Physics
- Created by: maano
- Created on: 31-05-17 17:22
Energy Stores
Kinetic - lion running, wind
Thermal - radiator, sun
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
Energy transfer diagrams
The law of Conservation: the total amount of energy going in, is exactly the same amount coming out.
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
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
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%
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
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
Current and circuit symbols
Learn these!
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.
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
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
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
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.
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
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
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
...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
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
Resistance adds up
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
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.
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
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.
- It is about 230V
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
.. 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
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
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
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
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
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
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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