electricity Flashcards
What is the circuit symbol for a switch?
switch - two circles with a line connecting them
What is the circuit symbol for a cell?
cell - a larger line and a smaller line parallel with a small gap between them
What is the circuit symbol for a battery?
battery - two cells with a dotted line joining them
What is the circuit symbol for a diode?
diode - circle with a line through it following the line of the wire, with a triangle with a line on it
What is the circuit symbol for a resistor?
resistor - rectangular box
What is the circuit symbol for a variable resistor?
variable resistor - resistor with arrow through it
What is the circuit symbol for a LED?
LED - diode with two small arrows coming off it
What is the circuit symbol for a lamp?
lamp - circle with X in it
What is the circuit symbol for a fuse?
fuse - rectangular box with line joining the wire going through it
What is the circuit symbol for a voltmeter?
voltmeter - circle with V in it (measures volts)
What is the circuit symbol for an ammeter?
ammeter - circle with A in it (measures amps)
What is the circuit symbol for a thermistor?
thermistor - resistor with line going through it that is flat just below the box
(used in thermostats)
What is the circuit symbol for a LDR?
LDR - resistor with circle around it and two arrows with the heads towards it
(used in outdoor lighting)
What is the function of a diode?
A diode makes the current in a circuit flow in only one direction.
What is the function of a resistor?
A resistor is fixed and restricts the current.
What is the function of a variable resistor?
A variable resistor means that the resistance can be changed and it restricts the current to a certain extent.
What is the function of a LED?
A LED is a diode that gives off light.
What is the function of a thermistor?
A thermistor changes the resistance based on temperature.
- as temperature increases, resistance decreases
What is the function of a LDR?
A LDR is a light dependent resistor.
- as light increases, resistance decreases
What is current? Give the unit.
Current is the flow of electric charge (electrons) around a circuit. It carries energy from the source and passes it to each component. It is measured in Amperes (A).
In what direction does current flow in a circuit?
Current flows in a circuit from the positive to the negative end of a cell.
What is potential difference? Give the unit.
Potential difference is the driving force that pushes a charge around a circuit. It is measured in volts (V). It is the energy per charge (Joules per Coulomb).
What is electrical charge? Give the unit.
Electrical charge is a property which experiences a force in an electric field. It is measured in Coulombs (C).
What is 1A, in terms of charge?
1A is 1C of charge flowing every second.
How do you calculate charge?
charge (C) = current (A) x time (s)
What is resistance? Give the unit.
Resistance is the potential difference required to drive a current through a component. It is measured in Ohms (Ω).
How do you calculate resistance?
resistance (Ω) = potential difference (V) / current (A)
What is an ohmic conductor? Give two examples.
An ohmic conductor is a component that obeys Ohm’s law (V=IR). For example, a wire or a resistor.
What is the relationship between potential difference and current across a resistor?
Current is directly proportional to the potential difference across a resistor (at a constant temperature), so the resistance remains constant.
When does resistance change across a diode?
If the current is reversed across a diode, the resistance increases.
When does resistance change across a bulb?
Some of the energy from the charge is transferred to heat, which causes the resistance to increase.
What is the I-V graph for an ohmic conductor (linear conductor)?
For an ohmic conductor, the I-V graph is directly proportional at a constant temperature.
current is y axis, voltage is x axis
What is the I-V graph for a filament lamp (non-linear conductor)? Why?
For a filament lamp, the I-V graph... - passes through the origin - curves off at the ends (like an S) This is because as current increases, the temperature increases as some energy is transferred to thermal, so the resistance increases as more room is taken up by ions so it's harder for them to move, so less current can flow per V, so graph gets shallower. current is y axis, voltage is x axis
What is the I-V graph for a diode (non-linear component)?
For a diode, the I-V graph…
- is flat on the left side of the y axis
- accelerates after passes (0,0) (like a _/)
This is because the current will only flow in one direction, so there is a very high resistance in the reverse direction, so no current flows.
current is y axis, voltage is x axis
What is a series circuit?
Series circuits have all their components connected in one loop of wire. If you remove or disconnect one component, the circuit is broken and the current stops flowing.
What is a parallel circuit?
Parallel circuits have their components connected in multiple loops of wire in parallel to each other. If you remove or disconnect one component, the other branches of wire are fine.
What is the rule for current in a series circuit?
series circuit
- the current is the same everywhere
What is the rule for potential difference in a series circuit?
series circuit
- potential difference of the power supply is shared between the components
- it is only shared equally if the resistance across each of the components is equal
What is the rule for resistance in a series circuit?
series circuit
- the total resistance across each component is the resistance of the circuit
What is the rule for current in a parallel circuit?
parallel circuit
- the current of the circuit is the current of each branch of wire added
What is the rule for potential difference in a parallel circuit? Why is this?
parallel circuit
- the potential difference is the same across each branch of wire
- it is shared within that branch
- this is because when it separates across the branches, at that point, the voltage has not yet been used up, so the driving force continues
What is the rule for resistance in a parallel circuit? Why is this?
parallel circuit
- the total resistance is the less than the resistance of the smallest resistor
- this is because there are more pathways for the current to take, so more total current will flow through the circuit, and R = V/I
What is a direct current?
A direct current only flows in one direction.
What is an alternating current?
An alternating current constantly changes direction.
What is the frequency of a wave?
The frequency of a wave is the number of wave cycles in one second, in Hertz (Hz).
What is mains electricity? Is the current direct or alternating? What is the potential difference and frequency?
Mains electricity is the electricity supply for most homes in the UK.
- alternating current
- 230V and 50Hz
What kind of cable is connected to an appliance in the UK?
Most electrical appliances are connected to mains electricity by a three-core cable.
What is each wire in a three-core cable composed of?
Each wire in a three-core cable is made of copper as it is a good conductor. Each has a coating of plastic as it is a great insulator.
Name the three parts of a three-core cable, their colours and their functions.
three-core cable
- live wire - brown, carries the alternating voltage from the supply at 230V
- neutral wire - blue, completes the circuit and carries the current away, 0V
- earth wire - green and yellow, a safety wire to stop the appliance from becoming live, so only carries current when there’s a fault, 0V
Why is the live wire dangerous? How is this danger reduced? Why does it not remove the danger completely?
dangers of three-core cable
- live wire can be dangerous when touched as the current will flow through a person into the earth to complete the circuit, so they’re electrocuted
- this can happen even when the switch is open as the first part of the live wire still has 230V
- if the appliance has a metal case and the live wire touches it, the case will become live
- the metal case is attached to the earth wire, which is connected to the ground, so if the case becomes live, the current will flow to the earth, the fuse melts and the current shuts off
- this doesn’t remove the danger completely as if the live and earth wire touch, a low resistance path to the earth is created
- a huge current will flow, which can result in a fire
What is electrical power? Give the units.
Electrical power is the rate of energy transferred, in Watts (W). 1W = 1J per second
How do you calculate electrical power? Give three equations.
power (W) = voltage (V) x current (A)
power (W) = current^2 (A) x resistance (Ω)
power (W) = energy transferred (J) / time (s)
What are everyday appliances designed for?
Everyday appliances are designed to bring about energy transfers.
eg. kettle - electrical energy to thermal energy
fan - electrical energy to kinetic energy of motor
When is work done in a circuit?
Work is done in a circuit when a charge flows.
Give two factors that affect the amount of energy transferred by an appliance.
affecting energy transferred by appliance
- the time the appliance has been switched on for
- the power of the appliance
How do you calculate the energy transferred in a circuit?
energy (J) = charge flow (C) x voltage (V)
What are power ratings?
Every electrical appliance has a power rating. This is the highest safe power they can operate at.
- the lower the rating, the less electricity is needed, so the cheaper it is (however, isn’t always usefully transferred)
- appliances which transfer thermal energy usually have a much higher power rating than those that transfer kinetic energy
What is the national grid?
The national grid is a system of cables and transformers, linking power stations to customers. This is how electrical power is transferred to customers in the UK.
How does the national grid increase the power to customers?
To increase the power in the national grid, we increase the V or I as P = VI, but increase V as it is cheaper to do.
What is the function of step-up transformers?
step-up transformers
- increase V from about 25,000V to 400,000V
- this reduces energy loss in the transmission cables
- less/the same current is used as it is less energy, so less is transferred to heat, which is wasteful, and it is less expensive
What is the function of step-down transformers?
step-down transformers
- decrease V from about 400,000V to 230V
- this makes the voltage safe for home usage
What happens when certain insulating materials are rubbed on each other? Give an example.
static electricity
when certain insulating materials are rubbed on each other (friction), they become electrically charged as electrons move from one object to the other. The numerical value of the charges on each are equal. They can then cause a force of repulsion or attraction.
eg. polyethene rod and cloth rubbed, electrons move from cloth to polyethene rod
How are sparks caused?
Sparks are caused by too much static electricity.
- as the charge builds on an object, the V between the object and the earth increases
- when the V gets large enough, electrons can jump between the gap, which earths the charge and cause a spark
- this can trigger an explosion
- higher V causes a stronger electric field between the charged object and the earth
- the strong electric field causes electrons in the air particles to be removed (ionisation) so the air is now a conductor, so the current can flow through it and cause a spark
What is an electric field?
An electric field surrounds a charged object. This gets stronger as you get closer to the object.
Draw the electric field for a positively charged object.
electric field for positive object
- circle with + in it
- lines with arrows facing AWAY from the circle
- the further away the arrows are from each other, the weaker the force
Draw the electric field for a negatively charged object.
electric field for negative object
- circle with - in it
- lines with arrows facing TOWARDS from the circle
- the further away the arrows are from each other, the weaker the force
