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Transcript
Electricity
Electric Charge
Can come from batteries and generators.
Some materials can be charged when they are rubbed – this charge
can be called electrostatic charge.
Polythene
Wool
Transfer of
electrons
More electrons than
normal: net negative
charge.
Perspex
Wool
Transfer of
electrons
More electrons than
normal: net negative
charge.
When the polythene is rubbed with wool it becomes ___________ charged. This is
because _________ are being transferred to the wool.
When the perspex is rubbed with the wool the electrons move onto it and it
becomes _________charged.
Two pieces of polythene will __________ each other because like charges
___________. A piece of polythene and a piece of perspex will __________ each
other because unlike charges _________.
Where does charge come from?
Can you name the parts of the atom?
This is the NUCLEUS.
ELECTRONS orbit the
nucleus and have a
negative charge.
PROTONS (positive
charge)and NEUTRONS
(no charge) are found in
the nucleus
Usually atoms have an equal number of protons and electrons, therefore the net charge is zero.
Rubbing, separates the charges by electron transfer, leaving one material positive and the other
one negative.
Conductors and insulators
Conductors
Insulators
Metals are good conductors – such as
copper, silver
Plastic, glass rubber
Have loosely held electrons
Have tightly held electrons
Electrons free moving
Electrons not free moving
Difficult to charge by rubbing
Easy to charge by rubbing
Charge by induction
Charged objects will attract uncharged objects close to them by inducing a
charge in them.
Charged objects that are brought close to uncharged objects induce charge –
as in the diagram – opposite charges are attracted to the rod.
Touching the sphere with a finger will Earth the sphere.
Earthing
If enough charge builds up on something, electrons can be pulled through the
air. This causes sparks.
Objects must be Earthed. Unwanted charge flows to Earth through a conducting
material.
The Van De Graaf Generator
i. carbon brushes remove electrons from
rubber band
ii. rubber band turned by motor
iii. electrons pass to metal dome
iv. dome becomes negatively charged
Homework
• Find out the units of charge
• Research and find at least two uses of
electrostatic charge
Questions on electrostatic charge
Give an example of where electrostatic charge might be a hazard.
How can a build-up of charge be prevented?
In the diagram a charged metal rod is held close to a metal can.
Copy the diagram and draw any induced charge on the can
Why is the can attracted to the rod even though the net charge is zero?
In which direction is the electron flow when your finger touches the
can?
d) What type of charge is left on the can after it has been touched?
4. Explain why a balloon becomes negatively charged after it has been
touched.
5. Why does the balloon stick to the ceiling?
1.
2.
3.
a)
b)
c)
Circuits
•Draw and interpret circuit diagrams containing sources, switches,
resistors (fixed and variable), lamps, ammeters, voltmeters,
magnetising coils, transformers, bells, fuses, relays
•Understand that the current at every point in a series circuit is the
same
Current
The conventional current direction is from +
to – and describes the flow f positive charge.
This was before the electron was discovered
and scientists realised that positive charge
did not flow round circuits
• Defined as a flow of charge (Q)
• Measured in Amperes/Amps
• Current measured using an ammeter
• In a circuit the current is not affected by the ammeters in it
• The current is the same at all points around the circuit
There is a link between charge and current
• Charge (Q) = current (I) x time (t)
Q
l
t
1 Coulomb/sec = 1 Ampere
2 Coulomb/sec = 2 Amperes
• If a current of 2A flows for 3 seconds the charge delivered is 6 Coulombs.
• The direction of the current is from a high electron potential to a low electron potential (from – to +)
Voltage
Defined as the amount of energy given to electrons in a current.
Measured in volts (V)
Also known as potential difference (PD). Energy is wasted inside a cell
when it is connected to a circuit so the maximum PD a cell can have is
when it is not connected to a circuit. This maximum PD is called the
electromotive force (EMF).
Voltage is measured using a voltmeter
In a circuit, voltmeters are always placed in parallel to the component over
which they are measuring the voltage difference.
The total voltage across the bulb (in the diagram) is equal to the voltage in
the main circuit.
No energy is lost in the main circuit but some is lost in the first bulb and
more in the second.
Measuring current and voltage
1.
2.
3.
Write the title and draw the circuit symbols on page 176
Make a parallel and series circuit – draw a diagram.
Compare two ways of wiring up two lamps and two switches in a circuit –
one in a parallel and one in a series circuit. Record your findings in a table:
Series
Circuit diagram
Appearance of lamps
Behaviour of current
Advantages
Disadvantages
Voltage across bulb A
Voltage across bulb B
Voltage across cell
Current between cell and bulb A
Current between cell and bulb B
Parallel
Resistance
the degree to which current is reduced when flowing
through a conductor
Resistance (Ω) = voltage (V)/Current (A)
V
I
R
Factors affecting resistance – Investigation
length, thickness, material, temperature
Why do bulbs get hot?
Plan an investigation of how
resistance is affected by length.
Apparatus
• circuit board
• bulb
• voltmeter
• ammeter
• constantan wire
• meter rule
• power supply
Think about what you will vary and what you will measure
Make a prediction of how you think the resistance will vary when
you change the length.
Investigation of factors affecting
resistance
1) Length of circuit
2) Size of material
3) Type of material
4) Temperature
Resistance and Ohm’s Law
 Aim: to investigate Ohm’s Law
In a metal conductor, current will vary with potential difference if the
temperature of the conductor is kept the same. This is Ohm’s Law:
o Current is proportional to the PD
 Method
o Draw a circuit diagram and describe how you will investigate Ohm’s Law.
o What are you going to vary (independent variable)?
o What are you going to measure(dependent variable?
o What are you going to keep the same in order to make this a fair test?
 Results: Copy and complete the
table and plot a graph of your results
 Conclusion: what does the graph tell
us?
PD (V) Current
(A)
Resistance
(Ω)
Resistance components
resistors are specially made to provide
resistance – useful in complicated circuits for
limiting currents and PDs
Variable resistors (rheostats)
vary resistance and therefore current
Thermistors
high resistance when cold and low when hot
Light-dependent resistors (LDRs)
high resistance in the dark and low in the light
Diodes
extremely high resistance in one direction and low in the
other
Current – PD graphs
The tungsten filament
 As the current increases the
temperature rises – increasing
the resistance
The semi-conductor diode
 A diode allows the current
to flow in one direction
only.
 Current is not proportional
to pd and if pd is reversed
the current is zero
Questions
1) 240V, 3A. Calculate the resistance
2) 70Ω, 12A. Calculate the voltage
3) 12000V 36000Ω. Calculate the current.
More questions on page 181.
Series and parallel circuits
•State that, for a parallel circuit, the current from the source is larger than the
current in each branch
•Recall and use the fact that the current from the source is the sum of the currents
in the separate branches of a parallel circuit
•Recall and use the fact that the sum of the p.d.’s across the components in a series
circuit is equal to the total p.d. across the supply
•State the advantages of parallel circuits
Pages 186 and 187 out of books
Questions page 187
Resistors in series and parallel
•Draw and interpret circuit diagrams containing diodes as rectifiers
•Give the combined resistance of two or more resistors in series
•State that the combined resistance of two resistors in parallel is less
than that of either resistor by itself
•Calculate the effective resistance of two resistors in parallel
Investigations
for each investigation
 Investigation 7 - Resistors
Resistors in a circuit
In series
In parallel
If two or more resistors are connected
in series their combined resistance
is added together – this is like
lengthening the wire in the circuit.
R = R1 + R2
Eg:
If two or more resistors are
connected in parallel they give a
lower resistance than any of the
resistors by themselves – this is
like using a thicker piece of wire
in your circuit:
1/R = 1/R1 + 1/R2
Eg:
R1 = 2Ω
R2=2Ω
R2=2Ω
R1 = 2Ω
Total resistance R
= R1 + R2 = 4Ω
Total resistance 1/R
= 1/R1 + 1/R2 = 1Ω
Electrical Energy
•Energy is measured in Joules.
•Electrical equipment always has a power rating. This tells us how
much energy (Joules per second) is required by the appliance.
•1 Joule per second = 1 Watt.
Electrical Energy
 Usual Power ratings;
 Filament light bulbs are normally 60 or 100W.
 An electric heater can be 3 KW.
 This means the heater can convert 3000 Joules per second
into heat (+ any wastage).
Electrical Energy
 The formula for finding power of electrical equipment is;
P = IV
Where;
P = Power (W)
I = Current (A)
V = Volts (V)
Electrical Energy
 What is the power of an electric iron, if a current of 7A is
obtained from a 230V supply?
Electrical Energy
P = IV
P = 7 x 230
P = 1610 W
Electrical Energy
 An electric oven has a power rating of 2KW What is the
current measured when the oven is used with a 230V supply?
Electrical Energy
 Calculating energy transferred
E = Pt
Where;
E = energy is Joules
P = Power
t = time
Electrical Energy
 We can also rewrite the equation.
 E = VIt
 Where;
 E = Energy
 V = Voltage
 I = Current
 t = time
Review
 You will need the following equations;
Q = It
Q = Charge
I = Current
t = time
GPE = mgh
GPE = Gravitational potential Energy
m = mass
g = gravity
h = height
E = Pt
E = energy is Joules
P = Power
t = time
Electrical power in resistors
 Electrical power dissipated in a resistor
P = I2 R
 Where
 P = Power (W)
 I = Current (A)
 R = Resistance ()
Electrical power in resistors
 Although we are discussing resistors, this also applies to
heating elements and all other components that have
resistance.
 When current flows through a resistor it has a heating effect.
Electrons lose potential energy when is changed to thermal
energy.
Electrical power in resistors
 What power is dissipated in a 5 resistor when the current
through it is a) 2A b) 4A?
Electrical power in resistors
20 W
b) 80 W
a)
Electrical safety
 Includes circuit breakers and fused plugs
Circuit Breakers – automatic safety switch switches off
with excess current
2. Fuses and fused plugs:
Fuse circuit symbol
1.



Deliberate weak link in a circuit for safety
Fuse wire melts if too much current flows
Prevents electrical fires
Fused plugs have several safety
features
1.
2.
3.
4.
5.
6.
7.
8.
Live wire – colour coded – brings live voltage to plug
Earth wire – safety wire – connects plug to Earth – excess current to Earth
Neutral wire – completes the circuit – kept at zero voltage
Plastic casing – electrical insulator
Fuse – placed in the live wire – wire in fuse melts with excess current
Cable grips hold cable firmly in place
Connecting screws – holds wire in place
Double insulation – has two plastic casings around main cable to plug
Hazards
Hazard
Danger
Damaged insulation
Possibility of electric shock
Cable overheating
Excess current in the cable causes plastic to
catch fire
Damp conditions
Water is an excellent electrical conductor
Short-circuit
Electrical current travels along path of least
resistance
Switches
Live current present is accidentally switched
on
Fuse Ratings
P
I
V
 All fuses have a fuse rating
 Use the power, current,
 The maximum current a fuse
voltage triangle to calculate
the fuse ratings
Example
A vacuum cleaner has a rating of
460W on the 230V mains.
Calculate the fuse rating.
Power = current x voltage
460W = current x 230V
Current = 2A
Here a 3A fuse should be fitted
can carry without melting
 Only certain values are
available: 1A, 3A, 5A, 10A,
13A.
 The fuse rating should always
be slightly higher than the
current in the wire
Questions on pages 193 and 195