charges move?

No just negative electrons

How are sparks caused?

As electric charge builds on an object the pd between the object and the earth increases
If the pd gets large enough, electrons can jump between the charged object and the earth
As well as any other earthed conductor

Electrostatic Forces

Like repel
Opposites attract

Electrostatic Attraction/Repulsion

Cause an object to move if they are able to do so
Non-Contact

Electric Field

Created around electrically charged objects

What Happens When you Place a Charged Object in an Electric Field

It feels a force
Causes the attraction and repulsion of electrostatic forces
Caused by the electric fields interacting with each other

Explain Sparks Through Electric Fields

Sparks are caused when there is a high enough pd between the charged object and the earth
High pd causes a strong electric field between the charged object and the earth
This causes ionisation where electrons get removed from air particles
Air is normally an insulator but when it is ionised it becomes conductive so a current flows
Causing a spark