Section 5 - Electricity and Currents Flashcards
What is current?
The flow of electrical charge around the circuit / the rate of flow of charge
Unit: Ampere, A
When will a current only flow/be produced?
If there is a potential difference across that circuit, and if the circuit is complete.
The current then depends/changes depending on the potential difference and resistance
What is potential difference (voltage)
The driving force that pushes the charge round / the energy transferred per coulomb of charge (between 2 points)
Unit: Volt, V
Higher Potential difference = higher current
What is resistance?
Anything that slows the current down
Unit: Ohm, Ω
Higher resistance = Lower current
What is the current caused by in metals?
Flow of electrons
What is the equation for current (amps) ?
Charge (coulombs / C) / Time
Q / t
What is the equation for potential difference?
For general
Potential difference = energy transferred
/charge moved
V = E / Q
1V = 1J/C
What is the equation for potential difference, current and resistance?
Potential difference = Current x Resistance
V (volts) = I (amps/A) x R (ohms/mikewasouski)
How does resistance increase with temperature?
(Most of time)
1) When an electrical charge flows through a component, it has to do work against resistance
2) This causes an electrical transfer of energy - some of this useful, but some dissipated to the thermal energy stores of the component and its surroundings
3) When a current flows through a resistor, the resistor heats up – cont
Why does a resistor heat up as a current passes through it?
Electrons collide with the ions in the lattice that make up the resistor as they flow through it. This gives the ions energy, which causes them to vibrate and heat up.
-> The more the ions heat up, the harder it is for electrons to get through the resistor (+collisions) - for a given P.d, current will decrease as the resistor heats up
->If the resistor gets too hot, no current will be able to flow
The only exeption to this is a thermistor which decreases res as temp +
What does an ammeter do?
Mesures the current (in amps) flowing through the component - Has to be put in series with component, not parralel
What does a voltmeter do?
Mesures the p.d. across the component. It must be placed in parralel to the component under testing
what does changing the output of a p.d. supply do? ( when testing a component)
Alters the current flowing through the circuit and the potential difference across the component
How to stay safe when investagating circuits?
Dont let it get too hot - unplug for a bit to let it cool down
What is a diode?
a circuit component that only allows current to flow in one direction
What does a thermistor do?
their resistance decreases as the temperature increases
- Make temp detectors e.g. car engine temp sensors and electronic thermostats
What is an LDR?
LDRs (light-dependent resistors) are used to detect light levels, eg in automatic security lights. Their resistance decreases as the light intensity increases.
How could you repeat the standard test circuit for a DIODE test?
After you’ve finished taking mesurements for a range of currents, remove the diode and swap its direction. You should find that the current cannot flow through the diode anymore
How could you repeat the standard test circuit for a Thermistor test?
Keeping the supply P.d constant, gradually heat the thermistor. (E.g. put it in a hot beaker of warm water)
-> You should find that as temp increases, the current through the thermistor increases as the resistance decreases
How could you repeat the standard test circuit for a LDR test?
Conduct your experiment in a dim room - keep supply of p.d. constant and slowly adjust the light level near the LDR (e.g. by using a lamp connected to a dimmer switch).
-> You should find as the light level gets brighter, the current through the LDR increases as the resistance decreases
What are series circuits?
where the different components are connected in a line, end to end (except voltmeters, which are always parralel but dont count as part of the circuit)
What happens if you remove one component in a series circuit?
The circuit is broken and they all stop working
-> not good, not used for stuff cus of this
In a series circuit
When more cells are in series what does this do?
Basic math - more cell = add them all = bigger p.d
In a series circuit
What is the rule for current?
The same everywhere, I1 = I2 = I3
-> Depends on the total p.d. and total resistance
In a series circuit
What is the rule for p.d?
The potential difference of the supply is shared between its components
-> P.d. for each component depends on its resistance
In a series circuit
What is the rule for resistance?
total resistance of the circuit increases as you add resistors
What happens in parralel circuits?
Each component is seprerately connected to the +ve and -ve of the supply (exept ammeters, which are always connected in series)
What happens if you disconnect a component in parralel circuits?
It will hardly affect the others at all
- This is how most things are connected e.g. household lights have to be turned off and on seperately
What do everyday circuits contain?
A mixture of series and parralel parts
What are the rules for a parralel circuit?
Potential difference
P.d is the same across all components
What are the rules for a parralel circuit?
Potential difference
P.d is the same across all components
What are the rules for a parralel circuit?
Current
Current is shared between branches. The total current flowing around the circuit is equal to the total of all the currents through seperate components
What are the rules for a parralel circuit?
Juction
There are junctions where the current either splits or rejoins
- total current going into a junction = total current leaving
What are the rules for a parralel circuit?
Resistance
Total resistance decreases if you add a second resistor in parralel
Why is resistance in a series circuit a total of the resistances?
Each time a resistor is added, it shares the total P.d.
-> The p.d. 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 decreases when a resistor is added.
Bigger resisitance = Bigger share of total P.d.
Current vs potential difference
Potential difference is the energy
Current is the movement
What happens if two resistors are in parralel?
Their resistance is less than the resistance of the smallest of the two resistors
In parralel, both resistors have the same P.d. as source, so the push of current is constant with added resistors.
By adding another loop (w/resistor) the current has more than one direction to flow in, increasing overall current flow around the circuit.
Cus V = IR (and p.d.) is the same, resistance decreases
What happens when an electrical charge does through a change in potential difference?
Energy is transferred ( as work is done against resistance)
How does energy supply work at the source?
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
What is the equation to find energy transferred to an electrical component?
E = V x I x t
E = Energy (Joules/J)
V = p.d. (Volts/v)
I = Current (amps/A)
t = Time (s)
The larger current through, or P.d. across a component, the more energy transferred to it
How do kettles transfer energy in its circuit?
Kettles transfer energy electrically from the mains (a.c.) supply to the thermal energy store of the heating element inside the kettle
Higher current = higher unuseful energy transfer
How do hand-held fans transfer energy in their circuits?
Energy is transferred electrically from the battery of the hand-held fan to the kinetic energy store of the fan’s motor
What are the disadvantages of heating in a circuit?
1) reduced efficiency - loss to thermal energy store
2) If temp is too high it can cause other components to melt - stops circuit working
How do fuses protect circuits?
When its too hot, the fuse melts and breaks the circuit
How is heating an advantage in circuits?
Its useful if you want to heat something
How does heating work as an advantage in the circuit of a toaster?
Toasters contain a coil of wire with a really high resistance. When a current passes through the coil, its temperature increases so much that it glows and gives off infrared radiation - this transfers energy to the bread and cooks it
Filament bulbs and electric heaters work in a similar way
What does the total energy transferred by an appliance depend on?
How long the appliance is on for and its power
(power = energy transfer per second)
OR P.d and current
In electricity
What is the equation for power?
Power (w/watts) = Energy transferred (J) / Time (s)
P = E/t
what is an appliance power rating?
The minimum safe power that they can operate at - their maximum operating power
The maximum amount of energy transferred between stores per second when the appliance is in use
How does a power rating help customers?
Lower power rating = the less electricity used in a given amount of time = cheaper to run
Higher power rating ≠ more efficient
-> They exist seperately
What is the equation for electrical power?
Electrical power (w) = Current (A) x p.d. (V)
P.d. = How much energy of each unit of charge transfers
Current = how much charge passes per unit time
What are the two types of electricity supplies / current?
Alternating current and direct current
What is happening in A.c. supplies?
the movement of the charges is constantly changing direction -> This is produced by alternating +ve and -ve current
This is the UK’s mains supply
What p.d. is the UK’s a.c. mains supply?
230V
What is the frequency of the UK’s a.c. mains supply?
50 Hz ( or cycles per second)
What things supply direct current?
Cells and batteries
What is direct current?
The movement of charges in only one direction. Its created by a direct voltage ( a p.d. that is only positive or negative, not both)
What are the main 3 cables in wires and their colors (of their insulation) ?
Neutral wire - blue
Live wire - Brown
Earth wire - Green and Yellow
Each with a core of copper and colored plastic coating
Brown
What does the live wire do?
The live wire carries the Potential difference. It alternates between a +ve and -ve voltage of about 230V
Blue
What does the neutral wire do?
The neutral wire completes the circuit - current flows back in this ( 0V )
Green and yellow
What does the Earth Wire do?
The Earth wire is for safety and protecting the wire. It carries the current away of something goes wrong and stops the appliance casting becoming live
At 0V
What is the p.d. between the live wire and the neutral wire?
(equals the supply p.d.) 230V for the mains
What is the p.d. between the live and earth wire?
230V for a mains-connected appliance
What is the p.d. between the neutral wire and earth wire?
0V
Why do plug sockets have switches connected to the live wire of the circuit?
So the circuit can be broken
-> an appliance becomes isolated and the risk of an electric shock is reduced
Why does touching a live wire cause an electric shock?
Your body is at 0V
-> If you touch a live wire, a large p.d. is produced across your body and a current flows through you
This causes a large electric shock which could injure or even kill you
Why can you still get an electric shock from an off plug socket or light switch?
A current isnt flowing, but there is still a p.d. in the live wire. Your body would provide a link between a supply and the earth, so a current would flow through you
Why is any connection between live and neutral dangerous?
It creates a low resistance path to earth, a huge current will flow, resulting in a fire
What helps to prevent electrical overloads?
Earthing and fuses
Why do surges occur?
Due to changes in a circuit (e.g. an appliance suddenly switching off or a fault in an electrical appliance)
What can current surges lead to?
Circuits and wiring in your appliances melting or causing a fire, and faulty appliances can cause deadly electric shocks
How does the earth wire and fuse prevent from current surges?
1) If a fault develops in which the live wire somehow touches the metal case, then because the case is earthed, too great a current flows through the live wire, the case and the earth wire
2) The surge in current melts the fuse when amount of current is greater than fuse rating. Fuses are connected to the live wire, so breaking the fuse breaks the circuit and cuts off the live supply
3) This isolates the whole appliance, making it impossible to get an electric shock from the case. It also prevents the risk of fire caused by the heating effect of a large current
this functions the same way household fuses work
How should fuses be rated?
As near as possible but just higher than the normal operating current
How do current and cable thickness relate?
Larger current = thicker cable needed ( to stop cable getting too hot and melting )
So then fuse rating increases with cable thickness
How do circuit brakers work?
1) Instead of melting a fuse, a large current may ‘trip’ (turn off) a circuit breaker
2) Circuit breakers turn off quicker than the time taken for a fuse to melt
3) They can also be reset - cheaper than replacting a fuse
4) But circuit breakers are more expensive than fuses
Theyre even safer than fuses
What are all appliances with metal casing given?
Theyre all ‘earthed’ to reduce the danger of electric shock
What is ‘earthing’
The casing being attached to the live wire
-> An earthed conductor can never become live
What is an appliance with plastic casing and no metal showing said to be?
Double-insulated
What does double-insulated mean?
Anything with double insulation doesnt need an earth wire - just a live and neutral
What are cables that only carry two wires (live and neutral) called?
Two-core cables
