Electricity Flashcards
What is electrical current?
Flow of electrical charge
What is the size of electric current?
The rate of flow of electrical charge
What is the equation linking charge flow, current and time?
Charge flow = current x time
How is current distributed in a single closed loop?
Same value at any point
What two things is the current through a component dependent on?
The resistance and potential difference across the component
What is the equation linking current, potential difference and resistance?
potential difference = current x resistance
What is the relationship between current through an ohmic conductor and potential difference across a resistor?
Directly proportional
Is the resistance of components such as lamps, diodes, thermistors and LDRs constant?
No, it changes with the current through the component
Does the resistance of a filament lamp increase or decrease with the temperature of the lamp?
The resistance of the filament lamp increases as the temperature does
T/F: A diode has a very high resistance in one direction
True, the current through a diode flows in one direction only, the diode has very high resistance in the reverse direction
Does the resistance of a thermistor increase or decrease as the temperature increases?
The resistance of a thermistor decreases as the temperature increases
Does the resistance of an LDR increase or decrease as light intensity increases?
The resistance of an LDR decreases as light intensity increases
How can LDRs be used in circuits?
Turning on lights when it gets dark
What is an Ohmic Conductor? State the condition required
- A conductor for which current and potential difference are directly proportional
- Resistance remains constant as current changes
- Temperature must be constant
List 4 components for which resistance is not constant as current changes
Lamps, diodes, thermistors, LDRs
What happens to the resistance of a filament lamp as the temperature increases? Why?
Resistance increases because ions in metal have more energy, so vibrate more, causing more collisions with electrons as they flow through the metal, creating greater resistance to current flow
What is different about current flow through a diode?
- Current only flows in one direction
- Resistance is very high in the other direction, preventing current flow
Give 2 examples of when a thermistor may be used
- in a thermostat to turn a heater on below a certain temperature
- in a freezer to turn on a cooler when the temperature gets too high
What are 2 ways a component can be connected in a circuit?
- series
- parallel
How does the p.d across 2 components vary when connected in series and parallel?
series - total p.d is shared
parallel - p.d is the same across each component
If 2 resistors are connected in parallel, what can be said about their combined total resistance?
total resistance is less than the smallest of the two individual resistances
If 2 resistors are connected in series, what can be said about their total resistance?
total combined resistance is equal to the sum of the 2 individual resistances
Describe the current in a series circuit
current is same in all positions (current only has one path to flow thru)
Describe the current in a parallel circuit
current is shared between the different branches, when the charge reaches a junction it splits
How should you connect an ammeter in a circuit to measure current?
in series with the component they’re measuring current through
How should you connect a voltmeter in a circuit to measure p.d?
in parallel to the component they’re measuring p.d of
Why is it advantageous to connect lamps in parallel?
if one lamp blows, the rest will be unaffected and can still receive current (circuit is still complete)
What is resistance measured in?
Ohms
What is charge measured in?
Coulombs
Is mains electricity an a.c supply or a d.c supply?
a.c supply
What does a.c stand for?
alternating current
What does d.c stand for?
direct current
Define a.c
current that continuously changes direction at a specific frequency
Define d.c
one directional current flow
What is the frequency and voltage if the UK mains electricity supply?
50 Hz
230V
How many wires are usually in the cables connecting electrical appliances to mains? Name these wires
- Live
- Neutral
- Earth
What is the insulator colour used on the Earth wire?
Yellow and green stripes
What is the insulator colour used on the live wire?
brown
What is the insulator colour used on the neutral wire?
blue
Explain when the Earth wire does and doesn’t carry a current
- normally no current flows through the Earth wire
- if a fault occurs in the appliance (surge/casing becoming live), current will flow to the ground
What potential is the neutral wire at?
0 volts
State the p.d between the live & earth wires
230V
What is the purpose of the neutral wire?
to complete the circuit by connecting the appliance back to the mains supply
For metal appliances, where is the Earth wire connected to? Why?
- metal casing of the appliance
- if live wire becomes loose and touches metal casing, current will flow through earth wire, preventing electrocution
State two equations for the power of a circuit. Give appropriate units.
P = I V
(watts, amps, volts)
P = I^2 R
(watts, amps, ohms)
State an equation linking energy transferred, power and time. Give appropriate units.
E = P t
(joules, watts, seconds)
State an equation linking energy transferred, charge flow and p.d. Give appropriate units.
E = Q V
(joules, coulombs, volts)
What two main factors does the amount of energy transferred by an appliance depend on?
- how long the appliance has been used for
- the power of the appliance
Describe the energy transfers in a battery powered torch
- battery converts chemical to electrical
- bulb converts electrical to light (waste - thermal)
Describe energy transfers in a battery powered motor
- battery converts chemical to electrical
- motor converts electrical to kinetic (waste - thermal from friction)
What 3 things determine the power of a circuit device?
- p.d across the circuit
- current thru circuit
- amount of energy transferred in a given time
What is the purpose of the National Grid?
link power stations to consumers so they have access to a source of electricity
What are the 2 types of transformers used in the National Grid?
step-up & step-down
Where are step-up transformers found in the National grid? What do they do?
- used when connecting power stations to transmission cables
- increase p.d
Where are step-down transformers found in he National grid? What do they do?
- used when connecting transmission cables to domestic buildings
- decrease p.d
Why do transmission lines transfer electricity at high potentials?
high p.d results in low current, low current means less energy is wasted as heat, making it more efficient
Why does the p.d need to be decreased between transmission lines and houses?
lower p.d is safer for domestic use and reduces the likelihood of severe electrocution, appliances are designed for 230V
What can happen when insulating materials are rubbed together?
they become statically electrically charged
Why can insulators become electrically charged when rubbed together?
electrons are rubbed from one material onto the other, material gaining electron becomes negatively charged, the one losing becomes equally positively charged
What happens when 2 electrically charged objects are brought close together?
exert a force on each other
What happens when 2 identically charged objects are brought close together?
exert a repulsive force on each other and repel
What happens when 2 oppositely charged objects are brought close together?
exert an attractive force on each other and attract
Give an example of a non-contact force
repulsive or attractive force acting between 2 electrically charged objects
What is an electric field?
a region in which a charged object will experience a non-contact electrical force
Where can electrical fields be found?
surrounding any charged object
Describe the electrical field around a charged particle
strongest closest to the object, decreases in strength as you move away from the object
What happens to the force between two charged objects when they are moved closer together?
force between them becomes stronger
In situations where sparks are unwanted, what precaution must be taken to prevent the build up of static charge?
any surfaces that are rubbing against each other should be earthed to allow the charge to flow off the materials
