Download Electrical energy, elastic potential energy

Electrical energy, elastic
potential energy
Efficiency of a small electric motor
Objectives
• To learn the energy changes that take place when a spring is
stretched or compressed
• To be able to calculate the energy stored in a stretched spring
• To know how to calculate electrical energy
• Some examples of calculations involving energy transfers
The extension of this spring is
0.5m
The spring stores…….
Elastic potential energy
The EPE stored in a stretched spring is given by the formula
Elastic potential energy = ½ k x2
‘X’ is the extension of the spring in
metres
‘k’ is called the spring constant. It is measured in N/m.
The stiffer the spring, the greater the value of ‘k’
At the top and bottom of the oscillation, the bob is not moving.
In the middle of the oscillation, the kinetic energy is a maximum
The total energy is constant
A spring, which has a spring constant k = 7.50 N/m, has been
stretched 0.40 m from its equilibrium position. What is the potential
energy now stored in the spring?
A spring with a spring constant k = 800 N/m has
been compressed, and 196 J of potential energy is
stored. What distance from equilibrium has the
spring been compressed?
Electrical energy
You should remember the formula for electrical power
Power = current X voltage
P = I
X V
You should also remember the relationship between power, energy and
time.
Power = energy ÷ time
So an expression for electrical energy is:
Electrical energy = current X voltage X time
E
= I
X V
X t
For example, consider an LCD TV that
is left on continuously for a week.
It uses 230 V and 4 A. The energy
transferred can be calculated using
the equation:
energy = current × voltage × time
energy = 4 × 230 × (60 × 60 × 24 × 7)
= 556,416,000 J
= 556 MJ
10 of 15
© Boardworks Ltd 2011
200g of a liquid at 21⁰C was heated to 51⁰C by a current
of 5A at 6V for 5 minutes. Neglecting heat losses,
calculate the specific heat capacity of the liquid.
A 50W heating coil is totally immersed in 100g of water. Assuming no heat
energy is transferred to the surroundings:
A)If the temperature of the water is 20⁰C when the heater is switched on, how
long would it take for the water to boil?
B)After the water has been boiling for 15 minutes it is found that the water in
the flask has decreased to 80g. Calculate a value for the specific latent heat of
vaporisation of the water.
(specific heat capacity of water is 4200J/kg⁰C)
• https://www.youtube.com/watch?v=ssMvUWKE7PI
13 of 15
© Boardworks Ltd 2011