P5 | Mindmap

Static electricity
1. There are electrostatic forces between charged objects
  1. The same charges repel
  2. Opposite charges attract
2. There is an electrostatic force of attraction between the positively charged nucleus and the negatively charged electrons
3. The outermost electrons are less strongly attracted to a nucleus and can be removed by rubbing
4. When to insulating objects are rubbed together they become charged because electrons are transferred from one object to another
  1. The object that has lost electrons will become positively charged
  2. The object that gained electrons will become negatively charged
Conductors and insulators
1. Metals are good electrical conductors because they have free electrons. This means there are a lot of charges that can move
2. Plastics are good insulators. There are few free electrons in plastics, so there are very few charges to move
Moving charges
1. When a bulb is lit in a circuit there is an electric current
2. The moving electrons or electric current transfer energy to light the bulb
3. The cell provides energy to the electrons. The electrons carry a negative charge so they will flow towards the positive terminal
4. Electric current is the rate of flow of charge or charge flowing per second
5. The more energy the charged particles receive from the cell the greater the current.
6. In an electric circuit charge is conserved and energy is transferred
Measuring current and voltage
1. An ammeter is used to measure current
  1. Connected in series
2. A voltmeter is used to measure voltage
  1. It is connected in parallel to a component
  2. It measures the difference in energy between the terminals of a battery or bulb.
    1. The difference in energy is known as the potential difference
    2. A potential difference of 1 volt means that 1 joule of energy is transferred in and out of elecrical form for each unit of charge
3. Power (W) = Voltage (V) x current(A)
Electrical resistance
1. The more resistance in a circuit the lower the current
2. The greater the voltage the larger the current
3. Resistance is a measure of how a conductor opposes the current. Its unit is ohm (omega sign)
4. Copper wires have a really low resistance
5. A variable resistor is a device that allows you to control the current by changing the amount of resistance wire in a circuit
6. Resistance in ohms = voltage / current
7. The current through a fixed resistor is directly proportional to the voltage across it
Series and parallel circuits
1. In series
  1. Components connected in a line
  2. The current is the same throughout all the components
  3. The more cells connected in series the higher the potential difference
  4. The potential difference across the components add up to the potential difference across the battery
  5. The p.d across each component will be proportional to its resistance
2. In parallel
  1. Are each connected separately to the power supply
  2. The charge has a choice of pathways, so the current is shared between the components
  3. The current to and from the power supply is the sum of the current through all the branches
  4. 2 or more resistors in parallel provide more paths for charges to move along than each resistor on its own, so the total resistance is lower
  5. The current through each resistor is inversely proportional to to its resistance
Thermistors and LDRs
1. Thermistor
  1. Is a semiconductor whose resistance changes with temperature
2. LDR
  1. Light Dependent Resistor
  2. Is a semiconductor whose resistance changes as the amount of light falling on it changes
Metals and semiconductors
1. In semiconductors as the temperature or light intensity increases there are more free electrons so the current is higher
2. When metal ions are hotter they vibrate more, therefore increasing the chance of collision with an electron, so the current decreases as the electrons can't move as fast
Making an electric current
1. A magnetic field is a space around a magnet in which the magnets forces act
2. A voltage is induced when a magnet is moved near a piece of wire. If the piece of wire is part of a circuit a current will flow
3. The direction of the current is reversed when the motion of a spinning wire is moved
4. The current will increase if the speed of spinning increases, a stronger magnet is used or there are more turns of wire
Generators
1. The coils of wire continuously 'cut' the magnetic field lines so a voltage is induced. This is called electromagnetic induction
2. A continuous supply of electricity is produced when there is continuous relative motion
3. Mains electricity is produced by generators that induce alternate voltage
4. When the coil is at a right angle to the field lines it cuts no field lines so there is no voltage induced.
Distributing mains electricity
1. Direct current (d.c) always flows in the same direction
2. Alternating current (a.c.) changes direction at regular intervals
3. Mains voltage = 230V as a frequency of 50hz
Motors
1. There is a circular magnetic field around a wire carrying a current
2. When a current flows in a wire that perpendicular to a magnetic field the wire experiences a force. If the wire is free to move it moves
3. M other - Magnetic field
4. F ucking - force
5. C unt - Current
6. The direction of the force is reversed if either the current or magnetic field is switched
7. The motor needs a split ring commutator to keep it spinning in the same direction
8. Without a commutator the motor will do a half turn and then stop as the current is parallel to the magnetic field
9. A split ring commutator switched the + and - around every half tun so the motor keeps spinning
Transformers
1. A transformer changes the voltage of an a.c. power supply
2. It consists of two separate coils around an iron core
3. The input voltage is fed into the primary coil
4. The output voltage is across the secondary coil
5. A step up transformer converts a low voltage to a high voltage. The primary coil will have less turns than the secondary coil
6. A step-down transformer converts a high voltage input to a lower voltage. The primary coils have more turns than the secondary.
7. The alternating current in the primary coil creates an alternating magnetic field around it.
8. The magnetic, soft iron core channels the magnetic field through the secondary coil
9. The alternating magnetic field will continuously cut through the wires in the secondary coil and an alternating voltage will be induced across the secondary coil
10. If the number of turns in the secondary coil is doubled the output voltage will double
11. primary coil voltage/turns of wire in primary = secondary coil voltage/turns of secondary coil

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