What should you consider when drawing a circuit?
- Being able to draw and interpret circuit diagrams using circuit symbols is an essential skill in the electricity topic
- Electric circuit diagrams require the following to work effectively:
- An energy source – This is a source of potential difference so a current can flow. This can be a cell, battery, or a power supply
- A closed path or a complete circuit – Electrons need to flow in a complete loop for a current to flow. A circuit can be open and closed using a switch
- Electrical components – These could act as sensors that respond to the environment (LDR, thermistor), or measure a value (ammeter, voltmeter), or transfer electrical energy to other forms of energy (LED, lamp). These must be drawn with the correct circuit symbol
- The key rules to remember are:
- An ammeter is always connected in series
- A voltmeter is always connected in parallel to the component the voltage is being measured
- The direction of current flow is always from the positive to the negative terminal of the power supply
What equation links Energy, Power and Time?
Appliances, power and energy
All electrical appliances transfer energy from one store to another, for example chemical energy in the fuel in power stations. This is transferred into kinetic energy in a fan or heat energy in a cooker.
The amount of energy transferred depends on the power (the energy transferred each second) and the amount of time the appliance is switched on for. The energy transferred by an appliance can be calculated using the equation:
energy = power × time
E=P×t
This is when:
- energy (E) is measured in joules (J)
- power (P) is measured in watts (W)
- time (t) is measured in seconds (s)
One watt is the power when one joule of energy is transferred in one second.
Time should be converted from minutes into seconds - this is done by multiplying the number of minutes by 60.
What is Power?
What is the equation that links power, voltage and current?
- Power is defined as
The rate of energy transfer or the amount of energy transferred per second
- The power of a device depends on:
- The voltage of the device
- The current of the device
- The power of an electrical component (or appliance) is given by the equation:
Watt is the same as Joules/Second
What is the equation that links power, resistance and current?
- The voltage across an electric device depends upon the current and resistance of that device
- Using the equations P = IV with V = IR, power can be written in terms of resistance, R:
What is amount of Energy Transferred by a Appliance dependenton?
- The amount of energy an appliance transfers depends on:
- How long the appliance is switched on for
- The power of the appliance
- A 1 kW iron uses the same amount of energy in 1 hour as a 2 kW iron would use in 30 minutes
- A 100 W heater uses the same amount of energy in 30 hours as a 3000 W heater does in 1 hour
What happens as Electricity is passed around a circuit into an appliance?
- As electricity passes around a circuit, energy is transferred from the power source to the various components (which may then transfer energy to the surroundings)
- As charge passes through the power supply it is given energy
- As it passes through each component it loses some energy (transferring that energy to the component)
Different domestic appliances transfer energy from batteries, such as a remote control
* Most household appliances transfer energy from the AC mains
* This can be to the **kinetic energy** of an **electric motor.** Motors are used in:
* **Vacuum cleaners - to create the suction to suck in dust and dirt off carpets**
* **Washing machines - to rotate the drum to wash (or dry) clothes**
* Refrigerators - to compress the refrigerant chemical into a liquid to reduce the temperature
* Or, in **heating** devices. Heating is used in:
* Toasters - to toast bread
* Kettles - to boil hot water
* Radiators - hot water is pumped from the boiler so the radiator can heat up a room
What is the equation that links Energy Transferred, Charge and Voltage?
- The electrical energy transferred also depends on the charge and potential difference:
E = Q × V
- Where:
- Q = charge in coulombs (C)
- V = potential difference in volts (V)
- When charge flows around a circuit for a given time, the energy supplied by the battery is equal to the energy transferred to all the components in the circuit
What do power ratings tell us?
- The power of an appliance is the amount of energy it transfers by electrical work every second
- This is normally the transfer of an electrical energy store to other energy stores, depending on the appliance
- Every electrical appliance has a power rating which tells you how much electricity it needs to work
- For example, a washing machine will require a lot more electricity than an iron because it is much heavier and more powerful
- The power rating for domestic electrical appliances is normally given on a label. This will include:
- The potential difference required to make the device work (eg. 230 V in the UK)
- The frequency of the supply (eg. 50 Hz in the UK)
- The power rating in Watts (this varies for each device)
- The higher the power rating, the quicker the change in stored energy
- For example, a 2000 W kettle means the kettle transfers 2000 J of energy per second from one store to another
- The different power ratings of various household appliances are listed in the table below as examples:
What is the National Grid?
- The National Grid distributes electricity across the UK
- It consists of a system of cables and transformers linking power stations to consumers (houses, factories and buildings)
- The transformers include:
- Step-up transformers which increase the voltage (and reduces the current) through the wires
- Step-down transformers which decrease the voltage (and increases the current) through the wires
What is the Benefits of the National Grid?
- When electricity is transmitted over large distances, the current in the wires heats them, resulting in energy loss
- By increasing the potential difference at which the electricity is transmitted, the same amount of power can be transmitted using a much smaller current (due to the equation P = IV)
- This results in less heat being produced in the wire and hence less energy loss
- Therefore:
- High PD means low current (less energy loss) for the same power
- Low PD means high current (more energy loss) for the same power
How are the cables of transmission lines made for efficiency?
To ensure that the minimum amount of power is lost from the cables:
- the cables are thick so that their resistance is low
- high voltages are used to reduce the current through the transmission lines
A low resistance and a low current mean that the transmission wires will not heat up as much. As a result, most of the power is delivered to the consumer, and not lost through the wires.
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What are Transformers?
- The type of current produced in power stations is alternating current (AC) which is transferred through to homes via the National Grid
- Transformers are used to increase and decreases the potential difference of the AC electricity before and after transmission across the National Grid
- They are formed of two coils of wire, called the primary and secondary coils, around a magnetic iron core
- A step-up transformer has more turns on the secondary coil than the primary
- A step-down transformer has more turns on the primary coil than the secondary
Step-up transformers are used to increase the potential difference from the power station to the transmission cables
* Step-down transformers are used to decrease the potential difference, to a much lower value, from transmission cables for domestic use
What does it mean to be Electrically charged”
- When certain insulating materials are rubbed against each other they become electrically charged
- This is called charging by friction
- The charges remain on the insulators and cannot immediately flow away
- One becomes positive and the other negative
- An example of this is a plastic or polythene rod being charged by rubbing it with a cloth
- Both the rod and cloth are insulating materials
How does plastic or polythene rod become charged by rubbing it with a cloth
- This occurs because negatively charged electrons are transferred from one material to the other
- The material, in this case, the rod, loses electrons
- Since electrons are negatively charged, the rod becomes positively charged
- As a result, the cloth has gained electrons and therefore is left with an equal negative charge
In both cases, the opposite charges will attract.
What happens when two charged particles are near each other?
- Therefore, an object becomes negatively charged when it gains electrons and positively charged when it loses electrons
- When two charged particles or objects are close together, they also exert a force on each other
- This force could be:
- Attractive (the objects get closer together)
- Repulsive (the objects move further apart)
- Whether two objects attract or repel depends on their charge
- If the charges are the opposite, they will attract
- If the charges are the same, they will repel
What is Static Electricity?
- Static electricity is the stationary electric charge which is produced by friction which causes sparks, or the attraction of other small objects such as dust or hair
- This is caused by the imbalance between negative and positive charges in two objects
- This only works for insulators, since in conductors, the charge will move through them instead of remaining stationary
- Static electricity and sparking is produced by rubbing surfaces which causes insulators to become charged by friction
What happens when electrons are transferred from one object to another?
- All objects are initially electrically neutral, meaning the negative (electrons) and positive charges are evenly distributed
- However, when the electrons are transferred, one object becomes negatively charged and the other positively charged
- This difference in charges leads to a force of attraction between itself and other objects which are also electrically neutral, by attracting the opposite charge to the surface of the objects they are attracted to
Why does a balloon stick to a wall when rubbed on a woolen jumper?
- One example is sticking a balloon to the wall:
- Rubbing a balloon on a woolen jumper transfers electrons onto the balloon by friction
- The balloon is now negatively charged whilst the jumper is left positively charged
- The wall is still neutral, however, when the balloon is placed near the wall, the positive charges in the wall are brought to the surface because they are attracted to the negative charge of the balloon
- Since opposite charges attract, the balloon sticks to the wall from only the electrostatic attraction
How is sparking caused?
- The build-up of electrostatic charge can be quite dangerous and can cause sparking (also known as an electric shock)
- A static electric spark occurs when two objects which are charged by friction and become oppositely charged and have a surplus of electrons so large that the electrons ‘jump’ across to an object that is neutral
- Since a current is the flow of electrons, this causes a small current to flow between the objects, called a spark
- An example of sparking is the small electric shock felt from touching a door handle, or another person, after walking on a vinyl floor or nylon carpet with rubber shoes or whilst wearing socks
How does Lightning occur?
- An extreme example of sparking is lightning
- In a storm, clouds move over each other causing them to become charged when electrons are transferred between them
- Since the ground is neutral, the negative charge jumps to meet the positive charges on the ground creating a big spark
- This is what is known as lightning
What is an Electric field?
What is the Strength of an electric field dependent on?
- A charged object creates an electric field around itself
- This is similar to the way in which magnets create magnetic fields
- This can be shown by electric field lines
- Fields lines always point away from positive charges and towards negative charges
The strength of an electric field depends on the distance from the object creating the field:
* The field is **strongest** **close** to the **charged object** - this is shown by the field lines being **closer together** * The field becomes **weaker further away** from the charged object - this is shown by the field lines becoming **further apart**
What will objects in an electric field experience?
- Objects in an electric field will experience an electric force
- Since force is a vector, the direction of this force depends on whether the charges are the same or opposite
- The force is either attractive or repulsive
- If the charges are the same (negative and negative or positive and positive), this force will be repulsive and the second charged object will move away from the charge creating the field
- If the charges are the opposite (negative and positive), this force will be attractive and the second charged object will move toward the charge creating the field
What does the size of the force depend on?
- The size of the force depends on the strength of the field at that point
- This means that the force becomes:
- Stronger as the distance between the two charged objects are closer
- Weaker as the distance between the two charged objects get further away
- The relationship between the strength of the force and the distance applies to both the force of attraction and force of repulsion
- Two negative charges brought close together will have a stronger repulsive force than if they were far apart
Which way do the electric field lines point in positive and negative spheres?
- Therefore, an electric field can be defined as:
A region in which a charged object will experience an electric force
- The electric field lines for a charged, isolated sphere, such as a spherical conductor:
- Point away from the centre of a positive sphere
- Point towards the centre of a negative sphere
