Transcript B3_Energy_transfers

Year 11 GCSE
Physics
(B3)
B3
ENERGY
TRANSFERS
LESSON 1 – Efficiency
LEARNING OUTCOMES:
• Calculate the net energy transfer from a number of
different transfers.
• Evaluate the efficiencies of energy transfer devices by
comparing energy input and useful energy output and use
the equation: efficiency = energy output energy input
LESSON 2 – Sensors
LEARNING OUTCOMES:
• Appreciate how LDRs and thermistors can be used with
electrical circuits to monitor light levels and temperature in
a building:
- circuits to include the sensor and a resistor in series
- light and temperature levels monitored via the voltage
across the resistor
- how changing the resistor value can affect the voltage
across it.
LESSON 3 – Specific Heat Capacity
LEARNING OUTCOMES:
• Appreciate that raising the temperature of one kilogram
of different materials requires the supply of different
quantities of energy and appreciate some of the effects of
materials having different specific heat capacities.
• Use the equation: change in internal energy (J) = mass
(kg) x specific heat capacity (J/kg/C or K) x
temperature rise (C or K).
• Appreciate and use the relationship between the change
in kinetic or potential energy and change in internal
energy.
SPECIFIC
HEAT
CAPACITY
On a HOT DAY at the beach:
The sand feels hotter than the sea in
the day.
BUT…
The sand feels cooler than the sea at
night.
WHY? Both get the same amount of
sunlight…
THE REASON IS:
Some materials heat up or cool down
faster…they are able to take in or
give out energy faster than others.
We measure this with something
called SPECIFIC HEAT CAPACITY.
APPLE PIE…
Which cools fastest…filling or crust?
When we heat an object up or cool
it down we cause it to gain or lose
heat energy:
The SPECIFIC HEAT CAPACITY of a
material is:
•A measure of how much energy it can
hold
•The energy needed to raise the
temperature by 1ºC.
Different materials have different
values of specific heat capacity.
The Specific Heat Capacity essentially
measures how much energy 1kg of a
material must gain/lose to go up/down in
temperature by 1C.
The SPECIFIC HEAT CAPACITY
(SHC) of a material can be worked out
using the formula:
Energy = Mass x SHC x Temperature change
For water, SHC = 4200 J/kg/ºC.
For aluminium, SHC = 680 J/kg/ºC.
Which will heat up/cool down faster?
As a formula with symbols:
E = m x c x 
Energy
change (J)
Mass
(kg)
Specific
Heat
Capacity for
a material
(J/kg/C)
Temperature
difference (C)
EXAMPLE 1:
How much energy is needed to heat 2kg
of oil up by 25C, if the SHC of the oil is
1000J/kg/ C?
SOLUTION:
E = mc
E = 2 x 1000 x 25
E = 50,000J
EXAMPLE 2:
Bartonium has an SHC of 200J/kg/ C. A
mass of 3kg is warmed up by a 50W
heater switched on for 5 minutes. What is
the temperature rise?
SOLUTION:
E = mc
 Pt = mc
 50W x 300s = 3kg x 200 x 
  = 15,000 / 600
  = 25 C
EXAMPLE 3:
How much energy is needed to heat 6kg
of water from a temperature of 0 C to 25
C , if the SHC of the water is 4200
J/kg/C?
SOLUTION:
E = mc
E = 6 x 4200 x 25
E = 630,000 J
EXPERIMENT:
• What is the Specific Heat Capacity (c) of an
aluminium block?
• How accurate is the experiment? Think about
limitations, errors, modifications, etc.
Electrical energy (IN):
E = V x I x t
Heat energy (OUT):
E = m x c x 
EXPERIMENT: Why is it poor?
•
•
•
•
•
•
Not all of the heater is inside the block
The heater is not very efficient (only 25% or less?)
The aluminium block is not insulated
Heat does not travel instantly or evenly through the block
Powerpack voltage is not correct (r)
Heater will take time to warm up
NB – Our measurements are probably quite precise but for
all of the reasons above our answer will not be exact or
accurate.
Specific Heat Capacity of aluminium = 650 J/kg/C.
LESSON 4 – Resonance
LEARNING OUTCOMES:
• Identify situations where resonance is happening:
• Recall that all objects vibrate with a natural frequency
• Describe how to measure natural frequency of the oscillator.
• Know that resonance occurs when an object is subjected to
a vibration at its natural frequency. Describe the conditions for
resonance in terms of a large amplitude resulting from the
driver frequency being equal to the natural frequency of the
oscillator.
• Appreciate some situations in which resonance is desirable
and some in which it is not.