Science Module P5 - Electric Circuits

BUILD UP OF STATIC IS CAUSES BY FRICTION

 1) When two insultating materials are rubbed togethe, electrons are scraped off one and dumped on another

2) This leaves a positive static charge on one and a negitive static charge on the other

3) Which way the electrons are transferred depends on the two materials involved

4) Electrically charged objects attract small objects placed near them

                 Example: Polythene and acetate rods being rubbed together with a cloth duster,

ONLY ELECTRONS MOVE - NEVER THE POSITIVE CHARGES

LIKE CHARGES REPEL, OPPOSITE CHARGES ATTRACT

POWER IS THE RATE OF ENERGY TRANSFER

ENERGY TRANSFERRED (in joules) = POWER (in W) X TIME (in S)

KILOWATT-HOURS (kWh) ARE "UNITS" OF ENERGY
ENERGY TRANSFERRED (in kWh) = POWER (in kW) X TIME (in H)

POWER RATING OF APPLIANCES

POWER = VOLTAGE x CURRENT

MORE EFFICIENT MACHINES WASTE LESS ENERGY

EFFICIENCY = ENERGY USEFULLY TRANSFERRED                X100                         TOTAL ENERGY SUPPLIED

ELECTRIC CURRENT IS A FLOW OF CHARGE ROUND A CIRCUIT

1) Electric current is the flow of charge. Its Units are amperes, A

2) In an electrical circuit the components and wires are full of charges that can move.

3) Circuit wires are usually metal with a plastic cover. Electric charge flows in a metal conductor because there are lots of electrons that are free to move around. Current can't flow in an insulator (like Plastic) because the electrons are fixed.

4) In a complete circuit, the battery pushes charge through the wires. The charge is not used up and does not disappear. 

CURRENT DEPENDS ON VOLTAGE AND RESISTANCE. its Units are Volts, V

INCREASE THE VOLTAGE = MORE CURRENT WILL FLOW .
INCREASE THE RESISTANCE = LESS CURRENT WILL FLOW .

ELECTRONS FLOW THE OPPOSITE WAY TO CONVENTIONAL CURRENT

Normally you would say that current in a circuit flows from positive to negitive.

But alas, electrons were discovered long after that was decided and they turned out to be negitive charged  - UNLUCKY.

This means they actually flow from -ve to +ve, opposite to the flow of "conventional current"

THE SLOPE OF A VOLTAGE-CURRENT GRAPH SHOWS RESISTANCE

V-I graphs show how the current in a circuit varies as you change the voltage.

1) The current throgh a resistor (at constant temperature) is proportional to the voltage

2) Different resistors have different resistances - the steeper the slope the lower the resistance.

3) The wires in an electric ciruit have such a small reistance that it's usually ignored.

RESISTANCE = POTENTIAL DIFFERENCE / CURRENT

Resistors get hot when current passes through them.

*When electrons move through a resistor they collide with stationary particles in the resistor

*These collisions cause the resistor to heat up, which changes its resistance

LIGHT-DEPENDANT RESISTOR or "LDR"

A LDR is a special type od resistor that changes its resistance depending on how much light there is:

1) In  bright light, the resistance falls

2) In darkness, the resistance is highest

                         example: Burgular detector

THERMISTER (TEMPERATURE-DEPENDANT RESISTOR)

A thermister is like an LDR - but its resistance depends on temperature.

1) In hot conditions, the resistance drops

2) In cool conditions, the resistance goes up

                        example: electronic thermostats

POTENTIAL DIFFERENCE IS SHARED
V1 = V2 + V3

CURRENT IS THE SAME EVERYWHERE

A1 = A2 = A3

RESISTANCE ADDS UP

R = R1 + R2 + R3

CELL VOLTAGE ADDS UP

two 1.5V cells in series would supply 3V in total.

INDEPENDANT AND ISOLATION

DEPENDANT DIFFERENCE IS THE SAME ACROSS ALL COMPONENTS
V = V = V

CURRENT IS SHARED BETWEEN BRANCHES:
A = A + A

RESISTANCE IS TRICKY

A circuit with two resistors in parallel will have a lower resistance then one on its own. Which means that a parallel circuit would have a higher current.

THE CURRENT THROUGH A COMPONENT DEPENDS ON ITS RESISTANCE

The resistance of a component controls how much current the voltage is able to push through it. All the components have the same p.d. across them, so the component with the most resistance hs teh lower current and teh component with the least resistance has teh highest current.

Mains supply is AC, Battery supply is DC

MOVING A MAGNET IN A COIL OF WIRE INDUCES A VOLTAGE

AC GENERATORS

Just turn the magnet and there's the current. As the magnet turns the magnetic field changes. Change the direction and reverse the magnetic field.

THREE FACTORS AFFECT THE SIZE IF THE INDUCED VOLTAGE

1) The STRENGTH of the MAGNET

2) The SPEED of movement

3) The number of TURNS on the COIL

TRANSFORMERS CHANGE THE VOLTAGE - but only AC VOLTAGES

step-up transformers - step the voltage up

step-down transformers - step the voltage down

TRANSFORMRS WORK BY ELECTROMAGNETIC INDUCTION

      Primary Voltage    =  Number of turns on Primary                                     Secondary Voltage         Number of turns on Secondary