# P5- Electric Circuits

A summary of the P5 module Electric circuits, taken from the OCR 21st Century science textbook

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• Created by: R_Hall
• Created on: 11-05-11 09:10

## Static Electricity

• Static electricity is electric charge that is not moving around a circuit  but has built up on an object
• Electrical effects can be produced by rubbing 2 materials together- the more it is rubbed, the stronger the effect
• It a lot of energy is stored (through rubbing), it may escape by moving to a nearby object, by jumping as a spark- it has been electrically charged
• If you rub 2 identical rods together they will repel each other. The forces they exert are very small
• If you rub 2 rods of different plastics together, they will attract as they have two different electrical charges (positive and negative)
• Charge is not made but is moved between the materials when rubbed. Rubbing does not make charges, it moves them
• In most materials, there are equal numbers of positive (protons) and negative (electrons)- making it neutral
• Around every charge there is an electric field, a region of space where the charge is felt
• Electric current is a flow of charges around an electric current
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## Simple circuits

• Electric circuit- A closed loop of conductors connected between the positive and negative terminals of a battery/ power supply
• Electric charges have to go around a circuits for it to work. A bulb goes on immediately when the circuit is completed
• The size of the circuit does not effect the time after circuit completion when the circuit works
• There are charges in all components of the circuit, completing the circuit just allows these charges to work
• The battery pushes these charges around together- chemical reactions inside the battery separate electrical charges (positive and negative on different terminals
• When connected, the charges on the battery terminals set up an electric field in the wires, this makes the free charges move slowly but together
• Conventional current - Charges flow from positive terminal to negative (not true)
• Electron flow- Charges flow from negative to positive (true)
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## Electric Current

• The bigger the current through a bulb, the brighter it glows
• An ammeter measures the size on an electric current. The reading (in amps, amperes or A) indicates the amount of charge going through the ammeter each second
• The current in a simple, single-loop circuit is the same everywhere
• The circuit transfers energy from the battery, to the bulb filament, then onto the surrounding (as light). The current enables this to happen, but is not used up
• To run several components from the same battery, the components need to be connected in a series circuit. The charges have to pass through each component in turn
• Connecting in parallel, this has the advantage of each bulb works independently without the other.
• At the junctions of the parallel circuit, the current splits into the 2 branches. The amounts in both branches added= total amount in the single wire before branching
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## Controlling the current

• Voltage- Measure of a battery's push on charges in an electric circuit
• The bigger the voltage- the bigger the push- the bigger the current
• Components in a circuit provide resistance to the flow of charge. The battery pushes against this resistance. Resistors are components designed to control the flow of charge
• The bigger the resistance, the smaller the current
• Connecting wires have little resistance, filaments in light bulbs have high resistance
• In metals, the moving charges are free electrons. As they move, they collide with the lattice of atoms in the wire, making them vibrate and and heat. In some metals, the electrons can pas through the lattice easily, bu in others it is harder- the resistance is higher.
• Ohms law- Current through a conductor is proportional to the voltage across it (constant temperature)
• Resistance of a conductor= Voltage across conductor/ Current through conductor
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## Controlling the current- Variable resistors

• Variable resistor= Resistance whose size can be changed steadily (volume of CD player)
• A variable resistor in a circuit controls the size of the current everywhere round the circuit loop
• A LDR (Light Dependent Resistor) has a large resistance in dark, but get lower as the light on it get brighter. Used to measure the brightness or switch on/off hen light changes
• Thermistor= changes resistance with temperature. Lower resistance when hotter. Used to make thermometers or switch a device on/off when temp. changes
• 2 resistors in series have a larger resistance than one on its own- battery has to push current through both
• Connecting 2 resistors in parallel= smaller total resistance- 2 paths charges can follow
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## Potential difference

• Voltmeter= a device used for measuring the potential difference between 2 points in a circuit
• Potential difference (p.d).= the difference of potential energy between any 2 points in a circuit
• In an electric circuit, the battery dos work on electric charges to lift them to a higher energy level. They then transfer energy as they drop back to their starting level
• The voltage of a battery= p.d. between its 2 terminals
• Battery with larger voltage= bigger p.d. across terminals/resistors
• If several resistors are connected in parallel to battery, pd across each is same, = to battery voltage
• In series, sums of pd= battery voltage
• In series, pd across each resistor depends on resistance. Biggest voltmeter reading across biggest resistance;more work has to be done to push charge
• The larger the resistance of the resistor; the smaller the current
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## Electrical power

• The power of a circuit= the rate at which energy is transferred from the battery to other components
• The power dissipated in an electric circuit depends on the current and the voltage
• Power (watts, W)= Current (amperes, A)/ Voltage (Volts, V)
• Watt is a unit of power. 1 watt= 1 joule per second
• Work done (or energy transferred, joule, J)= Power x Time (s)
• If the battery voltage increases, the electrical field in the wire gets bigger. The free electrons move 2x as fast
• When an electron collides with an atom, 2x as much energy is transferred and happen more regularly
• When voltage increased. collisions between electrons and lattice of atoms is harder and more frequent
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## Domestic appliances

• Electrical energy use is the amount of work done by the electricity supply on all the appliances run
• Work done on an appliance depends on its power rating and the time for which it is run
• Energy transferred when device is on (kilowatt- hour, kWh)= power rating (kilowatt, kW) x time (hour, h)
• On an electricity bill, 1 unit is 1 kilowatt- hour.
• In all electric appliances, some of the energy transferred does not end up where it is wanted, or in the incorrect form- it is not 100% efficient
• % efficiency= useful energy transferred / work done on the appliance x 100
• Power = current x voltage
• For all UK mains appliances, the operating voltage is 230 V
• A fuse is a short piece of wire that melts at a low temperature. An electric current through it heats it
• The length and thickness of the fuse are chosen so it melts if the current goes above the stated value.
• Fuses for main plugs come in 3A (for any appliance with a power rating below 690W) and 13A (for 690+ appliances)
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## An electricity supply

• Electromagnetic induction-the process in which a potential difference (or an electric circuit) is generated in a wire, when it is in a changing magnetic field- how generators work
• To generate a current, out can move a magnet in or out of a coil. The movement of the magnet causes a induced voltage across the ends of the coil (the coil acts as a small battery).
• If the coil is part of a closed circuit, the induced voltage makes a current flow
• If a magnet is rotated near the end of the coil, the magnetic field around the coil would constantly change direction
• The changing magnet induces a current in the coil in 2 directions- alternating current ac
• Direct current dc is always in 1 direction
• The size of the alternating current and voltage can be increased with a bigger magnet, faster rotations, coil with more turns and putting an iron core inside the coil
• In real generator, the magnet is rotated inside the coil , as it spins ac is generated
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## Distrubuting electricity

• A transformer is used to either step up or step down voltage to a level required
• A transformer works because the current in the primary coil produces an electric filed which passes through the secondary coil. The field is changing and therefore induces a voltage
• If a transformer has fewer turns on the secondary coil,It means it is a step down transformer. The output voltage will be less than the input voltage.
• Voltage across secondary coil/ voltage across primary coil = no. of 2ndary turns/ no. of primary turns
• The National Grid distributes electricity using a transformer
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very enlightening

alex:)

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thanks :D

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