P5 - Static Electricity
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
P5 - Electrical Energy
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
P5 - Electrical Current (1)
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 .
P5 - Electrical Current (2)
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"
P5 - Resistance (1)
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
P5 - Resistance (2)
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
P5 - Series Circuits
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.
P5 - Parallel Circuits
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.
P6- Mains Electricity (1)
Mains supply is AC, Battery supply is DC
MOVING A MAGNET IN A COIL OF WIRE INDUCES A VOLTAGE
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
P5 - Mains Electricity (2)
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