⭐️Topic SP10/SP11 - Electricity, static electricity and circuits Flashcards
What are the rules of current in a series circuit?
- it’s the same everywhere
- it reduces as more components are added
What’re the rules of potential difference in a series circuit?
Potential difference is shared between components so will reduce as more components are added
What are the current rules in a parallel circuit?
- current entering and leaving the circuit is the same
- current splits at a junction
- the more branches, the more current on each branch
What is the rules for potential difference in a parallel circuit?
Potential difference is the same on each branch and is shared between the components of each branch
Why do metals conduct electricity?
Because metals have few electrons in their outer shell which are weakly attracted to the nucleus and so there are many free electrons that can be pushed by the voltage from a battery
Where do electrons move to in a circuit?
Towards the positive terminal as they are negatively charged
What is the conventional direction of current?
Where in a circuit, the conventional current direction goes from the positive terminal to the negative terminal of the battery
What is current measured in? What with?
Amps using an ammeter
How do you use an ammeter?
You connect it in series to measure the current passing through a component or circuit
When current moves through a circuit does it change?
No. current leaving the positive terminal of a battery is the same as current arriving at the negative terminal as it’s conserved
What does current do at each junction of a parallel circuit?
It splits but total amount entering junctions is the same as total amount leaving
What does potential difference do in a circuit?
It ‘pushes’ the current around an electric circuit. It allows electrons to flow when applied across a component
The bigger the potential difference across a component…
The bigger the current
What is needed for a current to flow?
The circuit must be closed and must contain a good source of potential difference like a cell or battery
What is potential difference measured with and what’s the unit?
With a voltmeter and it’s in volts
How do you use a voltmeter?
You connect it in parallel to measure the potential difference across a component or circuit
What is charge? What is charge measured in?
The rate of the flow of current and is measured in coulombs (C)
What is 1 coulomb?
The charge that passes a point in a circuit when there is a current of 1 amp for 1 second
What is the current? What does it do?
The flow of electrons and so moves charged particles
What does the size of the current in a circuit tell you?
How much charge flows past that point each second
How can you calculate charge? Give units
Charge (C) = current (A) x time (s)
Also written as
Q = I x t
How is energy transferred in a circuit with a cell and a lamp starting from the cells store of energy?
- The cell has a store of energy
- Energy is transferred to the charge
- They charge can now transfer energy to the components in the circuit ; it has a store of potential energy (this means it has the potential to transfer energy to other components in the circuit)
- Energy is transferred from the charge as it moves through the lamp
- The lamp transfers energy to the surroundings by heating and by light
What is the potential difference of a cell?
The amount of potential energy the cell transfers to each coulomb of charge
How can you calculate energy transferred using charge and potential difference? Use units
Energy transferred (J) = charge moved (C) x potential difference (V)
Also written as:
E = Q x V
What is resistance and what is it measured in?
A measure of how hard it is for the current to flow measured in ohms
What does it mean is some wires and components have a large resistance?
They they need a larger potential difference to produce current through them than others
How can you calculate potential difference in terms of resistance and current? use units
Potential difference (V) = current (A) x resistance (Ω)
Also written as
V = I x R
What happens to total resistance of a circuit when resistors are connected in series and explain?
The total resistance is increased because the pathway becomes harder for current to flow through.
How is potential difference distributed across resistors in series?
It’s shared between resistors but not always equally but resistors with higher resistances will have a greater potential difference
When resistors are connected in parallel describe the total resistance
It’s less than the resistance of the individual resistors because there are now more pathways for the current
What can a circuits with a variable resistor be used for?
To check wether a resistor has the correct value or to measure an unknown resistance
How do you know that two variables are in direct proportion on a graph?
The line is straight and it goes through the origin
Describe what is meant by p.d and current being in direct proportion say why they are so
When p.d. changes across a fixed resistor, the current changes by the same % due to the resistance staying the same
Gives examples of components where resistance changes when p.d. Changes
Filament lamps and diodes
What causes current to flow through a filament lamp?
Potential difference
Explain why pd and current are not proportional across a filament lamp
The greater the p.d, the more current flows and the hotter and brighter the filament gets but this causes resistance to increase. This means when pd changes current does not change by the same % and so aren’t in proportion because of the change in resistance
Explain the graph showing current and potential difference for a diode
Means it has a low resistance if pd is in one direction and and high resistance if it’s in the opposite direction so current can only flow in one direction and is not directly proportional
Describe the resistance of a light dependant resistor in the dark and in the light
It has a high resistance in the dark but the resistance gets smaller when light intensity increases
Describe the resistance of thermistors in the heat and the cold
High resistance in low temps and low and as temp increase resistance decreases
Why do circuits warm up when there’s a current?
Because when a current passes through a resistor, energy is transferred as electrical work is done against the resistance and so energy transferred by heating and the resistor becomes warmer
Give examples where heating effect of circuits is not useful
In a computer or in plugs and wires as useful energy is being transferred from the circuit by heating
What happens to wasted energy in circuits (usually by heating)
The wasted energy is dissipated to the surroundings which gain thermal energy
Explain how resistors work in terms of electrons and ions
- Electrons flow through the lattice of vibrating ions in the resistor and collide with the ions
- The more collisions the electrons make with ions, the harder it is for them to pass through so the higher the electrical resistance
- When electrons collide with the ions, they transfer energy to them
How can resistance in circuits be reduced?
- By using wires made from metals with a low resistance such as copper. The thicker the wires, the lower the resistance.
- by cooling metals so that the lattice ions are not vibrating as much
How do you calculate the energy transferred in a circuit? Include units
Energy transferred (J)= current(A) x p.d (V)x time(s)
Why is it useful to use power?
As energy transferred by an electric current depends on the time taken
What is power? Give a unit
The energy transferred per second in watts (W) so 1W is a transfer of 1J per second
How can you calculate power?
Power(W) = energy transferred (J) / time taken (s)
How do you calculate electrical power and what does that mean? Use units
Electrical power (W) = current (A) x p.d(V) Which means the power transfer in a component is directly proportional to the p.d. Across it and the current through it
Give the energy stores and transfers in an electric fan
Store of chemical energy in the battery is transferred by electricity to the motor where its transferred to a store of kinetic energy in the fan, some energy will also be transferred by heating to the motor, wires and surroundings which gets dissipated by heating increasing a store of thermal energy
explain the energy transfers and stores in heating gloves
They will have a high resistance as energy stored in battery is transferred by electricity to the wires where it’s transferred by heating to a store of thermal energy in a wire. This energy is then is transferred by heating to the gloves and hands of the wearer and eventually dissipates to the surroundings
What resistance will the wire in heat up gloves have?
High resistance
How does electricity get to the mains? Use energy stores and transfers
In a power station, energy is transferred from a store of kinetic energy by electricity and this is carried to our homes through the national grid (a network or cables and wires)
How is energy transferred by electricity used in a washing machine?
The motor transfers energy to kinetic energy in the washing machine drum
What is direct current?
Where cells and batteries have a positive and negative terminal and the direction of movement stays the same in the circuit
What is alternating current and where is it commonly outputed?
It’s where the direction of current keeps changing and is the electricity in the mains
Why is mains electricity alternating current.
As it’s produces using generators that rotate causing current direction to keep changing
What is the frequency of the mains supply and what does this mean?
It’s 50 Hz as the alternating currents has 50 cycles per second
Describe the voltage in alternating current
the voltage changes causing it to peak voltage and then it changes to 0. It then increases to a peak in the opposite direction and then changes to 0
How many joules of energy is transferred by 3kw?
3000j of energy
In a plug what colours the earth wire, what does it do. What voltage is it usually
It’s green and yellow striped and connects to the metal parts of the appliance to a large metal spike or metal tubing that is pushed into the ground. It’s a safety feature. Usually at 0V if the circuit is correctly connected
In a plug what colours the neutral wire, what does it do and what voltage is it at
It’s blue
Acts as a return path to the power station and will be at a voltage of 0V providing the circuit is correctly connected
In a plug what current does the fuse generally carry?
3A, 5A or 13A
In a plug what colour is a live wire, what does it do and what’s the voltage on it?
It’s brown
Connects the appliance to the generators
Voltage of 230V
What is a fuse? What does it do?
It’s a tube with a thin wire inside.
Current passes through and the wire gets hotter and if the current exceeds a certain value it melts breaking the circuit and stopping the current
How is a fuse a safety feature?
As if the product did draw too much current, it can cause overheating of the wiring in either the walls or the appliance and so cause a fire but the wire in the fuse melts when there’s too much current In the first place
How is the earth wire a safety feature?
As metal parts of an appliance can be at high voltage if it develops a fault and touching the metal can give an electric shock but the earth wire sends the current into the ground instead of you as the metal parts are connected to it
What happens if a fault causes the live wire to touch a metal part in an appliance?
It makes a very low resistance circuit of 230V and 0V being the earth, causing a large current to flow to the earth which heats up the wire and could cause a fire so the current blows the fuse and cuts if the mains electricity supply
What are circuit breakers?
Detect a change in the current and safely switch off the supply so can be an alternative to fuses
What is an electric shock?
Where a current flows through you and into the ground
Give 2 advantages of circuit breakers over fuses
- once a fault is fixed, they can be switched back on again but a fuse has to be replaced
- they work very quickly so are more likely to save lives whereas a fuse takes time to melt and won’t prevent you getting a shock e.g. you touch a live wire
What is an electrical conductor?
Materials that are very good at transferring charge through them
What are electrical insulators?
Materials that are difficult to transfer charges through them?
What is an electrical insulator good at?
Allowing for charges to be collected
Why happens when you charge an object by friction?
Take a cloth and polythene rod as an example
- The rod and cloth are both neutral
- when rubbed together some of the negative electrons from the cloth are transferred to the rod
- the rod now has more negative than positive charges so is now overall negatively charged and the cloth is overall positively charged as it has less electrons
What is an electrostatic force?
Forces between charges that aren’t moving
How does a van de grand generator work?
A roller moves a built to charge a Large metal sphere which builds up as electrons transfer from one insulator to another
How do you get a ballon to stick to the wall?
- You rub it on an insulator where electrons are transferred so it has a charge
- day the ballon is negatively charged, when you stick it to the wall, electrons move back as they are repelled by the negative balloon and so the positive charge of the wall attracts the negative charge on the balloon
- this means the wall has been charged by induction as the wall isn’t normally like this and goes back to normal when negative charges in the ballon are no lover attracting the wall’s positive charge
How does the discharge sphere of a van de graft generator work?
Say the dome is positive
As spark occurs between the dome and the discharge sphere as electrons flow from positive to negative reducing the overall positive charge of the dome causing it to loose the excess charge
Give an example of being earthed
If you touch a conductor like a metal tap and there is a spark and feel a small electric shock, as electrons flow in whichever direction removes the excess charge this is an example of earthing
Give the steps to the formation of lightening
- There is friction between water, ice and air in storm clouds
- This causes a charge to build up with a positive charge at the top and negative at the bottom
- The ground is induced and becomes positively charged so the charge is earthed by the ground as there’s a rapid discharge of static electricity to become neutral which is seen as the lightening
How are electrostatics used in insecticide sprayers?
The insecticide is given an electrostatic charge so that they are attracted to the crops as they are oppositely charged allowing it to reach the crops quicker and it also spreads out more evenly over the crops as the droplets repel each other as they have the same charge
How is electrostatics used in paint guns?
By being electrostatic, surface space that’s coated with paint is increased because the paint drops will not touch each offer as they repel and they adhere better to the surface
How is electrostatics used in photocopiers?
The toner (black powder) has a negative charge and the drum under the gloss is positive allowing the toner which is attracted to the drum to be attracted to paper making a black and white image of the original
How do factory chimneys use electrostatics?
electrostatic smoke precipitators smoke dirty gas Past two electrodes giving it a negative charge and above that are positively charged plates which the soot is attracted to and so the plates collect it
What are the dangers of electrostatics in refilling aircrafts?
If fuel passing along the hose to the vehicle was allowed to build up a static charge, a resulting spark may ignite the fuel and so hoses are earthed to prevent this
What are the dangers of electrostatics in helicopters?
As they are a conductive body, they can be dangerous when flying as they are in a highly electrostatically insulated environment due to electrostatic charge generated so they are earthed
What is a force field?
The volume of space around an object in which another object can experience a force
When does a magnetic material feel a force from a magnet?
If it’s inside it’s magnetic field
What is an electric field?
The force field around a charged object
What are the characteristics of field lines in electric field diagrams?
- they never cross
- show where the field is strongest which is where the line is closer together
- show the direction of the force on a charge in the field
- start in a positively charged object and end on a negative one and if there’s only one object they keep going and become more widely spread
What is t point charge?
The single point where field lines start in an electric field diagram
More info
Watch my gcse science video on electrostatic fields
