Currents in Electrical Circuits, AQA P2.3

Currents in Electrical Circuits, AQA P2.3

1. Build up of static electricity is caused by friction

   • When certain insulating materials are rubbed together, negatively charged electrons will be scraped off one material and dumped on the other
     • This leaves a positive static charge on one and a negative charge on the other.
       • Which way the electrons are transferred depends on the two materials involved
         • Electrically charged objects attract small objects placed near them
           • The classic examples are polythene and acetate rods being rubbed with a cloth duster, as shown in the diagram

2. Positive and negative electrostatic charges are only ever produced by the movement of electrons

   • A positive static charge is always caused by electrons moving away elsewhere. The material that loses the electrons loses some negative charge and is left with an equal positive charge.

3. When two charged objects are brought together, they exert a force on each other. These forces decrease in strength was the distance between the two things increases.

   • Opposite charges are attracted to each other
   • Identical charges repel each other

4. Charges can move easily through conductors. Metals are good conductors.

5. Current is the flow of electric charge round a circuit. Current will only flow if there is a potential difference (voltage) across a component. Unit: Amperes, A.

6. Potential difference is the driving force that pushes the current around. Unit: Volts, V.

   • Potential difference is the work done (the energy transformed in joules) per coulomb of charge that passes between two points. It is given by the following formula: Voltage = work done/charge. Voltage and potential difference are interchangeable terms.

7. Resisitance is anything in a circuit which slows the flow of current down. Unit: Ohm, Ω.

8. The greater the resistance across a component, the smaller the current that flows (for a given potential difference across the component).

9. Total charge through a circuit depends on current and time.

   • Current is the rate of flow of charge. When current (I) flows past a point in a circuit for a length of time (t) then the charge (Q) that has passed is given by this formula, with current being measured in Amps, charge in Coulombs and time in seconds::
   • The greater the current, the greater the charge flowing.

10. Ammeters must be placed in series, while voltmeters must be in parallel.

11. Difference-Current graphs

12. Resistance increases as heat increases

    • This is because heat energy causes ions in conductors to vibrate more, making it more difficult for charge-carrying electrons to get through conductors - the current can not flow as easily with high resistances

13. YOU MUST KNOW OHM'S LAW!!!

14. Diodes are made of semiconductor materials such as silicon and have low resistance in the 'forward' direction and very high resistance is the 'reverse' direction

15. Types of resistor

    • Carbon film - obvious
    • Wire-wound - made of coils of resistive wire
    • Variable - can be set at different values
    • LDR - high resistance in darkness and vice versa
    • Thermistors - high resistance at low temperatures

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