physics

Question 1

Simone has bought a new kettle. The kettle uses electricity to heat up an element which then heats up the water. 

1. In the diagram to the right, draw labelled arrows to show the movement of warm water and cool water while the heating element is on. 

The heating element is at the bottom of the kettle, the water particles that touch the element will get hot by conduction. Once the particles get hot, they will start a convection current, causing all the water particles in the kettle to heat up. 

2. Explain how convection currents occur. 

In your answer you should refer to the arrangement and movement of particles as they heat up and cool down. 

Simone turns on the kettle to make a cup of tea. She pours in 1.45kg of water from the tap which is at a temperature of 20^oC. She turns on the kettle which eventually turns itself off once the water boils.

The temperature of the kettle is shown in the graph below. 

1. ^{Calculate the average power of the kettle while it is heating up the water.} 

Specific heat capacity of water is 4200 J kg^{-1 o}C^-1

2. Simone is going out hiking for the day and she fills an insulated drink bottle with hot water from the kettle. Several hours later, she opens the drink bottle to make a cup of tea and the water is still warm. 

An insulated drink bottle is a double walled bottle that has a layer of air in between the two walls of metal. 

Explain why the layer of air between the two walls of the drink bottle reduces heat transfer by conduction to the surroundings. 

In your answer, you should: 

• Explain, in terms of particle theory of matter, how heat transfer by conduction works.

• Compare and contrast the conduction through air and metal. 

Question 2

Enzo has a torch that uses two 2.5 V batteries connected in series. The torch has two lamps (bulb), each rated at 5 V, connected as shown in the circuit diagram below. The current through each lamp is 0.6 A. 

1. Calculate the resistance of each lamp.

2. The batteries power both lamps at the same time. 

Explain why both lamps glow with their rated brightness if connected as shown above.

Begin your answer by identifying what type of connection the above diagram shows. 

3. Calculate the amount of electrical energy used by both lamps in 90 minutes. 

Begin your answer by calculating the power output of each lamp. 

4. Enzo has been learning about electrical circuits at school and his class are modelling how an electric kettle works with the circuit show below in the photo and circuit diagram. 

The circuit has a coil of wire heating 100 g of water. 

Enzo turns on the circuit and leaves it until the water gets to 100^oC and boils. The water boils for another 8 minutes . The latent heat of vaporisation is 2 260 000 J Kg ^-1

Calculate the current flowing through the circuit. 

Question 3

Rocket Lab is a NZ organisation that launches rockets into space. A telecommunications company has contracted them to launch a satellite into orbit that helps with mobile phone coverage.

After launching and taking the satellite into space, the rocket falls back to the ground to be re-used. 

1. In the box below, write an equation to show the energy changes taking place when the rocket falls vertically back to earth. 

A month later the rocket is reused to launch another satellite into space. Three steps in the process are shown below. 

2. For each of the steps, explain if work is being done on the rocket or not. 

No calculations are needed.

1. The rocket is stationary on the launch pad. 

2. Rocket accelerating upwards.

3. The rocket falling back to earth at a constant speed.

3. Rocket lab have been developing a new type of rocket. They have built a mini version of it for some trial launches. 

During one of the trials, the 30 kg rocket reached a maximum height of 650 m before falling back to the ground. 

9. Calculate the gravitational potential energy of the rocket at its maximum height.

2. While falling, the rocket reaches a speed of 106 ms^-1 just before it hit the ground. Calculate its kinetic energy. 

3. Explain the differences between the gravitational potential energy at the top of the flight, and the kinetic energy just before it hit the ground.

When space craft re-enter earth’s atmosphere, the outside shell can get very hot. Ceramic material can be used to coat the outside shell to stop the inside of the spacecraft getting too hot and damaging equipment. 

Ceramic has a high specific heat capacity, and it is a thermal insulator. 

4. Explain why ceramic’s high specific heat capacity, and thermal insulator capabilities are important for its use in this example.