Topic 2 - Electricity Flashcards
Define
electrical current.
and give its unit
the rate of flow of charge
amps (A)
What is
potential difference?
(or voltage) and give its unit
the drivinf force that pushes the charge round
volts (V)
What is
resistance?
and give its unit
anything that slows the flow down
ohms (Ω)
What equation links current, charge flow and time?
charge flow = current x time
Q = It
Q: coulombs, C
I: amps, A
t: seconds, s
How would you draw a
fuse
for a circuit diagram?
a rectangle with the wire running through it
How would you draw a
variable resistor
for a circuit diagram?
a rectangle with a diagonal arrow going through it
How would you draw a
thermistor
for a circuit diagram?
a rectangle with a straight then diagonal line going through it
How would you a draw an
LED
for a circuit diagram?
a diode with two arrows pointing north east out of it
How would you drawn an
LDR
for a circuit diagram?
a small resistor with a circle around it with two arrows pointing into it, south east
What equation links
current, potential difference and resistance?
potential difference = current x resistance
V = IR
V: volts (V)
I: amps (A)
R: ohms (Ω)
Explain the I-V graph for an
ohmic conductor.
The current through an ohmic conductor (at a constant temperature) is directly proportional to potential difference so you get a straight line.
(looks like a y=x graph)
Explain the I-V graph for a
filament lamp.
As the current increases, the temperature of the filament increases, so the resistance increases. This means less current can flow per unit pd, so the graph gets shallower - hence the curve.
(looks like the middle bit of a sin graph)
Explain the I-V graph for a
diode.
Current will only flow through a diode in one direction, as shown. The diode has a very high resistance in the reverse direction.
What is the relationship between resistance and light intensity in an LDR?
resistance is inversely proportional to light intensity
What is the relationship between resistance and temperature in a thermistor?
resistance is inversely proportional to temperature
What is the voltage from the UK mains supply?
and what kind of supply is this?
230V
ac
What is the frequency of the ac mains supply?
50Hz
How are most electrical appliances connected to the mains supply?
by three-core cables
this means that they have three wires inside them, each with a core of copper and a coloured plastic coating
What is the purpose of the
live wire?
and what is its colour?
and pd?
the live wire provides the alternating potential difference from the mains supply
brown, about 230V
What is the purpose of the
neutral wire?
and what is its colour?
and pd?
the neutral wire completes the circuit - when the appliance is operating normally, current flows through the live and neutral wires
blue, around 0V
What is the purpose of the
earth wire?
and what is its colour?
and pd?
it is for protecting the wiring, and for safety - it stops the appliance casing from becoming live, it doesn’t usually carry a current - only when there’s a fault
green and yellow, 0V
What would happen if you touched a
live wire?
and why?
a large electric shock would be caused, which could injure you or even kill you
Your body is at 0V. If you touch the live wire, a large potential difference is produced across your body and a current flows through you.
What is the power of an appliance?
the energy that it transfers per second
What equation links
charge flow, energy transferred and potential difference?
energy transferred = potential difference x charge flow
E = VQ
E: joules, J
V: volts, V
Q: coulombs, C
What equation links
power, potential difference and current?
power = current x potential difference
P = IV
P: watts, W
I: amps, A
V: volts, V
What equation links
current, resistance and power?
power = current x current x resistance
P = IIR
P: watts, W
I: amps, A
R: ohms, Ω
What is the
national grid?
and what does it do?
a giant system of cables and transformers that covers the UK and connects power stations to consumers
it transfers electrical power from power stations anywhere on the grid to anywhere else on the grid where it’s needed
What equation explores current and potential difference in the coils of a transformer?
Vs x Is = Vp x Ip
What equation links
voltage and number of turns of wire
in a transformer?
Vp / Vs = Np / Ns
How do transformers make the national grid an efficient way of transferring energy?
for a given power, increasing the pd decreases the current, which decreases the energy lost by heating the wires and the surroundings
What is
static electricity?
charges which are not free to move
How are
sparks created?
(2 steps)
- As electric charge builds up on an object, the potential difference between the object and the earth (which is at 0V) increases.
- If the potential difference gets large enough, electrons can jump across the gap between the charged object and the earth.
(this usually happens when the gap is fairly small)
What are some rules about
electric field lines?
(3 rules)
- they go from positive to negative
- they’re always at a right angle to the surface
- the closer together the lines are, the stronger the field is
How do
electric fields explain sparks?
(4 steps)
- Sparks are caused when there is a high enough potential difference between a charged object and the earth.
- A high potential difference causes a strong electric field between the charged object and the earthed object.
- The strong electric field causes electrons in the air particles to be removed (known as ionisation).
- Air is normally an insulator, but when it is ionised it is much more conductive, so a current can flow through it.
