P5 - Electric Circuits

Static Electricity
1. some insulating materials can become electrically charged when rubbed against each other, the electrical charge then stays in the material. it doesn't move - the charge is STATIC
  1. charge builds up when electrons (with a negative charge) are rubbed off one material onto another.
    1. the material receiving the electrons becomes negatively charged and the one giving up the electron will become positive charged.
    2. if a perspex rod is rubbed with cloth, the rod will lose electrons and then become positively charged. the cloth gains electrons to become negatively charged.
    3. if an ebonite rod is rubbed with fur, the rod gains electrons to become negatively charged, he fur loses electrons to become positively charged
repulsion and attraction
1. when 2 charged materials are brought together, they exert a force in each other so they are either attracted or repelled.
  1. 2 materials with the same type of charge will repel each other. but, two materials with different charges will attract each other.
    1. if a positive rod is moved near another positively charged rod on a string, the suspended rod on the string will be repelled. the same will happen with 2 negatively charged ebonite rods.
    2. if a negatively charged rod is mode near to a positively charged suspended perspex rod, the suspended perspex rod ail be attracted - the same would happen if the rods were the other ways around,
2. When a car is spray painted, a panel of the car is positively charged and the paint is negatively chafed. the paint particles repel each other but are attracted to the positively charged panel. this will cause the paint to be applied evenly
electric circuits
1. an electric current is a flow of charge, measured in AMPERES (AMPS)
  1. In an electric circuit, the components and words are full of charges that are free to move. when a circuit is made, the battery causes these charges to move in a continuous loop. the charges are nit see up
2. in metal conductors there are lots of charges free to move. insulators however have few charges that are free to move.
  1. metals contain a free electron in their structure. the movement of these electrons creates the flow of charge - an electric current,
3. the amount of current flowing in a circuit depends on the resistance of the components in the circuit and the POTENTIAL DIFFERENCE across them
  1. Potential difference tells us the energy given to the charge and is another name for VOLTAGE. i is essentially a measure od the push of the battery on the charges in the circuit.
    1. for example, a 12 volt battery will transfer 12 joules of energy to every unit of charge. a potential difference of 3 volts across a bulb means that the bulb is transferring 3 joules of energy from every unit of charge - this energy is transferred as heat and light.
4. the greater the potential difference - voltage - across a component, the greater the current that flows through the component. two cells together provide a bigger potential difference across a lamp than one cell. this will make a bigger current flow.
5. 2 types:
  1. parallel circuits
  2. series circuits
Resistance and Current.
1. components such as resisters, lamps and motors resist the flow of charge through them - they have resistance
  1. work is done by the power supply and energy is transferred to the component.
    1. the grater the resistance of a component(s) the smaller the current that flows for a particular voltage. or, the greater the voltage needed to maintain a particular current.
      1. even the connecting wires in the circuit have some resistance, it is usually ignored because it is so small
        2 lamps together in a circuit with one cell have a certain resistance. including another cell in the circuit provides a greater potential difference. as a result, a greater current flows.
        adding resisters in series increases the resistance as the battery has to push through all the resisters.
        adding resisters in parallel reduces the total resistance and increases the total current because this provides more paths for the charges to flow along.
        when an electric current flows through a component, it causes the component to heat up. in a filament lamp, this heating effect is large enough to make the filament to glow in the lamp.
        as the current flows, the moving charges collide with the vibrating ions in the wore, giving them energy - this increase in energy causes the component to become hot.
2. Resistance which is measured in ohms is a measure of how difficult it is to get a current through a component at a particular potential difference or voltage. the formula for it is; Resistance = voltrage/ current
3. current voltage graphs:
  1. as long as the temperature of the resistor stays constant, the current through the resistor is directly proportional to the voltage across the resistor, regardless of which direction the current is flowing - if one doubles, the other also doubles.
4. The resistance of some materials depends on ENVIRONMENTAL conditions.
  1. the resistance of a thermistor depends on its temperature. its resistance will decrease as the temperature increases - allows more current to flow,
  2. The resistance of a light dependant resistor (LDR) depends on light intensity. its resistance decreases as the amount of light falling on it increases - allows more current to flow.
  3. as the resistance of a LDR and a thermistor can change, this will result in a change in the potential difference for all the other components in the circuit.
5. potential difference and current
  1. the potential difference across a component in a circuit is measured using a voltmeter in volts, connected in parallel across the component.
    1. the current flowing through a component in a circuit is measured in amperes (amos) (A) using an ammeter connected in series.
      1. when batteries are added in series, the total potential difference i the sum of all individual potential differences.
    2. when batteries are added in parallel, the total potential difference and current through the circuit remains the same but, each battery supplies less current - sharing the load = longer lasting batteries.

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