Electricity

Electricity

1. Static Electricity

   • When you rub two different insulating materials against each other they become electrically charged
     • This happens because negatively charged electrons move from on material to the other
       • The material that loses electrons becomes positively charged
         • The Material that gains the electrons becomes negatively charged
           • Two charged objects will repel each other if the charges are the same and attract each other if opposite

2. Current and Voltage In Electrical Circuits

   • Circuit Symbols
     • Circuit Symbols
   • For a circuit to work, two things are important
     • There must be a complete circuit
       • A complete circuit is one without any gaps
     • There must be no short circuits
       • Short circuits are ways of getting past the lamp without going through another component
   • Series and Parallel circuits
     • Series circuits are circuits where each component is connected one after the other in a single loop
       • The current that flows across the components is the same
     • Parallel circuits are circuits where components are connected in separate loops
       • The current is shared between each component connected in parallel
   • Electric current:
     • a flow of electric charge
       • No current can flow if the series is broken
         • Current is measured in amps (A) and it is measured using an ammeter which is connected in series with a component
           • The size of an electric current is the rate of flow of electric charge
             • It is calculated as: I = Q ÷ t
               • Where I is current (A), Q is charge in Coulombs (C) and t is time in seconds
     • When more cells are added to a circuit p.d. increases so more current can flow
   • Potential Difference
     • Potential Difference, or voltage is needed to make current flow and is often provided by cells or batteries
       • It is measured in volts, V
         • The potential difference across a component in a circuit is measured using a voltmeter which is connected in parallel
           • The potential difference between two points in an electric circuit is the work done when a coulomb of charge passes between the points
             • It is calculated as: V = W ÷ Q
               • Where V is p.d. (V), W is work done in joules (J) and Q is charge (C)
     • A typical cell produces 1.5V
       • If two or more cells are connected in series their p.d. is the sum of the two values
         • If their are connected in opposite directions then they are subtracted e.g. to cells in opposite directions will produce a p.d. of 0V meaning no current can flow

3. Resistance

   • There is a resistance to the flow of an electric current through most conductors which increase as the length of the wire increases or the thickness of the wire decreases
     • The moving electrons can collide with ions in the metal wire which makes it more difficult to flow which causes resistance
       • The resistance of a long wire is greater than that of a short wire because electrons collide more
         • The resistance of a thin wire is greater than the resistance of a thick wire because a thin wire has fewer electrons to carry the current
       • As the temperature increases the metal ions vibrate more so there are more collisions and the resistance increases
     • Resistance can of a component can be found by measuring the current flowing through it, and potential difference across it
       • The equation used to calculate this is V = I × R which can be rearranged as R = V ÷ I
         • Where V is potential difference, I is Amps and R is resistance in ohms, Ω
   • The current flowing through a resistor at a constant temperature is directly proportional to the potential difference across it
   • The resistance of a lamp increases as the temperature of its filament increases. The current flowing through a filament lamp is not directly proportional
   • The diode has a very high resistance in one direction which means that the current can only flow in the other direction
   • A light-emitting diode, LED, produces light when a current flows through it in the forward direction. LEDs are often used for indicator lights in electrical equipment such as computers and television sets. As LEDs use a much smaller current than other types of lighting, their use is increasing.
   • Thermistors are used as temperature sensors - for example, in fire alarms. Their resistance decreases as the temperature increases:
     • At low temperatures, the resistance of a thermistor is high and little current can flow through them
     • At high temperatures, the resistance of a thermistor is low and more current can flow through them
   • LDRs (light-dependent resistors) are used to detect light levels, for example, in automatic security lights. Their resistance decreases as the light intensity increases:
     • In the dark and at low light levels, the resistance of an LDR is high and little current can flow through it
     • In bright light, the resistance of an LDR is low and more current can flow through it

4. Household Electricity

   • AC and DC
     • Direct Current is current that only flows in one and it is supplied by batteries and cells
       • On an oscilloscope Direct Current would have a straight line
     • Alternating current is current that constantly changes direction and it is supplied by mains plugs
       • On an oscilloscope an Alternating current would show a wave
   • Cables and Plugs
     • A mains electricity cable has two or three inner wires made of copper, because copper is a good conductor, and covered by plastic, because it is a good insulator
       • The inner wires are colour: blue is neutral, brown is live, and thee green and yellow stripey wire is earth
     • The features of a plug are
       • A plastic or rubber case because they are both good insulators
       • Three pins are made from brass, which is a good conductor of electricity
       • A fuse between the live terminal pin which breaks the circuit if too much current flows
       • The cable is secured in the plug by a cable grip
     • To remember where each of the wires go: b(L)ue is (L)eft, b(R)own is (R)ight, and s(T)riped is (T)op
   • Fuses
     • Fuses protect electrical circuits and appliances by breaking if a fault in an appliance causes too much current to flow
       • The fuse contains a metal wire which melts easily, if the current going through the fuse is too great, the wire heats up until it melts and breaks the circuit
         • Fuses are made in standard ratings such as 3A, 5A and 13A
           • A suitable fuse is one that should melt just above the appliance's working current, for example: if the device works at 3A use a 5A fuse
     • Circuit breakers also protect circuits by detecting a difference in current between the live and neutral wires
       • They work much faster than fuses and don't need replacing, just turning on again
   • Earthing
     • Many electrical appliances have metal cases; the earth wire creates a safe route for the current to flow if the live wire touches the casing
       • You would get an electric shock if the live wire inside an appliance came loose and touched the metal casing
         • The earth terminal is connected to the metal casing so that the current goes through the earth wire instead of causing an electric shock
           • A strong current surges through the earth wire because it has a very low resistance breaking the fuse and disconnecting the appliance
             • Some appliances do not have earth wires this is because they have a plastic casing and there is no threat of an electric shock

5. Charge, Current and Power

   • Choosing appliances
     • Some appliances are more efficient at transferring energy than others
       • For example less energy is wasted as heat energy from energy-saving, such as LEDs, and compact fluorescent lamps (CFLs) than from filament lamps
     • Filament lamps contain a thin metal filament that glows when electricity passes through it, however, most of the electrical energy is transferred as heat energy instead of light energy so it is innefficient
       • Modern energy-saving lamps work in a different way; they transfer a greater proportion of electrical energy as light energy so is more efficient than the filament bulb
   • Power
     • Power is a measure of how quickly energy is transferred; the unit of power is watts, W
       • The more energy that is transferred in a certain time the greater the power
         • Power is calculated as P = E ÷ t
           • Where P is power (W), E is energy transferred (J) and T is time (S)
           • To calculate the power in an electrical appliance use P = I × V
             • Where P is power (P), I is the current (A) and V is potential difference (V)

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