# P1A physics revision notes

p1a physics revision notes

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## electric current

current : flow of electrons around a circuit

voltage: the difference in electrical energy between two points that makes a current flow.

electricity:  movement of the electrons around a circuit.

﻿charge:  (+ or - ) electons have an negative charge , charge carried by electrons.

﻿electrons move randomly when theres no charge.

﻿electrons go from - to +. conventional current goes from + to -

﻿electrons attracted to nucleus because nucleus is positively charged (+) whereas the electrons are negativley charged (-)

﻿incresing the voltage means more current will flow.

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## series and parallel circuits

series circuit

one path for electrons to flow.    electrons are attracted to the positive of the battery.   faster the electrons , the greater the current.

AMMETER: always connected in series

VOLTMETER: always connected in parallel.

parallel circuit :

two or more paths for the current to flow. current splits between each available paths. resistance reduces current. current splits in a paralell circuit.

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## alternating current (AC) and direct current (DC)

alternating current (AC) constantly changing direction. usually found in homes main socket.   lights plugged in AC dont flicker as the current changes direction so quickly.

conventional current.  flows form the positive to the negative.

electrical current :  is a flow of negaitively charged particles called the electrons which flow from (- to +) when current s measured in amps it is actually measuring the number of electrons flowing per second.

direct current (DC) : stays in the same direction. this is usually found in the batteries. batteries are made up of induvidual cells connected together. the most common cells are dry cells. -(non- chargeable) a chemical reaction inside the cell causes the current.

rechargeable batteries are used in mobile phones. solar cells is a cell that produces a DC current without a chemical reaction.

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## recharageable/ dry cells and capacity

the capacity of a cell is given in amp-hours . it measures how much energy a cell an store.

battery rating(amp hours ) / current (amps) = time in hour

rechargeable batteries (advantage) : cannot produce as much chemical pollution as dry cells and provided cheaper electrcity in the long run.   (disadvantge )  originally cost more, need a charging unit to storage power is usually less.

dry cells:  batteries slowly falls over time as you use them. the output of the rechargeable battery remains constant but drops quickly at the end.

in a dmmer switch, the main components is a variable resistor this is the same in stereos , wind screens , fans and wipers.

e.g. a camera has an LDR which changes the photo to make sure it is not too dark and too light.

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## generating electricty

around every wire theres a magnetic field. if its moved through the field a voltage is produced. voltage is the amount of electrical push that makes the current flow. moving the magnet or wire creates a voltage. this is called the electromagnetic spectrum or the dynamo effects.

if you move the magnet or the wire in the opposite direction then the voltage will also change direction. if you trun the magent around then the current wil also change direction. the current consists of negative charges moving from the negative to the positive. the voltage is measured in volts and is placed in parallel to the circuit. the current is measured in amps and the ammeter is placed in the series to the circuit.

resistance describes how good a material is at conducting electricity, it is measured in ohms. adding more compotents to a circuit will increase its resistance so it's harder for the current to flow and so it will be less. air dosent conduct electricy but in a thunderstorm the voltage is so high that it cause it to conduct.

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## magnets and resistance lamps and computers.

to increase the voltage and the current :

-move the magnet to closer to the wire

-add more coils to the wire

-use a stronger magnet

-move the magnet faster

the relationship between a current and voltage in a resistor is different to a filament. the resistors line is straight whereas the filament is curved. the voltage increase, the resisance increases.

advantages of using a computer:     possble to collect lots of information and sets of readings quickly 2. easier to measure and analyse data 3. thousands of readings can be safely recorded.

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## LDRs and thermistors and superconductors

thermistor: is a  type of a resistor that responds to change in the temperature. as the temperature iincreases, the resistance decreases.used in pools and fridges.

LDR: is a type of a resistor that responds to changes in light intensity. as the light increases the resistance decreases. used in cameras and alarms.

supercondoctors: can be used to make strong electromagnets , they are charged constantly so one always attracts and one reples.

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## electric motor

the direction of the wire can be changed by :

reversing the direction of the current through the wire.

reversing the direction of the permenant magnetic field.

every electric motor contains magnets and wire and they all pass current through the wires to the magnets and make them move.

the motor effect    the wire is near a permenant magnet and it will move ,the force of a single can be small but can be made larger and can make the wire move by increasing the size of the current., causing a stronger magnetiic field causiing it to move.

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## electric motor (continued )

as the coil spins the motor reverses the current in the coil every half turn , which keeps it in the same direction :

to make it faster :

increase the turns on the coils

increase strengths on magnets

how a motor works :

1. inside a magnetic field a coil of copper wire is wrapped around a core that spins on it's axis         2. the wire is free to spin without getting tangled up               3. the current always flows in the same direction but the current is continuosly reversing in the wires            4. a current is passed through the coil to make it a magnet        5. it interacts with the magents and spins.

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## fuses

brown wire :  is the live wire that carries into appliances

blue wire :  is the neutral wire that carries the  current away.

yellow and green striped wire :  is the earth that prevents the electrocution.

RCCB detects the differnces between the currents in the live wire and the neutral wire. there are saftey devices that measure the current flow. they are quicker to respond and saftey

the fuse is a thin piece of wire with a low melting point inside a glass or cermiac causing. if the wire gets too hot it melts and breaks the circiut. this protects the appliancesand stops it from overheating and catching fire. all appliances with a metal outreaching must be connected to the with an earth wire.when a fault occurs the whole circuit becomes live. the metal causing is attached to the earth wire. the current increases because the earth wire offers less resistance.

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## power

power is about time and work done. time is the rate. electrical motors convert electrical energy into kinetic energy. the rate of the conversion is called the power of the motor. power is measured in WATTS

power = current x voltage

efficinecy=useful output / total output x 100%

efficiency tells us how much of the energy we  have is useful. some energy is transfered into an unwanted form, it is not possible to achieve 1005 efficiency.

to calculate the units of electricty that something uses you need to know: the power of the rating (kw) , the time it is on for (h)   energy=power/time

totalcost = power x time x cost of 1kwh

payback time =cost of measure/ amount of money saved yearly.  payback time is the time it will take to save enough money to cover the cost of an energy saving measures

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