# Physics

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• Created by: maano
• Created on: 31-05-17 17:22

## Energy Stores

Kinetic - lion running, wind

Light - torch, candles

Gravitational potential - clouds, aeroplanes

Chemicals - food, batteries

Sound - music, tv

Electrical emergy - microwave, iron

Elastic Potential - Elastic band, bunjee jump

Nuclear - Nuclear Power station

Kids Hate Learning GCSE Energy Names

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## Energy transfer diagrams

The law of Conservation: the total amount of energy going in, is exactly the same amount coming out.

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## Work, power and spring constant

Work done = Force x Distance

Unit for Energy: JOULES (J)

Power= Energy transfered ÷ time

Unit for Power: Watts (W)

Elastic potential energy = 1/2 x k x e²

Elastic potential energy = 1/2 x spring constant x extension²

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## Kinetic and Gravitational potential

E = 1/2 x mass x volume²

Q.) A car of mass 2500kg is travelling at 20m/s. Calculate the energy in its kinetic energy store.

E = 1/2 x 2500 x 20² = 500,000

E = mass x gravity x height  OR    E = mgh

Q.) Emily goes up the Eiffel Tower. If she has a mass of 40Kg and the Eiffel Tower is 300m high, how much gravitational potential does she have at the top?

(g= 10N / kg)  Answer: mass x gravity x height

40 x 10 x 300 = 120,000J

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## Efficiency

The less energy that is wasted, the more efficent the energy transfer it.

Efficiency = Useful output  ÷ Total input

E.G: 36,000J of energy is transferred to a television. It transfers 28,800 of this energy usefully. Calculate the efficiency of this television.

e= useful output ÷ total input

28,800÷36,000= 0.8

Then convert it into a percentage by x100!!

0.8x100= 80%

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## Alternative efficiency equation

However, you might not know the energy input and output of a device so you wuld have to use the following equation:

Efficiency = Useful power output÷ Total power input

E.G: A blender is 70% efficient. it has a total input power of 600W. Calculate the useful power output.

Convert from a percentage to a decimal:  efficiency= 70% ÷ 100 =0.7

useful power output = efficiency x total power input

useful power output = 0.7 x 600 = 420W

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## Energy resources

Non-renewable resources will run out one day.

The three main fossil fuels are

• coal
• oil
• gas

Renewable energy sources will never run out and can be replenished.

The main sources are

• sun
• wind
• water waves
• hydroelectricity
• bio-fuels
• tides
• giothermal
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Learn these!

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## Total charge through a circuit depends on current

Electric current is a flow of electrical charge

Current is measured in amperes, A

Charge is measured in coulombs, C

Electrical charge will only flow round a complete (closed) circuit, if something is providing a potential difference e.g. a battery

Resistance is anything that slows down the flow of charge

Potential difference is sometimes called voltage

In a single, closed loop the current is the same everywhere in the circuit

The size of the circuit tells how fast the charge is flowing.

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## Charge flow

Charge flow, current and time are related by this handy equation:

Charge flow (C) = Current (A) X Time (s)     OR        Q = It

Example: A battery charger passes a current of 2 A through a cell over a period of 300 seconds. How much is transferred to the cell?

Just substitute the values into the equation above the calculate the charge.

Q =It =  2x300

= 600C

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## Resistance and V = IR

The current flowing through a component depends on the potential difference across it and the resistance of the component.

The greater the resistance across a component, the smaller the current that flows  (for a given potential difference across the component).

The formula linking potential difference and current is

(potential difference in volts (v),    current in amps (A),    resistance in ohms

V = IR

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## Ohmic conductors have a constant resistance

V=IR works for components with a fixed resistance. This means the resistance of the component doesn't change with current.

These components are called ohmic conductors.

Ohmic conductors only have a fixed resistance if their temperature dosen't change

Wires and resistors are examples of ohmic conductors

The current flowing through it is directly proportional to the potential difference across it

This means that if you multiply the potential difference by a certain amount, the current will be multiplied by the same amount.

E.G: if the potential difference doubles, the current doubles too

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## Some components have a changing resistance

The resistance of some components does change with current. E.G: a filament lamp or a diode.

Filament lamps contain a wire (the filament), which is designed to heat up and 'glow', as the current increases.

So as the current increases, the temperature of the filament increases.

Resistance increases with tempterature, so the resistance increases with current.

For diodes, the resistance depends on the direction of the current.

A diode will let current flow in one direction. It has a very high resistance in the opposite direction, which makes it hard for a current to flow that way.

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## Circuit devices- LDR (short for light dependent re

Some components resistance can depend on light and temperature- this can be handy

1.) The resistance of an LDR changes as the intensity of light changes

2.) In bright light, the resistance is low

3.) In darkness, the resistance is high

4.) LDRs have lots of uses including turning on lights when its dark

5.) This can be used in automatic night lights, or outdoor lighting

6.) They're also used in burgular detectors

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## The resistance of a Thermistor depends on Temperat

A thermistor is temperature dependent resistor

In hot conditions, the resistance drops

In cold conditions, the resistance goes up

Thermistors are used in car engines and central heating thermo stats

Thermostats turn the heating on when it's cool and off when it's warm

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## You can use LRDs and Thermistors in sensing circui

Sensing circuits can be used to automatically change the pd across the components depending on changes in the environment

This circuit is a sensing circuit used to control a fan in a room

The potential difference of the power supply is shared out between the thermistor and the fixed resistor

How much pd each one gets depend on their resistances

The larger a component's resistance, the more of the pd it takes

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## ...continued

This circuit means that the pd across the fan goes up as the room gets hotter. Here's why:

• As the room gets hotter, the resistance of the thermisor decreases
• The thermistor takes a smaller share of the pd from the power supply
• So the pd across the fixed resistor rises
• The pd across the fixed resistor is equal to the pd across the fan
• So the pd across the fan rises too, making the fan go faster

If you connected the fan across the thermistor instead, the circuit would do theopposite

The fan would slow down as the room gets hotter

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## Series circuits - All or nothing

In series circuits, the components are all connected in a line between the ends of the power supply.

Only voltmeters break this rule. They're always in parallel

If you remove one component, the circuit is broke, so all the components stop working

Potential difference is shared, Current is the same everywhere

In series circuits the total pd of the power supply is shared between all the components

If you add up the pd across each component, you get the pd of the power supply

The bigger a component's resistance, the bigger its share of the total pd

If you connect cells in series, their pds add together to make the total pd across the circuit

In series circuits the same current flows through all components

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In series circuits, the total resistance of two components is found by adding up their resistances

R(total) is the total resistance of their circuit

R1 and R2 are resistances of the components > R(total) = R1 + R2

Why adding resistors in series increases the total resistance of the circuit:

• Adding a resistor in series means the resistors have to share the total pd
• This means the pd across each resistor is lower, so the current through each resistor is lower  (V =IR)
• The current is the same everywhere
• So the total current in the circuit is reduced when a resistor is added
• This means the total resistance of the circuit has gone up
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## Parallel circuits - Every component connected sepe

In parallel circuits, each component is seperately connected to the ends of the power supply.

Only ammeteres break this rule, they're always in series

If you take out one of the loops in a parallel circuit, the things in the other loops won't be affected

This means things in parallel can be switched on and off without affecting each other

Everyday circuits often include a mixture of series and parallel parts

Current is shared, potential difference is the same everywhere

• In parallel circuits all components get the full source pd
• So the potential difference is the same across all components
• This means that identical bulbs connected in parallel will all be at the same brightness
• In parallel circuits the total current in a circuit is equall to the sum of all currents through the seperate components
• At junctions, the current either splits or rejoins
• The total current going into a junction must equal the total current leaving it.
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## Electricity in the home

There are two types of electricity supply - alternating and direct currents.

Mains supply is AC, Battery supply is DC

An alternating potential difference is a potential difference that is constantly changing direction.      It produces an alternating current (ac)

In an alternating current, the current (flow of charge) is also constantly changing direction

The UK mains supply (the electricity in your home) is an ac supply at around 230V

The frequency of the ac mains is 50Hz (Hertz)

Direct current (dc) is a current that is always flowing in the same direction

It's created by a direct potential difference.

The direction of a direct potential difference is always the same

Cells and batteries supply dc

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## Most cables have three separate wires

1.) Most electrical appliances are connected to the mains supply by the three-core cables

2.) They have three wires covered with plastic insulation inside them

3.) They have coloured so that it is easy to tell the different wires apart

LIVE WIRE - BROWN

• The live wire provides the alternating potential difference from the mains supply.

NEUTRAL WIRE - BLUE

• This completes the circuit
• Usually, current flows in through the live wire and out through the neutral wire
• It is around 0V
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## .. CONTINUED

EARTH WIRE - GREEN AND YELLOW

• The earth wire is a safety wire
• It stops the appliance becoming live:
• It is connected to the metal casing of an appliance
• If a fault causes the live wire to touch the casing, the current flows away through the earth wire
• It is also at 0V
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## The live wire can give you an electric shock

There is a pd between the live wire and your body (which is at 0V)

Touching the live wire can cause a current to flow through your body

This can give you a dangerous electric shock

Even if a switch is turned off (the switch is open), touching the live wire is still dangerous.

-This is because it still has a pd of 230V

Any connection between live and earth can be dangerous

The pd could cause a huge current to flow, which could result in a fire

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## Energy is transferred from cells and other sources

When a charge moves around a circuit, work is done against the resistance

Whenever work is done, energy is transferred

When the work is done by a charge, the energy is transferred electrically

Electrical appliances transfer energy to components in the circuit when a current flows

Kettles transfer energy electrically from the mains supply to the thermal energy store of the heating element inside the kettle

Energy is transferred electrically from the battery of a handheld fan to the kintetic energy store of the fan's motor

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## Energy transferred depends on the power

1/ The total energy transferred by an appliance depends on how long the appliance is on for and its power

2/ The power of an appliance is the energy that it transfers per second

3/ So the more energy it transfers in a given time, the higher its power

4/ The amount of energy transferred by electrical work is given by:

Energy transferred(J) = Power (W) x Time (s)    OR   E = Pt

5/ Appliances are often given a power rating. This is the power that they will work at

6/ The power rating tells you how much energy is transferred between stores when the appliance is real

7/ An appliance with a higher power will cost more to run for a given time, as it uses more energy

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## More on power

Potential difference is energy transferred per charge passed

As a charge moves around a circuit, energy is transferred to or from it

The energy transferred by a component depends on the potential difference

Energy transferred (J) = Charge flow (C) x Potential difference (V)     OR    E=QV

Power also depends on current and potential difference. The power of an appliance can also be found by using =  Power (W) = Current² x Resistance       OR         P = I²R

E.G: A motor with a power of 1250W has a resistance of 50 Ohms. Calculate the current flowing through the motor.

First rearrange the formula P=I²R to make I the subject                                              P÷R=I²  so I²=P÷R

Divide both sides by R                                                                                            I= √P÷R

Find the sqaure root of both sides                                                                            I = √1250÷50 = √25

Now just plug in the numbers                                                                                  = 5A

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## Electricity is distributed via the national grid

The national grid is a giant system of cables and transformers that cover the UK

It transfers power from power station to consumers (anyone who is using electricity) across the UK

Electricity production has to meet demand

Throughout the day, the amount of electricity used (the demand) changes

Power stations have to produce enough electricity for everyone to have it when they need it

More electricity is used when people get up in the morning, come home from work or school and when it starts to get dark and cold outside

Power stations often run at well below their maximum power output so, that they can increase their power if needed

This means that the national grid can cope with a high demand, even if another station shuts down without warning

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## The national grid uses a high pd and a low current

The National grid transfers loads of energy, so the power has to be very high

To transmit this huge amount of power you need either a high potential difference or a high current

This is because P=VI

A high current means loads of energy is lost to thermal energy stores as the wires heat up

So the national grid transmits electricity at a very high pd. For a given power, the higher the pd the lower the current. This reduces the energy lost, making the national grid an efficient way of transferring energy.

Step-up transformers are used to increase the pd from power stations to electric cables

Step-down transformers bring the pd back down to safe levels before the electricity gets to homes

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