Section 5: Electricity & Circuits Flashcards
What is electric current?
The rate of flow of charge
What allows current to pass through metals?
Delocalised electrons
What happens to current if there is no source of potential difference?
No current flows
What happens to current if there is a source of potential difference?
Current flows
What is resistance?
Anything that slows down the flow of charge
What does current through a component depend on?
The components resistance and potential difference across the component
What happens to current if resistance increases?
The greater the resistance, the smaller the current
What can be used to change the current in a circuit?
Variable resistor
What is potential difference?
The energy transferred per unit of charge that passes between two points in a circuit
What is another name for potential difference?
Voltage
What is the unit of measure for charge?
Coulombs, C
What is the unit of measure for current?
Amperes, A
What is the unit of measure for resistance?
Ohms, Ω
What is the unit of measure for potential difference?
Volts, V
What is 1 volt equal to?
1 J/C OR 1 joule per coulomb
What happens to current in an open switch?
Current doesn’t flow
What happens to current in a closed switch?
Current flows
What are the three different current-potential difference graphs?
- Fixed resistor at a constant temp
- Filament lamp
- Diode
What happens in a fixed resistor at a constant temperature graph?
Current is directly proportional to potential difference SO resistance doesn’t change
What type of graph is the fixed resistor at a constant temperature graph?
Linear
(Straight line going through origin)
What happens in a filament lamp graph?
Current increases SO temperature of filament increases SO resistance increases
What does a filament lamp graph look like?
Non linear
Plateaus at bottom and top but straight line in the middle
What happens in a diode graph?
High resistance in one direction SO current only flows in the other direction
What does a diode graph look like?
Non linear
Line starts straight then its gradient rapidly increases
What components are the ones with a changing resistance when current through them varies?
Filament lamp
Diode
LDR (light dependent resistor)
Thermistor
CIRCUIT SYMBOLS MADE ON IN PERSON FLASHCARDS
What does resistance depend on in a LDR (light dependent resistor)?
Light intensity
When in there lower resistance in a LDR (light dependent resistor)?
When there’s a brighter light
When in there higher resistance in a LDR (light dependent resistor)?
When there’s a dimmer light
What does resistance depend on in a thermistor?
Temperature
When in there lower resistance in a thermistor?
Hotter temperatures
When in there higher resistance in a thermistor?
Cooler temperatures
What do resistance graphs look like for LDRs?
Resistance on y axis
Light intensity on x axis
The line goes downwards
The gradient is steep but gradually lowers
What do resistance graphs look like for thermistors?
Resistance on y axis
Temperature on x axis
The line goes downwards
The gradient is steep but gradually lowers
How is each component connected in series circuits?
Connected in a line
End to end with a power sourxe
What is current like at different points in a series circuit?
It’s the same everywhere
Current stays constant in a series circuit and doesn’t split between branches (as there aren’t any)
What is potential difference like at different points in a series circuit?
The total source of potential difference (the voltage supplied by the power source) is shared between components
What is resistance like at different points in a series circuit?
Total resistance of the components = the sum of their resistances
E.g. Rtotal = R1 + R2
What happens when a resistor is added in a series circuit?
The total resistance of the circuit is increased
What happens if a component breaks in a series circuit?
No current will flow
How is each component connected in a parallel circuit?
Separately connected on each different branch of the circuit
What is current like at different points in a parallel circuit?
Total current flowing around in the circuit = sum of the current flowing through each branch of the circuit
E.g. Itotal = I1 + I2
The total current entering a junction (new branch) = total current leaving the junction (new branch)
What is potential difference like at different points in a parallel circuit?
The potential difference across each branch in a circuit is the same as the potential difference/voltage supplied by the power source
E.g. V1 = V2 = Vtotal
What is resistance like in a parallel circuit?
The total resistance of resistors in parallel circuits, are less than the resistance of the smallest resistor
What happens if you add a resistor into a parallel circuit?
The total resistance of the circuit will decrease
What happens if a component breaks in a parallel circuit?
The components on other branches will still work as long as current still flows through them
What equipment measures potential difference/voltage? How is it put into a circuit?
A voltmeter - put in parallel to the lamp or component
What equipment measures current? How is it put into a circuit?
An ammeter - put in series of the circuit (not in parallel to the component)
What equipment measures resistance? How is it put into a circuit?
An resistor - put in series of the circuit (not in parallel to the component)
What is the correlation between work being done and charge?
When charge flows, work is done (so energy is transferred)
What happens when energy is transferred electrically?
It’s then stored in an energy store
E.g. light switch energy stored in thermal energy store of heating element of a kettle
What happens to some energy in circuits?
Some energy is dissipated to thermal energy stores when a current does work against resistance
Has a heating effect
How does a current working against resistance create heat?
- Current flows through a resistor
- Electrons collide with, and energy transfer to, the lattice of ions in the resistor
- Ions vibrate more, so energy in thermal energy store increase s
- The resistor heats up
Advantages of the heating effect caused by current working against resistors?
Used in electric heaters
Disadvantages of the heating effect caused by current working against resistors?
Causes energy losses in circuits
What is power?
Energy transferred per second
What is a power rating?
The maximum amount of energy transferred between stores per second when the appliance is in use
What is the correlation between current, potential difference and power?
As current through or potential difference across a device increases, the power increases
What are the two types of electricity supply?
- Alternating current (a.c.)
- Direct current (d.c.)
What is alternating current?
Current where the movement of charge constantly changes direction
What is alternating current produced by?
An alternating voltage, where positive and negative ends of the potential difference keep altering
What is alternating current used in?
Used as the UK’s main power supply
What is direct current?
Current where movement of charge is only in one direction
What is direct current produced by?
A direct voltage, where the potential different is only positive or only negative NOT BOTH
What is direct current supplied by?
Cells and batteries
What are the three key facts about the UK Mains Supply?
Has an a.c. supply
Frequency of 50Hz
Voltage of around 230V
What are the three core cables?
- Live wire
- Neutral wire
- Earth wire
What is the function of the live wire?
Provides alternating potential difference from mains supply
What is the function of the neutral wire?
Completes the circuit
What is the function of the earth wire?
Safety - stops appliance casing becoming live
When does current flow through the earth wire?
When there’s a fault
What is the potential difference between live wire and neutral OR earth wires?
230 V
What is the potential difference between neutral wire and earth wires?
0 V
How do electric shocks happen?
- Human makes contact with the live wire
- Large potential difference produced across body
- Current flows through body
- Electric shock - injury or even death
What can happen when the live wire makes a connection with Earth?
It could cause a fire
It’s very dangerous
What can make a live wire safe?
If a case of appliance is put on it
What happens to a fuse and circuit breaker if it’s connected to a live wire and the wire has a surge of current? What does the lead to?
Fuse can melt
OR
Circuit breaker trips
Leads to the circuit breaking appliance being isolated - preventing fires and shocks
What fuses are there?
3A
5A
13A
What fuse do you choose if you know the operating current of a device?
The next one up
E.g. if a phone charger has an operating current of 2.1A, it’ll need a fuse of 3A
COMPONENTS & CIRCUITS CORE PRACTICAL: How can a single component can be investigated?
- Set up a parallel or series circuit
- Put the voltmeter in parallel to the component you’re investigating and an ammeter in series to the component
- Vary output of potential difference of the source
- Take several pairs of readings for the current and voltage of the component
- Calculate the resistance by doing voltage/current
- Repeat the steps but re arrange the circuit (e.g. by adding more branches or components) and see how that affects the results
COMPONENTS & CIRCUITS CORE PRACTICAL: In conclusion, what happens in series and parallel circuits when the source had an increased potential difference?
Series circuit: total current through the circuit also increases
Parallel circuit: total current through the circuit also increase
COMPONENTS & CIRCUITS CORE PRACTICAL: In conclusion, what happens to the potential difference across each component in a series and parallel circuit?
Series: the potential difference supplied is shared between components
Parallel: the potential difference across the component is the same as the supplied potential difference
COMPONENTS & CIRCUITS CORE PRACTICAL: In conclusion, what happens to the total current through a circuit in a series and parallel circuit?
Series: it’s the same everywhere and decreased as more components are added
Parallel: its equal to the sum of the current in the branches and increases as components are added
