Electricity Flashcards
What is electric current?
The flow of electrical charge.
State the equation linking charge
Q = I t
Charge (Coulombs), Current (Amperes),
Time (Seconds)
What can be said about the value of current at any
point in a single closed loop?
Current is the same at all points in a
closed loop.
What equation should be used to calculate potential
difference if current and resistance are known?
V = I R
Potential Difference (V), Current (A),
Resistance (Ω)
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 four components for which resistance is not
constant as current changes.
- Lamps
- Diodes
- Thermistors
- Light Dependant Resistors (LDRs)
What happens to the resistance of a filament lamp
as the temperature increases? Why?
● Resistance increases
● 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?
● The current only flows in one direction
● Resistance is very high in the other
direction, preventing current flow
State what happens to the resistance of a thermistor
as temperature increases.
The thermistor’s resistance decreases.
Give two 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 becomes too high
State what happens to the resistance of a LDR as
light intensity decreases.
The LDR’s resistance increases.
Give an application for a LDR.
● Street lights often use LDRs
● When light levels become too low, the
light gains sufficient current to turn on
What are the two ways that a component can be
connected in a circuit?
- Series (same loop)
- Parallel (adjacent loop)
If two resistors are connected in parallel, what can
be said about their combined total resistance?
Their total resistance is less than the
smallest of the two individual
resistances.
If two resistors are connected in series, what can be
said about their total resistance?
Their total combined resistance is equal
to the sum of the two individual
resistances
Describe the current in a series circuit.
In a series circuit, the current is the same
at all positions since the charge only has
one path to flow through.
Describe the current in a parallel circuit.
In a parallel circuit, the 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?
Ammeters should be connected in series
with the component that they are
measuring current through.
How should you connect a voltmeter in a circuit to
measure potential difference?
Voltmeters should be connected in
parallel to the component that they are
measuring the potential difference of.
Why is it advantageous to connect lamps in parallel?
If one lamp blows, the rest will be
unaffected and can still receive current
(i.e. the circuit is still complete).
If one lamp blows, the rest will be
unaffected and can still receive current
(i.e. the circuit is still complete)
● Alternating current: Current that
continuously changes direction at a
specific frequency
● Direct current: One directional current
flow
What is the frequency and voltage of the UK mains
electricity supply?
● Frequency: 50 Hz
● Voltage: 230V
Explain when the Earth wire does and doesn’t carry
a current.
● Under normal circumstances, no current
flows through the Earth wire
● If a fault occurs in the appliance (such as a
surge or the casing becoming live), current
will flow to the ground
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?
● Earth wire is connected to the metal casing
of the appliance
● If live wire becomes loose and touches the
casing, the current will flow through the
Earth wire, preventing electrocution
State two equations for the power of a circuit. Give
appropriate units.
P = I V P = I² R
Power (Watts), Current (Amperes)
Potential Difference (Volts), Resistance
(Ohms)
State an equation linking energy transferred, power
and time. Give appropriate units.
E = P t
Energy (Joules), Power (Watts), Time
(Seconds)
State an equation linking energy transferred, power
and time. Give appropriate units.
E= Q V
Energy (Joules), Charge (Coulombs),
Potential Difference (Volts)
What two main factors does the amount of energy
transferred by an appliance depend on?
- How long the appliance is being used
for - The power of the appliance
Describe the energy transfers in a battery powered
torch.
● Battery converts chemical energy into
electrical energy
● Bulb converts electrical energy into light
as well as waste energy in the form of
heating
What three things determine the power of a circuit
device?
- The potential difference across the
circuit - The current through the circuit
- The amount of energy transferred in a
given time
What is the purpose of the National Grid?
To link power stations to consumers so
that they have access to a source of
electricity.
What are the two types of transformers used in the
National Grid?
- Step-Up Transformers
- Step-Down Transformers
Where are step-up transformers found in the
National Grid? What do they do?
● Step-Up Transformers are used when
connecting power stations to
transmission cables
● They increase the potential difference
Where are step-down transformers found in the
National Grid? What do they do?
● Step-Down Transformers are used in
connecting transmission cables to
domestic buildings (like houses)
● They decrease the potential difference
Why do transmission lines transfer electricity at high
potentials?
● A high potential, results in a low current
● The lower the current, the less energy
that is wasted as heat
● Therefore it is more efficient
Why does the potential need to be decreased
between transmission lines and houses?
● Lower potentials are safer for domestic
use and reduces the likelihood of
severe electrocution
● Appliances are designed for 230V
