Static Electricity

Static Electricity

1. Build up of static is caused by friction

   • When certain insulating materials are rubbed together negatively charged electrons will be scraped off one and dumped on the other
     • Theyll leave a positive static charge on one and a negative static charge on the other
   • Electrically charged objects attract small objects near them

2. Only Electrons move never positive charges

   • Positive and negative electrostatic charges are only ever produced by the movement of electrons. The positive charges do not move
   • A positive static charge is always caused by electrons moving away elsewhere. The material that loses electrons loses some negative charge and is left with an equal positive charge

3. Like Charges repel opposite charges attract

   • When two charged objects are brought together they exert a force on one another
   • Two things with opposite charges are attracted to each other
   • Two things with the same electric charge will repel each other
   • These forces get weaker the further apart the two things are

4. Charges can move easily through Conductors

   • Metals are good conductors

5. Current and Potential difference

   • Current is the flow of electric charge round the circuit. Current will only flow through a component if there is a potential difference across that component (A)
   • Potential difference if the driving force that pushes the current round (V)
   • Resistance is anything in the circuit which slows down the flow down (ohms)
   • The greater the resistance across a component the smaller the current that flows (for a given potential difference across the component)
   • Total charge through a circuit depends on current and time
     • When current flows past a point in a circuit (I) for a length of time (t) then the charge (Q) that has passed
       • Current = Charge / time
         • I = Q / t
   • Potential difference (P.D.) is the work done per unit charge
     • The potential difference (or voltage) is the work done (the energy transformed measured in J) per coulomb of charge that passes between two points in an electrical circuit
       • P.D. = Work done / Charge
         • V = W/Q
     • So the potential difference is the amount of energy that is transferred by that electrical component per unit of charge
     • Voltage and potential difference mean the same thing

6. Circuits the basics

   • Circuit symbols
     • Fuse
     • LED
     • Diode
     • LDR
     • Thermistor
   • Standard test circuit
     • If you want to know the resistance of a component
       • Find the resistance by measuring current through and the potential difference across the component
     • The ammeter measures the current (A) it has to be placed in series never in parallel
     • Voltmeter measures the potential difference (V) must be placed in parallel around the component under test

7. Resistance and V= I xR

   • Potential different current graphs
     • Diode - Current will only flow through a diode in one direction as shown. The diode has very high resistance in the opposite direction
     • Different resisters - The current through a resistor is directly proportional to P.D.. Different resistors have different resistances
     • Filament lamp - As the temperature increases the resistance increases hence the curve
   • Resistance increases with temperature
     • When electrical charge flows through a resistor some of the electrical energy is transferred to heat energy and the resistor get hot. This heat energy causes ions in the conductor to vibrate more. With the ions moving around its harder for the charge carrying electrons to get through the resistor. The current cant flow as easily.
   • Resistance, Potential difference and Current
     • Potential Difference = current x resistance
       • V = I x R
     • If the graph curves it means the resistance is changing

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