Exercises: Electric Current & Its Effects | Class 7 Oxford Science: Chapter Notes, Worksheets & Tests PDF Download

Objective Type Questions

When we connect two cells to form a battery, we should connect the positive terminal of one cell to the negative terminal of the other cell.
This is because the positive terminal of a cell is connected to its positive electrode which is the source of electrons, while the negative terminal is connected to its negative electrode which is the sink for electrons. Thus, when we connect two cells in series, we want the electrons to flow from the negative electrode of one cell to the positive electrode of the other cell, which requires connecting the positive terminal of one cell to the negative terminal of the other cell.

Q.2. In an electric circuit, the connecting wire should be made of a _______(conductor/insulator). 

In an electric circuit, the connecting wire should be made of a conductor.
A conductor is a material that allows electric current to flow through it easily, such as copper or aluminum wires. In contrast, an insulator is a material that does not allow electric current to flow through it easily, such as rubber or plastic. Since the purpose of a connecting wire in an electric circuit is to carry electric current from one component to another, it needs to be made of a conductor.

Q.3. A device that is used to open or close an electrical circuit is called a ________(fuse/switch). 

A device that is used to open or close an electrical circuit is called a switch.
A switch is a mechanical device that can be used to connect or disconnect an electric circuit. It typically consists of a metal contact that can be moved by a lever or button, and when the contact is closed, it allows electric current to flow through the circuit, while when it is open, the circuit is broken and no current flows.

Q.4. A.___________(fuse/switch) works on the heating effect of electric current. 

A fuse works on the heating effect of electric current.
A fuse is a safety device that is used to protect an electric circuit from excessive current. It consists of a thin wire or strip of metal that melts when the current passing through it exceeds a certain value, thus breaking the circuit and preventing further damage. This happens because the wire heats up due to the current flowing through it, and eventually reaches a temperature where it melts.

Q.5. In an electric bell, the electromagnet is magnetised when the switch is _________(on/off). 

In an electric bell, the electromagnet is magnetized when the switch is on.
An electric bell typically consists of an electromagnet, a metal armature, a spring, and a gong. When the switch is turned on, electric current flows through the coil of the electromagnet, which magnetizes it and attracts the metal armature towards it. This causes the armature to strike the gong, producing a sound. When the switch is turned off, the current stops flowing and the electromagnet demagnetizes, allowing the spring to pull the armature away from the gong.

B. Choose the correct option.

Very Short Answer Type Questions 

Tissue

Q.2. Thin wire that glows in an incandescent light bulb ___________

Filament

Q.3. A device that limits the amount of current flowing in a circuit ___________

Resistor

Q.4. A long wire that has been wound many times into a tightly packed coil ___________

Solenoid

Q.5. The part of an electric bell that is attracted to the electromagnet, when switched __________

Armature

Short Answer Type Questions 

Four components of an electric circuit are:

(a) Battery (symbol: +ve and -ve terminals)

(b) Switch (symbol: open and closed)

(c) Bulb (symbol: circle with a cross inside)

(d) Wire (symbol: straight line)

Q.2. Draw a circuit diagram where an electric current will flow. Add a component that will  help open and close the circuit and label it. 

A circuit diagram where an electric current will flow can be drawn as follows:

Battery (+) ---- Switch ---- Bulb ---- Battery (-)

The component that will help open and close the circuit is the switch, which can be labelled as 'S'.

Q.3. Explain 'heating effect of electric current! 

Heating effect of electric current is the process of producing heat when an electric current passes through a conductor. This effect occurs due to the resistance of the conductor, which converts electrical energy into heat energy. The amount of heat produced depends on the resistance of the conductor and the amount of current flowing through it.

Q.4. What is 'magnetic effect of electric current'? 

Magnetic effect of electric current is the production of a magnetic field around a conductor when an electric current flows through it. This effect is due to the movement of electrons in the conductor, which creates a magnetic field. The strength of the magnetic field depends on the amount of current flowing through the conductor and the distance from the conductor.

Q.5. What is the purpose of having an electromagnet in an electric bell?

The purpose of having an electromagnet in an electric bell is to produce a magnetic field that attracts and releases a metal hammer, which strikes a bell to produce sound. When an electric current flows through the coil of the electromagnet, it creates a magnetic field that attracts the metal hammer towards the bell. When the current is switched off, the magnetic field disappears and the metal hammer is released, striking the bell to produce sound.

Long Answer Type Questions 

Two situations when an electric current will not flow through a circuit are:

a) Open circuit: In this situation, the circuit is incomplete or broken, which means there is a gap in the circuit and the current cannot flow through it. For example, if a wire is cut or a switch is turned off, the circuit will be open and the current will not flow.

b) Short circuit: In this situation, the circuit has a low resistance path that bypasses the load, which means the current flows through the path of least resistance and not through the load. For example, if the positive and negative terminals of a battery are connected directly without any load in between, the circuit will be short and the current will not flow through the load.

Q.2. Explain the working of an electric fuse. 

An electric fuse is a safety device that is used to protect electrical appliances and circuits from damage due to excessive current. It works by breaking the circuit when the current exceeds a certain limit. The fuse consists of a wire or a thin strip of metal that melts when the current exceeds the rated value. When the wire melts, the circuit is broken and the current stops flowing. This prevents the appliance or circuit from overheating and catching fire.

Q.3. Draw a rough figure of a simple electromagnet. How can we find out if it is magnetic or not? 

A simple electromagnet can be made by wrapping a wire around a piece of iron or steel and connecting the ends of the wire to a battery or power source. When the current flows through the wire, it creates a magnetic field that magnetizes the iron or steel. To find out if the electromagnet is magnetic or not, we can use a compass. If the compass needle is deflected when brought near the electromagnet, it means that the electromagnet is magnetic and is generating a magnetic field.