Electricity

1. This happens because negatively charged electrons move from on material to the other
   1. The material that loses electrons becomes positively charged
      1. The Material that gains the electrons becomes negatively charged
         1. Two charged objects will repel each other if the charges are the same and attract each other if opposite

Adjunto:

• Circuit Symbols

1. There must be a complete circuit
   1. A complete circuit is one without any gaps
2. There must be no short circuits
   1. Short circuits are ways of getting past the lamp without going through another component

1. Series circuits are circuits where each component is connected one after the other in a single loop
   1. The current that flows across the components is the same
2. Parallel circuits are circuits where components are connected in separate loops
   1. The current is shared between each component connected in parallel

1. a flow of electric charge
   1. No current can flow if the series is broken
      1. Current is measured in amps (A) and it is measured using an ammeter which is connected in series with a component
         1. The size of an electric current is the rate of flow of electric charge
            1. It is calculated as: I = Q ÷ t
               1. Where I is current (A), Q is charge in Coulombs (C) and t is time in seconds
2. When more cells are added to a circuit p.d. increases so more current can flow

1. Potential Difference, or voltage is needed to make current flow and is often provided by cells or batteries
   1. It is measured in volts, V
      1. The potential difference across a component in a circuit is measured using a voltmeter which is connected in parallel
         1. The potential difference between two points in an electric circuit is the work done when a coulomb of charge passes between the points
            1. It is calculated as: V = W ÷ Q
               1. Where V is p.d. (V), W is work done in joules (J) and Q is charge (C)
2. A typical cell produces 1.5V
   1. If two or more cells are connected in series their p.d. is the sum of the two values
      1. 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

1. The moving electrons can collide with ions in the metal wire which makes it more difficult to flow which causes resistance
   1. The resistance of a long wire is greater than that of a short wire because electrons collide more
      1. 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
   2. As the temperature increases the metal ions vibrate more so there are more collisions and the resistance increases
2. Resistance can of a component can be found by measuring the current flowing through it, and potential difference across it
   1. The equation used to calculate this is V = I × R which can be rearranged as R = V ÷ I
      1. Where V is potential difference, I is Amps and R is resistance in ohms, Ω

1. At low temperatures, the resistance of a thermistor is high and little current can flow through them
2. At high temperatures, the resistance of a thermistor is low and more current can flow through them

1. In the dark and at low light levels, the resistance of an LDR is high and little current can flow through it
2. In bright light, the resistance of an LDR is low and more current can flow through it

1. Direct Current is current that only flows in one and it is supplied by batteries and cells
   1. On an oscilloscope Direct Current would have a straight line
2. Alternating current is current that constantly changes direction and it is supplied by mains plugs
   1. On an oscilloscope an Alternating current would show a wave

1. 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
   1. The inner wires are colour: blue is neutral, brown is live, and thee green and yellow stripey wire is earth
2. The features of a plug are
   1. A plastic or rubber case because they are both good insulators
   2. Three pins are made from brass, which is a good conductor of electricity
   3. A fuse between the live terminal pin which breaks the circuit if too much current flows
   4. The cable is secured in the plug by a cable grip
3. 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

1. Fuses protect electrical circuits and appliances by breaking if a fault in an appliance causes too much current to flow
   1. 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
      1. Fuses are made in standard ratings such as 3A, 5A and 13A
         1. 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
2. Circuit breakers also protect circuits by detecting a difference in current between the live and neutral wires
   1. They work much faster than fuses and don't need replacing, just turning on again

1. 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
   1. You would get an electric shock if the live wire inside an appliance came loose and touched the metal casing
      1. The earth terminal is connected to the metal casing so that the current goes through the earth wire instead of causing an electric shock
         1. A strong current surges through the earth wire because it has a very low resistance breaking the fuse and disconnecting the appliance
            1. Some appliances do not have earth wires this is because they have a plastic casing and there is no threat of an electric shock

1. Some appliances are more efficient at transferring energy than others
   1. For example less energy is wasted as heat energy from energy-saving, such as LEDs, and compact fluorescent lamps (CFLs) than from filament lamps
2. 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
   1. 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

1. Power is a measure of how quickly energy is transferred; the unit of power is watts, W
   1. The more energy that is transferred in a certain time the greater the power
      1. Power is calculated as P = E ÷ t
         1. Where P is power (W), E is energy transferred (J) and T is time (S)
         2. To calculate the power in an electrical appliance use P = I × V
            1. Where P is power (P), I is the current (A) and V is potential difference (V)