CBSE Class 7 Science Electricity and Electric Circuits Worksheet Set 01

Answer: (a) energy. Electricity is a form of energy that can be changed into light, heat and motion.

Teacher's Note:
1. Electricity is not matter because it has no mass.
2. It can be converted into other forms of energy.

Answer: (c) silver. Metals such as silver allow current to pass easily.

Teacher's Note:
1. Silver is one of the best conductors known.
2. Plastic is an insulator.

Answer: (a) together. Positive and negative charges are found together in matter.

Teacher's Note:
1. Matter normally has equal positive and negative charges.
2. Separating them needs an effort such as rubbing.

Answer: (b) ammeter. It is connected in series in the circuit.

Teacher's Note:
1. A voltmeter measures potential difference, not current.
2. The unit of current is the ampere.

Answer: electric current.

Teacher's Note:
1. Its symbol is A.
2. Smaller currents are given in milliampere.

Answer: electric current in a circuit.

Teacher's Note:
1. It is always connected in series.
2. Its symbol is a circle with the letter A.

Answer: electric charges (electrons in a metal wire).

Teacher's Note:
1. In metals the moving charges are electrons.
2. They carry negative charge.

Answer: negative, positive.

Teacher's Note:
1. Conventional current is shown in the opposite direction.
2. The two ideas should not be mixed up.

Answer: The earth wire carries any leaking current safely into the ground. This prevents a person from getting an electric shock if the metal body becomes live.

Teacher's Note:
1. Earthing is a safety feature for appliances with metal bodies.
2. It gives the current an easy path to the ground.

Answer: Insulating materials such as rubber do not allow current to pass through them. The gloves protect the person from an electric shock.

Teacher's Note:
1. Never repair a live wire.
2. Switch off the supply first.

Answer: In a parallel connection each appliance gets the full supply voltage and works independently. If one appliance is switched off or fails, the others keep working.

Teacher's Note:
1. In series, one break stops the whole circuit.
2. Parallel wiring avoids this problem.

Answer: It cuts off the supply automatically when the current becomes too large. This protects the wiring and appliances from damage and fire.

Teacher's Note:
1. MCB stands for miniature circuit breaker.
2. A fuse melts but an MCB can be switched on again.

Answer: A short circuit occurs when the live wire and the neutral wire touch directly, for example when the insulation wears off. A very large current then flows suddenly and can cause sparks or fire.

Teacher's Note:
1. The resistance in the path becomes very small.
2. This is why a fuse or MCB is used.

Answer: Electric current is the flow of electric charges through a conductor. Its SI unit is the ampere (A).

Teacher's Note:
1. Current is measured with an ammeter.
2. Always write the unit with the symbol.

Answer: Chemical energy stored in the substances inside the cell is converted into electrical energy.

Teacher's Note:
1. A cell works as long as the chemicals can react.
2. When they are used up, the cell is dead.

Answer: Conductors are materials that allow electric current to pass through them easily. Examples are copper and aluminium.

Teacher's Note:
1. Metals are good conductors.
2. Graphite and salt solution also conduct.

Answer: Insulators are materials that do not allow electric current to pass through them easily. Examples are rubber, plastic and dry wood.

Teacher's Note:
1. Insulators are used to cover wires.
2. Glass and cloth are also insulators.

Answer: 1. Make a circuit with a cell, a bulb and two loose ends of wire.
2. Touch the free ends to the two sides of the material to be tested so that it completes the circuit.
3. If the bulb glows, the material is a conductor. If the bulb does not glow, it is an insulator.

Teacher's Note:
1. Test each material the same way for a fair result.
2. Use a bulb or tester of low power for safety.

Answer: Examples are a fan, a tube light, a refrigerator, a television, a mixer-grinder and a washing machine.

Teacher's Note:
1. Name at least four items.
2. Add what each one does if you have space.

Answer: Conductors: copper wire, iron nail, aluminium foil, steel spoon.
Insulators: plastic comb, rubber eraser, wooden ruler, glass bottle.

Teacher's Note:
1. Test them with a simple circuit if you can.
2. Keep the two lists separate.

Answer: The switch is the part used. It breaks or completes the circuit, so it stops or allows the flow of current.

Teacher's Note:
1. A circuit works only when it is closed.
2. An open switch gives a break in the circuit.

Answer: Connect the positive terminal of the cell to one terminal of the switch. Connect the other terminal of the switch to the bulb and join the second terminal of the bulb back to the negative terminal of the cell. When the switch is closed the bulb glows.

Teacher's Note:
1. The cell, switch and bulb form a closed loop.
2. Use standard symbols when you draw it.