What is the commercial unit for electrical energy? Calculate energy consumption and cost for a 2 kW heater used 5 hours daily for 30 days at 200 units/kWh.

$$\text{Energy per day} = \text{Power} \times \text{Time per day}$$ $$\text{Energy per day} = 2 \, \text{kW} \times 5 \, \text{h} = 10 \, \text{kWh}$$

$$\text{Total Energy} = \text{Energy per day} \times \text{Number of days}$$ $$\text{Total Energy} = 10 \, \text{kWh} \times 30 = \boxed{300 \, \text{kWh}}$$

$$\text{Total Cost} = \text{Total Energy} \times \text{Cost per kWh}$$ $$\text{Total Cost} = 300 \, \text{kWh} \times 200 \, \text{units/kWh} = \boxed{60000 \, \text{units}}$$

Question 2:

Figure 1 shows a bucket of sand suspended by a string. (Neglect the weight of the bucket)
(Assuming the mass of sand is 2 kg and $g = 10 \, \text{m/s}^2$ from the blurry text.)

a) Copy the diagram and show the direction of the tension in the string and the weight of the sand.

• The weight of the sand acts vertically downwards from the center of the sand.
• The tension in the string acts vertically upwards along the string, opposing the weight.

b) Calculate the tension between the bucket and the state of the sand.
The tension in the string supports the weight of the sand.

$$\text{Weight} (W) = \text{mass} (m) \times \text{acceleration due to gravity} (g)$$ $$W = 2 \, \text{kg} \times 10 \, \text{m/s}^2$$ $$W = \boxed{20 \, \text{N}}$$

The tension in the string is equal to the weight of the sand.

c) If the sand is in equilibrium, calculate the support provided by the string. (Neglect the weight of the bucket)
Since the sand is in equilibrium, the support provided by the string (tension) is equal to the weight of the sand.

$$\text{Support} = \text{Weight of sand} = \boxed{20 \, \text{N}}$$

Question 3:

Figure 2 above shows a simple electrical circuit connected to a low voltage battery.

a) Copy the diagram and show on it the direction of current and the magnetic flux pattern on the cardboard.

• The direction of current flows from the positive terminal of the battery, through the switch, upwards through the straight wire passing through the cardboard, and back to the negative terminal.
• The magnetic flux pattern on the cardboard consists of concentric circles centered on the wire. Using the Right-Hand Grip Rule, if the current is flowing upwards through the cardboard, the magnetic field lines will be in a counter-clockwise direction when viewed from above.

b) Name one rule that can be used to determine the direction of the magnetic field around the wire.
The rule used is the Right-Hand Grip Rule (or Maxwell's Corkscrew Rule).

c) State one thing that can be done to increase the strength of the magnetic field.
To increase the strength of the magnetic field around the wire, you can increase the current flowing through the wire.

Question 4:

Infra-red (IR) rays, ultra violet (UV) rays and visible light are all part of the electromagnetic spectrum.

a) Arrange the above radiations in order of increasing frequency.
In order of increasing frequency, the radiations are:
Infra-red (IR) rays, Visible light, Ultra violet (UV) rays.

b) Name a detector each for infra-red and ultra violet radiations.

• Detector for Infra-red (IR) radiation: Thermopile (or bolometer, IR camera).
• Detector for Ultra violet (UV) radiation: UV sensor (or photographic film, fluorescent screen).

c) Give two properties that are common to all electromagnetic (EM) waves.

1. All electromagnetic waves are transverse waves.
2. All electromagnetic waves travel at the speed of light in a vacuum ($3 \times 10^8 \, \text{m/s}$).