Download P2.3 P2.4 Electricity - School

P2: Electrical Circuits
Charging objects
- - -
• A balloon rubbed
on a sleeve will
gain electrons
from the sleeve
and become
negatively
charged.
• The sleeve
becomes
positively
charged since it
loses electrons
Qu: If the sleeve has gained 100000
positive charges, how many electrons
have been gained by the balloon?
Rubbing your feet on a
nylon rich carpet and
then touching a
radiator will result in a
shock. How?
Electrostatic Forces
A gold leaf
electroscope
uses this
principle to
measure the
amount of
charge. The
more charge
the further the
gold moves
away from the
metal stem
Qus:
1. Why does the girl’s hair (opposite) stand on end?
2. Why does a balloon rubbed on your sleeve pick
up smalll pieces of paper?
Current:
• A flow of charges
Electron
flow
• Electrons in wires (negatively charged)
• Ions in solutions (positive and negatively charged)
Conventional
current
• measured in amperes (A)
• Found using an ammeter connected in series
• Charges are always present but they need a
battery OR mains to push them along
Scientists making life complicated?
Electrons flow from the negative electrode (terminal) of the battery round the
circuit to the positive terminal… however, historically scientists thought the
charges were positively charged so the current is often shown flowing the other
Way [CONVENTIONAL CURRENT!]
Is current used up?
• NO! But the chemical energy stored in the battery is as it
is transferred to the components/ appliances.
Series circuits:
Parallel circuits:
•The current is the same at all points
•If one component breaks then the
charges stop flowing so no current
•Found in some Christmas lights (much
to your parents annoyance if one bulb
is faulty!)
•The flow of charge splits at the
branches flowing away from the battery
and rejoins flowing towards the battery
•If one component breaks then the
components in the other branches
carry on working
How does the electrical energy get
reduced?
• Every appliance has a power rating showing how many joules per
second it transfers (measured in watts)
• The electrical energy transferred to the appliance will be either
useful (e.g. light for a lamp) OR wasted (e.g. heat for a lamp). The
more useful energy the more efficient
efficiency % = useful energy/ total energy X 100
• The longer the appliance is on for the more energy is transferred
and so the more it costs.
Energy
Transferred = Power rating X
(kW)
(kWh)
Time used
(h)
The electricity board set a cost per kWh used…
This is shown by your electricity meter.
Changing the current…
By changing the voltage…
• The bigger the voltage of the
battery/ supply the bigger
the current (since each
charge gets more energy…
a bigger push!)
• Voltage is measured in volts
(V)
• Measured using a voltmeter
By changing the
resistance…
• The bigger the resistance
the smaller the current
(since the flow of charges
are opposed… a bit like
friction!)
•Resistance is measured in
ohms (Ω)
•Calculated from current
and voltage using…
Ohm’s Law
Current (A)
Voltage (A)
Calculate the resistance above using the Ohm’s Law
Equation
• Current is
proportional to
the voltage for
resistors
• The
resistance is
given by the
gradient
Changing the resistance…
• Variable resistors
• controls resistance using a slider OR a
dial
• Thermistors
• Resistance decreases as it gets hotter
• Useful for thermostats
• Light dependent resistors (LDR)
• Resistance decreases as it gets lighter
• Useful for the automatic switching on/off
of lights during the day/ night
Resistors in circuits
• Series
– Just add the resistance of each
Calculate the total resistance
opposite…
• Parallel
– Resistance will be less than the individual resistors
– For the boffins…
1/R=1/R1+1/R2
Calculate the total resistance
opposite…
Potential difference
• We can still measure a voltage across a component like a lamp
even though a lamp doesn’t “push” the charges… this is the
potential difference across the component.
Series
parallel
Power
We can increase the power (work done per second/ watts)
by…
• Increasing the potential difference (VαP)
• Increasing the current (IαP)
• To calculate the power we can use…
P=V x I
Penguins visit Iceland
• However we often rearrange this to find the current e.g.
for a suitable fuse…
P/V = I
The power of an appliance is normally written on the back
and the voltage in the UK is normally 230V for mains.
Finding the correct fuse
• Fuses can be found in lots of different
ratings but 3A and 13A are the most
common
• Fuses have a low melting point wire which
melts when too large a current flows
Brass cap
Car fuses
Ceramic tube
Low melting point wire
A wii console has a power rating of 36W and uses a 12V supply.
Calculate the current and then explain why the two fuses mentioned
are unacceptable
Generating electricity
• If a wire (or even better a coil of wire) cuts the
field lines of a magnet a potential difference is
formed… the size of which is determined by
•
•
•
•
The strength of the magnet
The number of turns on the coil
The speed of the movement
The core in the coil; soft iron is best
• process: electromagnetic induction note how the
voltmeter needle goes from side to side…. An
alternating current (a.c.)
Transformers
• These do not need moving parts to induce a
potential difference
• A coil with an AC supply will induce another AC
provided the two are joined via a soft iron core
• You can step up the voltage OR step down the
voltage BUT not alter the power.
Voltage on secondary = number of turns on secondary
Voltage on primary
Vs = Ns
Vp Vp
number of turns on primary
If a primary coil has 100 turns and
the secondary has 1000 turns what
will be the secondary voltage if the
primary voltage is 20V?
National Grid
• Uses transformers to minimise energy loss by
keeping voltages high but current very low so
there is less heating of the wires on the way to
our homes
Electricity generated at
25000V is stepped up
to 400000V for
distribution
Local substations step
down the voltage to
33000V for industry and
230V for homes