Edexcel Physics IGCSE Topic 2: Electricity Summary Notes PDF

Edexcel Physics IGCSE Topic 2: Electricity Summary Notes (Content in ​bold​ is for physics only) This work by PMT Education is licensed under https://bit.ly/pmt-cc https://bit.ly/pmt-edu-cc CC BY-NC-ND 4.0...

Edexcel Physics IGCSE Topic 2: Electricity Summary Notes (Content in ​bold​ is for physics only) This work by PMT Education is licensed under https://bit.ly/pmt-cc https://bit.ly/pmt-edu-cc CC BY-NC-ND 4.0 https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc Energy and voltage in circuits Current Current​ I is measured in ​amperes (A)​ and is the ​rate of flow of charge​ at a ​point​ in the circuit. The current is given by I=Q/t, where Q is measured in ​coulombs (C) ​and t in ​seconds (s). In metals, current is due to a ​flow of electrons.​ In solutions it can be the flow of ions. Conventional current is the rate of flow of ​positive​ charge - this is in the ​opposite​ direction to the flow of electrons because electrons are ​negatively​ charged. Current is ​conserved​ at a ​junction ​in a circuit because charge is always conserved. Current is measured with an ​ammeter ​connected in ​series ​with the component. Potential difference Potential difference ​V is measured in​ volts (V where 1 V = 1 JC​ ) ​and is the ​work done per unit -1​ charge​ in moving​ between two points​ in a circuit. The potential difference is given by V=E/Q. It is measured with a ​voltmeter​ placed in​ parallel ​across the component. The higher the potential difference, the greater the current (​V = IR​). Resistance The ​resistance​ of a component is measured in ​ohms (Ω) ​and is given by the potential difference across it divided by the current through it, i.e. ​R=V/I​. The greater the resistance, the harder it is for current to flow through the component. In an ​ohmic conductor ​(such as a ​resistor at a constant temperature​),​ ​the current is directly proportional to the voltage (i.e. it has constant resistance). In a non-ohmic conductor (such as a filament lamp​), the resistance changes as the voltage and current changes. In a filament lamp, this is because as the ​current increases​ through the filament, so does the temperature,​ which means ​electrons and ions vibrate more​ and ​collide more, increasing resistance. A​ thermistor​ is a resistor whose resistance decreases as the​ temperature​ increases. A ​light dependent resistor​ is a resistor whose resistance decreases as ​light intensity​ increases. Electric circuits Series: Components are connected ​end to end​ in one loop The ​same current​ flows through every component https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc  The ​potential difference is shared​ across each component - depending upon their resistance (i.e. the sum of the p.d.s across the components is equal to the total p.d. across the supply) - components with a higher resistance have a greater PD across. The total resistance in series is the ​sum of the resistances​ of each component​ R​ = R​ + R​ T​ 1​ 2 … Parallel: Components are connected to the power supply in ​separate branches The ​current is shared ​between each branch (i.e. the sum of the currents in the separate branches is equal to the current through the source) - because charge can only flow one way. The ​potential difference​ is the ​same​ across every branch Connecting lamps in parallel is advantageous because if one breaks, current can still pass through the rest. Mains electricity Dangers of electricity Hazards: Damaged insulation​ – contact with the wire due to gaps in the insulation can cause an electric shock​ or pose a​ fire hazard​ by creating a short circuit. Overheating of cables​ – high currents passing through thin wire conductors cause the wires to heat up to very high temperatures which could ​melt the insulation​ and cause a fire. Damp conditions – water can conduct a current so wet electrical equipment can cause an electric shock. Fuses and circuit breakers: A ​fuse​ is a thin piece of​ wire ​which overheats and ​melts​ if the ​current is too high, protecting the circuit.​ They have a current ​rating​ which should be slightly higher than the current used by the device in the circuit. The most common are 3A, 5A and 13A. Circuit breakers ​consist of an automatic ​electromagnet​ switch which ​breaks the circuit ​if the ​current rises over a certain value.​ This is better than a fuse as it can be ​reset​ and used again, and they operate ​faster. Earthing metal cases: Earth wires create a ​safe route​ for current to flow through in the case of a ​short circuit, preventing electric shocks. Earth wires have a​ very low resistance​ so a strong current surges through them which breaks the fuse and disconnects the appliance. Double insulation: Appliances with ​double insulation ​have either ​plastic casings ​completely covering their electrical components, or have been designed so that the earth wire ​cannot touch​ the metal casing, preventing them from giving an electric shock. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc Electrical transfer of energy Energy, measured in ​joules (J)​, is transferred from ​chemical ​energy in the ​battery ​to​ electrical energy used by ​circuit components ​and then to the ​surroundings. The ​power​ of a component is measured in ​watts (W) ​and is given by P=IV (by using V=IR, this can be shown to be equivalent to P=I​2​R and P=V​2​/R)​. ​Using this equation, the energy transferred is given by E=IVt. Alternating current and direct current In a​ direct current,​ the current only flows in ​one direction​ whereas in an​ alternating current,​ the current continuously ​changes direction. Mains electricity ​is an alternating current (a.c.) whereas the current supplied by a ​cell or battery is direct current (d.c.). https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc Electric charge Charge​ is measured in coulombs, C. There are ​positive​ and ​negative​ charges; ​opposite ​charges attract​ and ​like​ charges​ repel. Atoms are composed of protons, electrons and neutrons. Protons have a charge of ​+1​, electrons have a charge of ​-1​ and neutrons have a charge of ​0​. Charging a body involves the ​addition​ or ​removal​ of ​electrons - ​charging atoms creates ions. Conductors​ such as ​metals​ allow electrons to flow through them whereas ​insulators ​such as ​plastics​ impede the flow of electrons. o When two insulators are ​rubbed​ together, friction causes electrons to move from one to the other and they become charged. The material that loses electrons becomes ​positively charged​ and the material that gains electrons becomes negatively charged​. o The ​magnitude​ of the charge on each material is equal, since they lose/gain the same number​ of electrons. o For example, when a​ rod​ is rubbed with a ​cloth,​ electrons are transferred from the rod onto the cloth and the rod becomes positively charged. The charges cannot move within the insulator so they build up - this is known as ​static electricity​. Consequences of static electricity can be seen in a number of phenomena. Lightning: ○ Electrostatic charge can build up on clouds due to ​friction​. ○ When this charge becomes large enough, the clouds ​discharge​ through the air to the earth. This results in ​lightning​. Charged balloon on a wall: ○ A positively charged balloon will stick to a wall if moved close enough. ○ Positive charges in the wall are​ repelled by the balloon​ and move to other parts of the wall. This leaves a ​negative charge​ on the area of the wall closest to the balloon. ○ The ​attraction​ between the ​negatively charged wall​ and the​ positively charged balloon​ makes the balloon stick. Comb picking up bits of paper: ○ Rubbing a comb against an​ insulator ​will cause it to pick up an electrostatic charge due to the ​transfer of electrons​. ○ The charge on the comb ​repels ​like-charged in the paper, leaving the paper closest to the comb with an electrostatic charge ​opposite to the comb​. ○ This end of the paper is then ​attracted​ to the comb. Electrostatic phenomena caused by the movement of electrons have many useful applications but also pose many risks. Dangers of electrostatic charges include: o Static charges pose a risk of ​electric shock​. If a person touches an object with a large amount of static charge, electrons will flow through the person’s body to the earth​. o When ​fuelling aircraft and tankers, ​if enough charge builds up on the vehicle or pump it can create a ​spark.​ This can ignite the fuel and cause a ​fire or explosion. For safety, an ​earthing ​wire can be attached so that the charge instead flows into the earth. Safety measures when using electrostatic charges include: o Earthing​ involves offering electrons an ​alternative pathway​ to the earth. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc  o This prevents too much electrostatic charge form ​building up​ on the surface of an insulator. Less electrostatic charge​ reduces the risk​ of electric shock, or the harm it can cause. Uses of electrostatic charges include: o In an ​inkjet printer, ​droplets of ​ink​ are ​charged ​and pass between ​two charged metal plates,​ one of which has a positive charge and the other a negative charge. The droplets are attracted to the plate with the opposite charge and repelled by the plate with the same charge and ​deflected​ towards a specific place on the paper. o In a ​photocopier, ​the image of a document is projected onto a ​positively charged plate; ​where light falls onto the plate, the charge leaks away. ​Negatively charged toner particles ​are attracted to the remaining positive areas. Paper is then placed over the plate and the toner is transferred to it, making the photocopy. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc