B3 ENERGY TRANSFERS

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B3ENERGYTRANSFERS
(B3)

• Year 11 GCSE Physics

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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

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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.

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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.

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SPECIFIC HEAT CAPACITY
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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
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THE REASON IS Some materials heat up or cool
down fasterthey are able to take in or give out
energy faster than others. We measure this with
something called SPECIFIC HEAT CAPACITY.
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APPLE PIE
Which cools fastestfilling or crust?
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• When we heat an object up or cool it down we
  cause it to gain or lose heat energy

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• 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.

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• 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.

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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?
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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)
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• 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
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• 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

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• 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
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• 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 ??
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• 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.

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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.