Keywords
Energy cannot be created or destroyed only transformed from one form to another or transferred between objects
Law of conservation of energy
A small rocket has a mass of 2.6kg and a weight of 26.0N. Explain the difference between mass and weight.
Weight is a downward force due to gravity that an object experiences, while mass is a measure of the amount of matter that an object has.
Name the three different heat transfer methods.
•Conduction
•Convection
•Radiation
Explain how decreasing the resistance of a variable resistor affects the current in the series circuit
Current increases
When the resistance of the variable resistor is decreased, the total resistance of the circuit decreases accordingly, because it is a series
circuit. As the supply voltage is a constant, the current from the supply increases
What loses its head in the morning but gets it back at night?
A Pillow
How much an electrical component restricts the flow of current
Resistance
An 85kg worker climbed to the top of the ramp. This took 8s.
Calculate the work done by the worker to get to the top of the ramp and therefore the power exerted. Include units.
F = 85 x 10
850N
W = Fxd
850 x 1.2 = 1020J
P = W/t
1020/8 = 130W (2sf)
Explain why it would take more energy to raise the temperature of humid air from 16°C to 21°C than it did for dry air.
SHC of dry air = 1006 Jkg-1oC-1
SHC of water = 4200 Jkg-1oC-1
The specific heat capacity of water is higher than that of air. This means more energy is required to heat moist air, as the water contained in the air requires more energy (per kilogram) to increase the temperature by the same amount.
Aaron notices the maximum power rating for his heater is 2000 W. The voltage supply in his house is 240 V.
He sets the dial to 50% maximum.
Calculate the current in the heater circuit
P = VI; I = P/V
1000/240 =
4.17A (or other rounded answer form the correct equation)
Who makes it, has no need of it. Who buys it, has no use for it. Who uses it can neither see nor feel it. What is it
A coffin
Specific heat capacity
The energy required to heat 1kg of a substance by 1C without changing its state.
A small rocket has a mass of 2.6kg and a weight of 26N. the rocket had gained 1950J of potential energy at the maximum height. It then fell back to the ground. What was the maximum speed it could reach just before hitting the ground.
Ep = Ek
Ep = 1950J
Ek = 1/2 mv2
1950 = 1/2 x 2.6 x v2
v = √(1950 x (2/2.6))
v = 38.7m/s
A heating curve of iron is plotted. There are two flat sections, one at 1500 0C and a second at 2850 0C.
What do each of these represent ?
Melting and Boiling points
An office uses a seperate 24V power supply to run a set of LED lamps for a large desk. The bulbs are connected in parallel to the power supply. All bulbs have 20Ω of resistance.
Calculate the current through each lamp.
V = IR; I = V/R
24/20 = 1.2A
What has hands but cannot clap?
A clock
Temperature
The average kinetic energy of the particles
A parachutist with a total mass of 63kg jumps from a plane.
What is the parachutists gravitational potential energy when the plane is 3 500m above sea level.
Ep = mgh
63 x 10 x 3500
Ep = 2 205 000J
During winter Felix uses an electric heater to heat his house. His house contains 600kg of dry air.
Calculate the amount of heat energy required to raise the temperature of all the air from 16C to 21C
SHC of dry air = 1006 Jkg-1oC-1
SHC of water = 4200 Jkg-1oC-1
E = mcΔT
600 x 1006 x 5
3018000J
Draw a series circuit with the following components:
9V batter, open switch, 2 identical lamps, voltmeter across the battery and an ammeter.
Peer judgement
Poor people have it. Rich people need it. If you eat it you die. What is it?
Nothing
Latent heat of fusion
The amount of energy required to change state from solid to liquid or from liquid to solid without a change in temperature
A crane is lifting wood. The cable broke and 150kg of wood fell 12m to the ground. The wood had a kinetic energy of 15 000J as it landed on the ground. This is different to the energy when it was hanging from the crane.
Calculate the difference between the kinetic energy of the wood just before hitting the ground and the energy the wood had when it was hanging from the crane
Ep = mgh
150 x 10 x 12 = 18000J
18000 - 15000 = 3000J
At night Tina closes the curtains. This layer of air between the curtain and the window reduces heat loss.
Explain in terms of particle theory of matter why air reduces heat transfer by conduction compared to glass.
Because particles in air are much more spaced out than in glass, conduction through air is much less effective than through glass.
This makes the layer of air between the curtains and the window a thermal insulator
Calculate the energy used by one lamp in one hour if the overall resistance of the series circuit is 150Ω. A voltmeter across the battery reads 9V.
V = IR; I= VR
9/150 = 0.06A
P = VI
0.06 x 4.5 = 0.27W
E = Pt
0.27 x 3600
972J
What is always on its way but never arrives
Tomorrow