Physics GCSE

a quick set of revising notes about topics p1a and p1b used to help me revise!! :D

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Electric Current

ELECTRIC CURRENT is a FLOW of ELECTRONS round a CIRCUIT!!

if you INCREASE THE VOLTAGE - then MORE CURRENT will flow, if you DECREASE THE RESISTANCE - then LESS CURRENT will flow

current is similar to a flow of water - the pump acts as the electricity supply, the constriction acts as the resistance. if you turn on the pump, you increase the flow, if you put in more constrictions, you decrease the flow.

ELCTRONS flow the OPPOSITE WAY to CONVECTIONAL CURRENT - it flows from negative to positive, NOT vice versa!!

AC (sqiggly line) keeps CHANGING DIRECTION, DC (straight line) always flows in the SAME DIRECTION.

AC = WIGGLY DC = STRAIGHT

^^ don't forget that, and remember, the amount of CURRENT you get depends on the VOLTAGE of the power supply and the RESISTANCE of the appliance you're running.  

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Generating Electricity

Generators use something called the 'dynamo effect' to make elctricity.

ROTATING A MAGNET in a COIL OF WIRE induces a VOLTAGE

 when you create voltage & current in a conductor by rotating a magnet in/near a coil of wire, its called ELECTROMAGNET INDUCTION.                        as you turn the magnet, the magnetic field through the magnetic field, the coil changes, this change induces a voltage - so a current flows in the wire. when that happens, the voltage reverses - so all the current flows in the opposite direction. - when you turn the magnet constantly (e.g. anti-clockwise) the voltage will reverse every half-turn, making an AC current.

FOUR FACTORS affect the SIZE od the induced VOLTAGE                                     1) the STRENGTH of the MAGNET                                                                         2) the AREA of the COIL                                                                                             3) the NUMBER OF TURNS on the COIL                                                                   4) the SPEED of the movement

how GENERATORS work - they generate current by electromagnet induction (see above). all generators need something to do the turning, this could be anything from a steam-driven turbine to a waterwheel! a dynamo is a special type of generator, often used in bike lights, here the magnet is rotated - not the coil. dynamo is attached to the wheel, as you turn the wheels, you tun the magnet inside the dynamo. 

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current, voltage and resistance

INVESTIGATING how CURRENT VARIES with VOLTAGE:                                       1) An AMMETER is used to measure current. always connected in a SERIES CIRCUIT.                                                                                                     2) a VOLTMETER measures the voltage. always connected in a PARALLEL CIRCUIT.                                                                                                                 3) the voltage is supplied from the cell doesn't change. you use a variable resistor which you adjust to pick up different values for the current, then for each value you pick using the variable resistor, MEASURE THE VOLTAGE across the component.

EXAMPLES:

FIXED-VALUE RESISTOR: if the component had a fixed resistor, you would get results that show that RESISTANCE is CONSTANT. if you plotted a graph with the results, you'd get a straight line - showing that the current is proportional to the voltage.

FILAMENT LAMP: if the component was a a filament bulb, you would get results that show the resistance isnt constant - it increases as the current increases. the resistance is bigger because as the temp. increases, its resistance increases. in the graph theres a curve - current not proportional to voltage. 

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Current, voltage and resistance - CONTINUED!

RESISTANCE EQUATION:

voltage = current X resistance

(V = I X R)

EXAMPLE: circuit has a 4 ohms resistor, voltmeter reads 6V, what is the current through the ammeter?                                                                                                ANSWER: 6V divided by 4 OHMS = 1.5A

COMPONENTS can be WIRED in SERIES or PARALLEL:                                        1) in SERIES CIRCUITS, components can be connected in a line, between positive and negative of the power supply. to workout the TOTAL RESISTANCE, just add the sum of the resistances if separate components. the same current flows through all parts of the circuit.                                                    2) in PARALLEL CIRCUITS, components are separately connected from positive to negative of the supply. the current flows along each branch of the circuit depending on the resistance. the lower the resistance of the branch, the bigger the current that'll flow through it. the total current is the total of all the currents in the separate branches. the TOTAL RESISTANCE is always less than the branch with the smallest circuit.

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Circuit Diagrams

two things are vital for a circuit to work, they are:                                                       1) must be a complete diagram                                                                                2) there must be NO short circuits

To check for a complete circuit, follow a wire coming out of the battery with your finger. You should be able to go out of the battery, through the lamp and back to the battery.

To check for a short circuit, see if you can find a way past the lamp without going through any other component. If you can, then there is a short circuit and the lamp will not light.

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