Exam practice

The Sun is a natural source of light.

Wood is a non-luminous object. (It is also opaque, but that word is not in the box.)

An object that lets light pass through it easily is called a transparent object.

A rainbow has seven colors.

A torch is a man-made source of light. (“Five” is not needed.)

A luminous object is a thing that gives out its own light. Examples: the Sun and a burning candle. (A star, a torch that is on, or a fire are also right.)

• A transparent object lets light pass through it easily. We can see clearly through it. Example: glass.
• An opaque object lets no light pass through. We can’t see through it at all. It makes a dark shadow. Example: wood.

Any two:

• Stick tracing paper, butter paper or oiled paper on it.
• Stick a frosted plastic sheet on it.
• Paint it with a thin coat of white paint.
• Rub a thin layer of soap on it.

A rainbow needs drops of water in the air and sunlight at the same time. After rain, many drops still float in the air. When the Sun comes out, its light goes into the drops and splits into seven colors. So a rainbow appears. With no drops in the air, sunlight can’t split, and there is no rainbow.

Fog is made of tiny drops of water floating in the air. It is translucent. It lets only a little light through, and scatters it about. So the driver can’t see the road, other cars or people clearly. That makes accidents more likely. Drivers go slowly and switch on fog lights.

Any one of these:

• On a sunny morning or evening, stand with the Sun behind you. Spray a fine mist of water from a hose or a spray bottle in front of you. A rainbow appears in the spray.
• Do Activity 7.4: shine sunlight on a mirror under water in a dish, and catch the colors on white paper.
• Hold a glass prism, or a glass of water, in a sunbeam, and look at the light it throws on the floor.

A radio is a source of sound.

To make a sound, an object must vibrate.

Good etiquette about sound must be followed even inside the classroom.

Beating a metal object makes a sharper sound than beating a wooden one.

Staying in a noisy place for a long time may cause mental illness. (“Hoarse” is not needed.)

Sound is a form of energy made when an object vibrates. We hear it when the vibration travels to our ears.

We give it energy by hitting, beating, blowing, plucking or shaking it. This makes it vibrate: shake quickly back and forth. The vibration makes the air around it vibrate, and that travels to our ears as sound.

A gentle hit makes the drum’s skin vibrate only a little: a soft sound. A hard hit makes the skin vibrate much more: a loud sound. The more force we use, the bigger the vibration and the louder the sound.

For example: vehicles and their horns, loudspeakers, TVs and radios played loudly, a pressure cooker’s whistle, barking dogs, a mill or a factory nearby, building work, people shouting, wedding bands.

• Drivers should stop honking: a horn doesn’t clear a jam, and the patients need quiet.
• Put up “No horn” signs near the hospital, and fine drivers who honk.
• Traffic police should guide the traffic and clear the jam.
• Make a separate lane for ambulances.
• Plant trees and build a wall between the road and the hospital, to block some noise.

A dry cell is a source of electricity.

A path made of a source of electricity, conductors, a switch and electrical equipment is called an electric circuit.

If you see a bare wire, it should be covered with an insulator (insulating tape).

The switch is on in a closed circuit.

Small children should not play with electrical equipment. (“Electricity” is also right. “Off” is not needed.)

An electric circuit is the complete path through which electricity flows. It is made of a source of electricity (a cell), conducting wires, a switch, and a device that uses electricity (like a bulb).

• In a closed circuit, the switch is on. In an open circuit, the switch is off (or a wire is loose).
• In a closed circuit, electricity flows and the bulb lights. In an open circuit, electricity doesn’t flow and the bulb stays dark.

Any two:

• Never put your fingers into a plug or socket.
• Never touch electrical things with wet hands.
• Cover bare wires with insulating tape.
• Call an electrician to repair anything electrical.
• Turn off the main switch in a fire, a storm or a power cut.

When the switch is off, there is a gap in the circuit. It is an open circuit. Electricity can’t flow across the gap, so none reaches the bulb.

• a source of electricity, like a cell or battery;
• conducting wires to make a path;
• a device that uses electricity, like a bulb;
• usually a switch, to turn it on and off (when there is a switch, it must be on).

All of them joined in one complete path, with no gap.

Two things are missing:

1. There is no cell. Put a dry cell in the gap in the bottom wire.
2. The switch is open. Close it.

Then the path is complete, and the bulb lights.