CGP P2 Flashcards
What is current
. Electrical current is the flow of Electrical charge
. Electrical Charge will only will only flow round a complete (closed) circuit if there is a potential difference, so a current can only flow if there’s a source of potential difference.
The unit of current is ampere A
Current in a single loop
In a single, closed loop, current has the same value everywhere on the circuit
Potential difference
Potential difference (voltage) is the driving force that pushes charge around. it’s unit is volt V
What is resistance
Resistance is anything that slows the flow down. Measured in ohms
Current flowing through a component
The current flowing through a component depends on the potential difference across it and the resistance of the component
Size of the current
The size of the current is the rate of flow of charge. When current flows through a point in a circuit for a length of time, then the charge that has passed is given by the formula:
Q = It
Q = Charge flow (coulombs, c)
I = Current (Amps, A)
t = Time (Seconds, s)
More charge passes around the circuit when a larger current flows
Reminder
. Circuit diagram symbols
. Investigate wire length
. Experimenting components I-V characteristic
. Sensing circuits
. Circuit diagram calculate current passing through circuit
. Investigating resistance practical
. Electric field lines
What is the formula linking potential difference and current
V = IR
V = Potential difference
I = Current
R = Resistance
What are the factors that can affect the resistance of a circuit
. If the components are in series or parallel
. The length of wire used in a circuit
Anmeter
. Measures the current (in amps) flowing through the test wire
. Must always be placed in series with whatever you’re investigating
Voltmeter
. Measures potential difference across the test wire in volts
. Must always be placed in parallel around whatever you’re investigating - not around any other bit of circuit e.g battery
Resistance of Ohmic Conductors
. The resistance of ohmic conductors (wire or resistor) doesn’t change with the current. At a constant temperature, the current flowing through an ohmic conductor is directly proportional to the potential difference across it
Resistance in other resistors or components
. The resistance of some resistors and components does change e.g. diode or filament lamp
Electrical charge through a filament lamp
. When an electrical charge flows through a filament lamp, it transfers some energy to the thermal energy store of the filament which is designed to heat up.
Resistance increases with temperature, so as current increases, the filament lamp heats up more and resistance increases
Current through a diode
. For diodes, the resistance depends on the direction of the current, they will happily let current flow in one direction, but have a very high resistance if it is reversed
What does the term ‘I-V characteristic’ refer to
. The term refers to a graph which shows how the current (I) flowing through a component changes as the potential difference (V) across it is increased.
What is the ‘I-V characteristic’ of linear components
. A straight line e.g. a fixed resistor
What is the ‘I-V characteristic of Non-linear components’
. Non-linear components have a curved I-V characteristic e.g. filament lamp or diode
LDR - Light Dependent Resistor
. An LDR is a resistor that is dependent on light intensity
. In bright light, resistance falls
. In darkness, resistance is the highest
. They have lots of applications including automatic night lights, outdoor lighting and burglar detectors
Thermistor
. A thermistor is a temperature dependent resistor
. In hot conditions the resistance drops
. In cool conditions, the resistance goes up
. Thermistors make useful temperature detectors e.g. car engine temps and thermostats
LDR’s and Thermistors in sensing circuits
. Sensing circuits can be used to turn on or increase the power to components depending on the conditions they are in
. The circuit on the right is used to control a fan in the room (reminder)
. The fixed resistor and fan will have the same potential difference because they’re connected in parallel
. The pd of the power supply is shared between the thermistor and the loop made up of fixed resistor and fan according to their resistances
. The bigger a component’s resistance, the more potential difference it takes
Connecting a component in a sensing circuit across a variable resistor
. if you connect a bulb in parallel to an LDR, the pd across both the LDR and the bulb will be high when it’s dark and the LDR’s resistance is high.
. The greater the potential difference across a component, the more energy it gets
. So a bulb connected across an LDR would get brighter as the room got darker
Series circuits
. In series circuits, the different components are connected in a line, end to end, between the +ve and -ve of the power supply (except for voltmeters which are connected in parallel, but they don’t count as part of the circuit)
. If you remove one component, the circuit is broken and they all stop.
. This is not hand and few things are connected in series
Potential difference in series
. In series circuits, the total potential difference of the power supply is shared between the various components. So the potential differences round a series circuit always add up to equal the source potential difference
V(total) = V1 + V2 + V…
Current in series
. In series circuits, the same current flows through all components I1 = I2 = I3 = I…
. The size of the current is determined by the total pd of the cells and the total resistance of the circuit i.e. :
I = V / R
Resistance in series
. In series, the total resistance of two components is just the sum of their resistances : R(tot) = R1 + R2
. This is because by adding a resistor in series, the two resistors have to share the total pd.
. The potential difference across each resistor is lower, so the current through each resistor is also lower. In a series circuit, the current is the same everywhere, so the total current is reduced when a resistor is added.
This means the total resistance of the circuit increases
. The bigger a component’s resistance, the bigger it’s share of the total potential difference
Cell potential differences
. Cell potential differences add up
. There is a bigger potential difference when more cells are in series, if they are all connected the same way
. For example, when two cells with a potential difference of 1.5V are connected in series they supply 3V between them
Parallel circuits
. In parallel circuits, each component is separately connected to the +ve and -ve of the supply (except ammeters which are always connected in series)
. if you remove or disconnect one of them, it will hardly affect the others
. Everyday circuits often include a mixture of series and parallel parts
Potential difference in parallel
. In parallel, potential difference is the same across all components : V1 = V2 = V3 = V…
. This means that identical bulbs connected in parallel will all be at the same brightness
Current in parallel
. In parallel circuits, the total current flowing around the circuit is equal to the total of all the currents through the separate components
. In a parallel circuit, there are junctions where the current either splits or rejoins. The total current going into a junction has to equal the total current leaving
. If two identical components are connected in parallel, then the same current will flow through each component
Adding a resistor in parallel
. If you have two resistors in parallel, their total resistance is less than the resistance of the smallest of the two resistors
. In parallel, both resistors have the same potential difference across them as the source
. This means the ‘pushing force’ making the current flow is the same as the source pd for each resistor you add
. But by adding another loop, the current has more than one direction to go in
. This increases the total current that can flow around the circuit. Using V = IR, an increase in current means a decrease in the total resistance of the circuit
What are the two types of electricity supply
. Direct current - dc
. Alternating current - ac
Alternating current
. In alternating current supplies, the current is constantly changing direction. Alternating currents are produced by alternating voltages in which the positive and negative ends keep alternating
. The UK mains supply is an ac supply at around 230V
. The frequency of the ac mains supply is 50 Hz
Direct current
. Cells and batteries supply dc
. Direct current is a current that is always flowing in the same direction
. It’s created by direct voltage
Mains supply cable
. Most appliances are connected by the mains supply by three-core cables. This means that they have three wires inside them, each with a core of copper and a colored plastic coating
What are the color’s of the wires in the mains supply cables
. Live wire - brown
. Neutral wire - blue
. Earth wire - green/yellow
Live wire - brown
. The live wire provides the alternating potential difference (at about 230V) from the mains supply
Neutral wire - blue
. Completes the circuit and carries away current - electricity normally flows in through the wire and out through the neutral wire. It is around 0V
Earth wire - green/ yellow
. For protecting wiring and for safety, stops the appliance from becoming live. It doesn’t usually carry a current - only when there’s a fault.
It is usually at 0V
Live wire risks
. Our body like the earth is 0V. This means that if you touch the live wire, a large potential difference is produced across your body and a current flows through you
. This causes a large electric shock
. Even if a plug or socket is switched off, there is still a danger of an electric shock. A current isn’t flowing but there’s still a pd in the live wire.
. If you made contact with the live wire, your body would provide a link between the supply and the earth, so a current would flow through you
Live and earth wire risk
Any connection between the live and earth wires can be dangerous. If the link creates a low resistance path to earth, a huge current will flow, which could result in a fire
Why does a moving charge transfer energy
. The charge does work against the resistance of the circuit (work done is the same as energy transferred)
Electrical appliances
Electrical appliances are designed to transfer energy to components in the circuit when the current flows
. Kettles transfer energy electrically from the mains ac supply to the thermal energy store of the heating element inside the kettle
. Energy is transferred from the battery of a handheld fan to the kinetic energy store of the fan’s motor
Appliance energy use
. No appliance transfers all energy completely usefully.
. The higher the current, the more energy is transferred to the thermal energy stores of the components (and then surroundings)
What is the formula for amount of energy transferred by electrical work
Energy transferred (J) = Power (W) x Time (S)
E = PT
Power
. The total energy transferred by an appliance depends on how long the appliance is on for and its power
. Power is the amount of energy that is transferred per second
Potential difference and power
. When an electrical charge goes through a change in potential difference, then energy is transferred
. Energy is supplied to the charge at the power source to ‘raise’ it through a potential
. The charge gives up this energy when it ‘falls’ through any potential drop in components elsewhere in the circuit
. This means that a battery with a bigger potential difference will supply more energy to the circuit for every coulomb of charge which flows around it, because the charge is raised up “higher” at the start
Potential difference is energy transferred per charge passed
Formula for Energy and Potential difference
Energy (J) = Charge flow (C) x Potential difference (V)
E = QV
Formula for Power, Current and Potential Difference
Power (W) = Potential Difference (V) x Current (A)
P = VI
What is the National Grid
. The National Grid is a giant system of cables and transformers that covers the UK and connects power stations to consumers
. It transfers electrical power from power stations anywhere on the grid (the supply) to anywhere else on the gird it is needed (the demand - homes etc.)
Electricity Production and demand
. Throughout the day, electricity usage (demand) changes. Power stations have to produce enough energy for everyone
. The can predict when electricity will be most used though like when it gets dark or cold
. Power stations run at well below their maximum power output, so there’s spare capacity to cope with a high demand, even if there’s a shut-down of a nearby station.
. Lots of smaller power stations are kept in standby just in case
National grid energy usage
. To transmit high amounts of power, you need a high potential difference or high current
. The problem with high current is that you lose loads of energy as the wires heat up to the thermal energy store of surroundings
. it’s cheaper to boost the pd and keep current as low as possible
. For a given power, increasing pd decreases the current which decreases energy lost by dissipated energy. This makes an efficient way of transferring energy
Transformers
. To get the pd to be higher, this requires transformers, big pylons and huge insulators
. The transformers step up the potential difference at one end, for efficient transmission, and bring it down to safe usable levels at the other end
. The potential difference is increased using a step-up transformer
. The potential difference is decreased using a step-down transformer
Static electricity
. Static electricity is all about charges which are not free to move e.g. in insulating materials. This causes them to build up in one place and often ends with a spark or shock when they do finally move
What causes static build-up
. A build up of static is caused by friction
. When certain insulating materials are rubbed together, negatively charged electrons will be scraped off one and dumped on the other
. This will leave the materials electrically charged, with a positive static charge on one and an equal negative static charge on the other
. Which way the electrons are transferred depends on the two materials involved
. The classic examples are polyethene and acetate rods being rubbed with a cloth duster
Too much static
. As electric charges builds on an object, the potential difference between the object and the earth )which is 0V) increases
. If the potential difference is large enough, electrons can jump across the gap between the charged object and the earth - this is the spark
. They can also jump to any earthed conductor that is nearby - which is why you get static shocks getting out of a car
. A charge build’s up on the car’s metal frame and the charge travels through you into the earth when you get out
. This usually only happens when the gap is fairly small (But not always - lightning is a big spark)
Electrostatic forces
. When two electrically charged objects are brought close together, they exert a force on one another
. Two things with opposite electric charges are attracted to each other
. Two things with the same electric charges will repel each other
. These forces get weaker the further apart the two are
. These forces will cause the objects to move if they are able to do so (electrostatic attraction / repulsion)
How do we see electrostatic forces between two objects
. Suspend a rod with a known charge from a piece of string (so it is free to move)
. Placing an object with the same charges nearby will repel the rod
. An oppositely charged object will attract the rod
Electric fields
. An electric field is created around any electrically charged object
. The closer to the object, the stronger the field is
. You can show an electric field by using field lines
Charged objects in electric field
. When a charged object is placed in the electric field of another object, it feels a force
. This force causes attraction or repulsion
. This force is caused by the electric field of each charged object interacting with each other
. The force on an object is linked to the strength of the field it is in
. As you increase the distance between charged objects, the strength of the field and the force between them gets smaller
Sparking
. Sparks are caused when there is a high enough potential difference between a charged object and the earth (or an earthed object)
. A high potential difference causes a strong electric field between the charged object and the earthed object
. The strong electric field causes electrons in the air particles to be removed (ionization)
. Air is normally an insulator, but when it is ionized it is much more conductive, so a current can flow through it. This is the spark
