Electric circuits- helpsheet Potential difference, current and resistance are all related by the equation; The total resistance in a circuit is the sum of the resistors in the circuit e.g. 6 ohms + 10 ohms = 16 ohms A variable resistor can alter the resistance in a circuit and is useful for things like controlling volume or light dimmer switches Current is the flow of charge. The size of the current is the rate of flow of electric charge. Current is measured using an Ammeter in series with the circuit. In a series circuit the current is the same everywhere. In a parallel circuit the current is split when it reaches a branch. The circuit needs to be complete for the current to flow Metal atoms (ions) in a wire have delocalised electrons which are free to move and carry the electric current around the circuit. The electrons moving around the circuit collide with the ions. This is called resistance. Components with high resistance (e.g. filament bulbs) often get hot. This is because when the electrons collide with the ions they transfer energy as heat (and light), this then causes the ions to vibrate more increasing the resistance by making it harder for the electrons to pass through without collisions. I = Q t I = current in amperes (amps), A Q = charge in coulombs, C t = time in seconds, s Potential difference (voltage) is measured between two points and is the work done (energy transferred) per coulomb of charge that passes between the points. Potential difference is measured using a voltmeter connected in parallel across a component. The sum of potential difference across one branch is equal to the sum of the potential difference of the cells or battery. In a parallel circuit the potential difference across a single component in a branch is equal to the potential difference of the cell. V = W Q V = potential difference in volts, V Q = charge in coulombs, C W = work done in joules, J A A 6V X M V 3V 2V V 4V V 3V V = I x R V = potential difference in volts, V I = current in amps, A R = resistance in ohms,  6 ohms10 ohms The current through a resistor (at a constant temperature) is directly proportional to the potential difference across a resistor. The resistance of a bulb increases as the temperature of the filament increases The current through a diode flows in one direction. It has a very high resistance in the opposite direction. An LED (light emitting diode) emits light when a current flows in the forward direction An LDR (light-dependent resistor) resistance decreases as light intensity increases. (DARK = high resistance, low current, LIGHT = low resistance, high current) A thermistor’s resistance decreases as the temperature increases.( HIGH TEMP = low resistance, high current, LOW TEMP = high resistance, low current)

Household electricity- helpsheet Current in a simple circuit is direct (dc) because it only flows in one direction, whereas current from the mains supply is alternating current (ac) because it alters direction. dc- one directioac- alternating direction Household electricity has a potential difference of around 230V and a frequency of 50 Hz (Hertz), so changes direction 50 times in a second. The live terminal (pin) potential difference varies between a high positive value and low positive value. The neutral terminal (pin) has a potential difference close to earth which is zero. In a plug and cable; B l ue is neutral and goes on the l eft S t riped is Earth and goes to the t op B r own is live and goes to the r ight This is the symbol for a fuse. A fuse will melt when the current is too high A plug is designed with the following; Plastic casing outside is an insulator Brass pins are good conductors Fuse melts if the current is too high (thus breaking the circuit) Cables are made of copper because they are bendy and good conductors Cables have plastic coating because they are good insulators Appliances with plastic casings sometimes only have 2 cables (no Earth) as they are double insulated and have been designed so that the live wire cannot touch the casing. This is because the plastic acts as an insulator so the chance of electric shock is minimal. The period of an AC supply is the time taken for one complete oscillation. You can find this by looking at the time between one peak and the next. e.g. In the oscilloscope trace, one horizontal division represents 5 ms (five milliseconds). There are four divisions between two adjacent peaks, so the period is: 4 × 5 = 20 ms The frequency of an AC supply is the number of oscillations per second. You can find it from the period: Frequency = 1 ÷ period (Remember to convert to seconds if needed. In this example, 20 ms = 20 ÷ 1000 = s.) Frequency = 1 ÷ = 50 Hz The circuits in your house are earthed outside Appliances with metal cases need to be earthed inside the case. This is just in case a live wire comes loose- the current will then pass through the earth wire and not you when you touch it Circuit breakers switch off the current when there is a fault When the switch is open it stays open until it is reset Residual current circuit breakers (RCCB) work faster because they cut off current in a live wire when it is different from current in a neutral wire. When an electrical charge flows through a resistor, the resistor gets hot. A lot of energy is wasted in filament bulbs as heat. Less energy is wasted in power-saving lamps such as Compact Fluorescent Lamps (CFLs). There is also a choice when buying new appliances in how efficiently they transfer energy. The rate at which energy is transferred by an appliance called the power P = power in watts, W E = energy in joules, J t = time in seconds, s P = E t Energy transferred, potential difference and charge are all related by.. P = power in watts, W I = current in amps, A V = potential difference in volts, V P = I x V E = energy in joules, J V = potential difference in volts, V Q = charge in coulombs, C E = V x Q