Transcript P5 – Electric Circuits

P5 – Electric Circuits
Static Electricity
• When two objects are rubbed together and become charged,
electrons are transferred from one object to the other
• Repulsive forces between similar charges
• Attractive forces between opposite charges
• Explain simple electrostatic effects in terms of attraction
and repulsion between charges
Electric Current
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Current is a flow of charge
Measured in amperes (Amps)
Wires, bulbs, etc are full of charges that are free to move
The battery causes these free charges to move
These charges are not used up but flow in a continuous
loop
Conductors and Insulators
• Conductors
– Electrons free to
move
– EG: Metals
• Insulators
– Electrons are not
free to move
– EG: Plastic
Resistance
• Components (for example, resistors, lamps, motors)
resist the flow of charge through them
• The larger the resistance in a given circuit, the
smaller the current will be
• The resistance of connecting wires is so small that it
can usually be ignored
Resistors Get Hot
• Resistors get hotter when electric current passes
through them
• This heating effect is caused by collisions between the
moving charges and stationary atoms in the wire
• This heating effect makes a lamp filament hot enough to
glow
Thermistors
• Low Temperatures = High Resistance
• High Temperatures = Low Resistance
Light Dependent Resistors
• Low Light
• Bright Light
= High Resistance
= Low Resistance
Circuit Symbols
1. Open Switch
2. Closed Switch
3. Lamp
4. Cell
5. Battery
6. Voltmeter
7. Resistor
8. Ammeter
9. Variable resistor
10. Thermistor
11.Light dependent resistor (LDR)
Series and Parallel
• Two (or more) resistors in series have more resistance
than one on its own, because the battery has to push
charges through both of them
• explain that two (or more) resistors in parallel provide
more paths for charges to flow along than one resistor
on its own, so the total resistance is less and the current
is bigger
Ohm’s Law
When the Resistance is Constant
• The current is directly
proportional to the
voltage.
• IE: if you double the
voltage, the current
will also double.
Voltage / Potential Difference
• The larger the voltage of the battery the bigger the
current
• The voltage of a battery (measured in V) provides a
measure of the ‘push’ of the battery on the charges
in the circuit
• Voltmeters are
connected either side of
the component you are
measuring.
Voltage / Potential Difference
• The energy given or taken away from the
charge as it moves between two points
Adding Batteries
• When you add
batteries in Parallel,
the Voltage and the
Current stay the
same
• When you add
batteries in Series the
Voltage and Current
increase
Series Circuits
1. Current through each component is the same
2. The voltage across the components add to the
voltage across the battery (The total energy
transferred to each unit of charge by the
battery must equal the amount transferred
from it to other components)
3. The voltage is largest across the component
with the greatest resistance (more energy is
transferred by the charge passing through a
large resistance)
Parallel Circuits
1. Voltage across each component is equal to the
voltage of the battery
2. Current through each component is the same as
if it were the only component present
3. Total current from (and back to) the battery is
the sum of the currents through each of the
parallel components
4. Current is largest through the component with
the smallest resistance, because the same
battery voltage causes more current to flow
through a smaller resistance than a bigger one.
Transformers
• Changing Current in a coil = Changing Magnetic Field
• This changing magnetic field can induce a voltage in
a neighbouring coil
Transformers
• Transformer = two coils of wire wound on an iron core
• A transformer can change the size of an Alternating
Voltage
Transformer Equation
Secondary Voltage = Number of Coils Secondary
Primary Voltage Number of Coils Primary
Generator
• A magnet or electromagnet is rotated within a coil of wire (or
Rotating coil inside a magnet) to induce a voltage across the ends of
the coil
•
The size of the induced voltage can be increased by:
1. Increasing the speed of rotation of the magnet (or coil)
2. Increasing the strength of the magnetic field
3. Increasing the number of turns on the coil
4. Placing an iron core inside the coil
Alternating Current
• The induced voltage across the coil of a
generator changes during each revolution of
the magnet or electromagnet and explain that
the current produced in an external circuit is
an Alternating Current (a.c.)
Alternating v Direct Current
• When the current is always in the same
direction, it is a direct current (d.c.), e.g. the
current from a battery
• Mains electricity is an a.c. Supply
• a.c. is used because it is easier to generate than
d.c., and can be distributed more efficiently
Mains Electricity
• Mains electricity is produced by generators
• Mains supply voltage to our homes is 230 volts.
Energy Transfer
• When electric charge flows through a
component (or device), energy is transferred
to the component
• Energy (J, kWh)
• Power (W, kW)
• Time (s, hr)
• A joule is a very small amount of energy, so a
domestic electricity meter measures the
energy transfer in kilowatt hours
Power
• Power is a measure of the rate at which an
appliance or device transfers energy
Cost of Electricity
• Calculate the cost of electrical energy given the
power, the time and the cost per kilowatt hour
• Multiply the Energy by the by the cost per kWh.
Efficiency of Electrical Appliances
• Efficiency (%) = (Useful Energy / Total Energy) x 100