OCR Physics 21st Century (new) P5- Electric Circuits

Build-up of static is caused friction 

• When two insulating materials are rubbed together, electrons are scraped off one and dumped on the other.
• Electrons are negatively charged.
• So this leaves a positive static charge on one (electrons scrapped off) and a negative static charge on the other (gained electrons).
• Which way the electrons are transferred depnds on the two materials involved.
• The classic example are polthene and acetate rods are being rubbed with a cloth duster.

Only electrons move- never the positive charges
When electrons are removed from particles to particles are left positively charged - these charged particles are called ions.

Both +ve and -ve electrostatic charges are only ever produced by the movement of electrons- the negatively charged particles. The positive charges definitely do not move. A positive static charge is always caused by electrons moving away elsewhere.

Like charges repel, opposite charges attract

Two things with opposite electric charges are attracted to each other.
Two things with the same electric charge will repel each other.

When you rub two insulating materials together a whole load of electrons get dumped together on one of the insulators, which becomes negatively charged. They try to repel each other, but can't move apart because their positions are fixed. The patch of charge that results is called static electricity because it can't move.  

Problems with static electricity:

• Attracts dust
• Clings to clothes and crackles
• Bad hair days! 

Current depends on voltage and resistance

• Current is measured in amps
• Voltage measured in volts
• Resistance (r) measured in Oms  Ω

If you increase the voltage -- then more current will flow
If you increase the resistance --- less current will flow

 POWER (w) = VOLTAGE (v) x CURRENT (A) 

Power is the rate of energy transfer
Work is also done because energy is transferred.
 

P.d tells us how much energy is transferred to or from each unit of charge as it moves between two points. 
The battery transfers energy to the charge as it passes -- that's the 'push' that moves the charge around the circuit.
Components transfer energy away from the charge as it passes
When energy is transferred, work is done.

A volmeter is used to measure the potential difference between two points.

Voltmeters are used in parrellel with a component so it can compare the charge before and after passing through the component.  

Calculating Resistance:

Resistance ( Ω)  = Voltage (V)
      Current (A)

Resistors get hot when current passes through them
When electrons move through a resistor, they collide with stationary positive ions in the resistor. These collisons make the ions vibrate more, which causes an increase in temperature. A filament lamp contains a peice of wire with a really high resisitance. When current passes through it, it's temperature increases so much that it glows - which is the light you can see.

Light- dependant resisitors (LDRs)
... special type of resistor that changes its resistance depending on how much light there is

BRIGHT LIGHT = LOWER RESISTANCE
DIM LIGHT = HIGHER RESISTANCE

used in automatic lights

 Thermistors- temperature dependant resistors

HOT TEMPERATURES = LOWER RESITANCE 
COLD TEMPERATURES = HIGH RESISTANCE

used in electronic thermostats 

• Potential difference is shared
• Current is the same everywhere
• Resistance adds up
  R= R1+R2+R3
• Cell voltage adds up 
• Cell current is equal throughout the circuit 

• Voltage is the same accross all components
  V1= V2=V3 
• Total current is shared between branches
  A= A1+A2 
• Resistance is weird 
  R < R1 and R < R2
• The current through a component depends on it's resistance.
  Each component in the circuit is seperately connected to the battery. This measns the current is as if that was  the only component in the citcuit.
  The resistance of a component controls how much current the voltage is able to push through it.
  THe component with the least resistance has the largest current
  This is because in a parallel circuit, all the components have the same P.D. across them -- the same P.D causes a larger current to flow through a smaller resistance than through a bigger one.
• Celll voltages don't add up 
• Cell current adds up 

Mains supply is AC, Battery supply is DC.

Main supply = 230 volts
... produced by generators through electromagnetic induction.

AC is used because,..

• easier to generate
• easier to distrubute over a long distance

Moving a magnet into a coil of wire induces a voltage

• called electromagnetic induction
• As you move the magnet, the magnetic field through the coil changes - this change in the magnetic field induces a voltage across the ends of the coil.
• If the ends of the wire are connected to make a closed circuit then a current will flow in the wire.
• The direction of the voltage depends on which way you move the magnet.

AC generators - just turn the magnet and there's a current

• In a generator, a magnet rotates in a coil of wire. As the magnet turns, the magnetic field through the coil changes - this change in the magnetic field induces a voltage, which makes a current flow in the coil.
• When the magnet is turned through half a turn, the direction of the magnetic field through the coil reverses. When this happens,  the voltage reverses, so the current flows in the opposite direction around the coilo of wire.
• If the magnet keeps on turning in the same direction - clockwise for example- then the voltage keeps on reversing every half turn and you get an AC current.

Four Factors affect the size of the induced voltage:

• Add an iron core inside the coil
• Increase the strength of the magnetic field
• Increase the speed of rotation
• Increase the number of turns on the coil

Change the voltage - but only AC voltage
Transformers are used to change the size of the voltage - they use electromagnetic induction to 'step up' or 'step down' the voltage. They have two coils of wire, the primary and secondary coils, wound around an iron core.
The alternating current in the primary coil causes changes in the iron core's magnetic field, which induces a changing voltage in the secondary coil.

STEP-UP TRANSFORMER step the voltage up (increase it) They have more turns on the secondary coil than the primary coil.

STEP-DOWN TRANSFORMER step the voltage down (decreases it). They have more turns on the primary coil than the secondary coil. 

Work by Electromagnetic induction
The magnetic field in the iron core constantly changes direction. This induces an alternating voltage in the secondary coil (with the same frequency as the primary). The relative number of turns on the two coils deteermines whether the voltage is higher or lower. 

If you supplied direct current (D.C) to the primary coil, you'd get nothing out of secondary coil at all. There'd still be a magnetic field in the iron core, but it wouldn't be constantly changing, so there'd be no induction in the secondary coil, because you need a changing field to induce a voltage. 
TRANSFORMERS ONLY WORK WITH AC.

The transformer equation

Voltage across primary coil         = Number of turns in primary coil
Voltage across secondary coil Number of turns on a secondary coil.

A Magnetic field is a region where magnetic materials (like iron and steel) and also wires carrying currents experience a force acting on them.

A current- carrying wire creates a magnetic field
There is a magnetic field around a straight, current-carrying wire.
The field is made up of concentric circles with wire in the centre.

A rectangular coil reinforces the magnetic field
If you bend the current- carrying wire round into a coil, the magnetic field looks like this. The circular magnetic fields around the sides of the loop reinforce each other at the centre.If the coil has lots of turns, the magnetic fields from all the individual loops reinforce each other even more.

A current in a magnetic field experiences a force 
Beacuse of it's magnetic fields, a current carrying wire or coil can exert a force on another coil, or on a permenant magnet. When a coil is put in a different magnetic field, the two magnetic fields affect one another. The result is a force on the wire. To feel the full force, the wire has to be 90 degrees to the lines of of the magnetic field it is placed in.  

Fleming's Left-Hand Rule tells you which way the force acts

• Point your first finger in the direction of the Field and you seCond finger in the direction of the Current.
• Your thuMb will then point in the direction of the force (Motion)

Magnetic Fields make current-carrying coils turn
If a rectangular coil of wire carrying a current is placed in a uniform magnetic field, the force will cause it to turn. This is called the motor effect. You can use fleming;s left hand rule (LHR), to work out which way the coil will turn.

The Simple Electric Motor
Two forces acting on the side arms of the coil. THese forces are just the usual forces which act on any currnet-carrying wire in a magnetic field. Because the coil is on a spindle and the forces act one up and one down, it rotates. The split-ring commutator is a clever way of swapping the contacts every half turn. This reverses the direction of the current every half-turn to keep the coil rotating continuously in the same direction. Otherwise, the direction of the force would reverse every half turn and the coil would change direction every half-turn instead of fully rotating.

Anything that uses rotation can be powered by an electric motor
Lots of devices use rotation. They all work by using an electric motor in a similar way. Link the coil to an axle, and the axles spins round. THis can be used to power DVD player, car wheels and washing machines.