Class 7
SCIENCE
Electric Current and its effects
Q1 What is meant by electric current?
A ns-Electric Current : Flow of electrons.
Q2 What is an electrc circuit?
Ans -Electric Circuit : The closed path in which electric current flows.
Q3 Under what conditions a circuit is said to be complete?
Ans-Current can flow through a circuit only if it is complete i.e. all its parts are made up of a
conductor.
Q4 What is a switch?
Ans -Switch is used to close and open a circuit. When switch is ‘ON’ circuit is complete and
current flows through it and when it is ‘OFF’ circuit breaks (air is an insulator) and flow of
current stops.
Q5 Draw the symbols for various Components of an electric circuit ?
Cell
:
Battery :
Source of electric current.
Combination of 2 or more cells
Connecting Wires – generally insulated Copper wires.
Electric Bulb –
Key or switchOpen keyClosed key
Note : When cells are connected to form a battery positive terminal of one is always connected
to negative terminal of the other.
Q6 What is a circuit diagram?
Ans—In an electrical Circuit when we replace electrical components with their symbols we call
it Circuit diagram.
Q7 Draw the circuit diagram corresponding to the following electrical circuit.
Ans-Q8 What do mean by heating effect of current?
Ans--When current flows through a metal wire it gets heated up, this is called heating effect of
current.
e.g. a glowing electric bulb becomes warm.
Q9 Name few appliances which use heating effect of current.
Ans--Appliances using heating effect of current
Electric Iron, room heater, electric heater, geyser, hair dryers etc.
Q10 What are filaments of a bulb and a heater made up of?
Ans--In a bulb there is a thin wire called the filament made up of tungsten. When current
passes through it, it glows and gets heated up.
In a heater there is a coil of wire called element which is made up of nichrome. When current
passes through element it becomes red hot and give out heat.
Q11 What are the factors on which amount of heat produced depends?
--The amount of heat produced in a wire depends on
-(A) Its material – better conductors produce less heat;
e.g. Elements of electric appliances are made of nichrome whereas connecting wires are made
up of copper which is a better conductor so they do not get heated up as the element.
-(B) Its length – as length increases heat produced increases;
e.g. elements are coiled in heaters to increase length
.
-(C) Thickness – as thickness increases heat produced decreases;
e.g. lead wires of iron, heater etc. are thicker than normal connecting wire so that they don’t get
heated up easily.
Q12 What is an electric fuse?
Ans---Electric Fuse - A Safety device in domestic circuit used to prevent electrical fire due to
short circuit or overloading.
Q13 How does an electric fuse work?
Ans--They are wires made from special material which melt quickly and break when large
electric current are passed. (usually they are alloys of tin and lead)
When current exceeds safe limit fuse wire is overheated and it melts breaking the circuit.
Fuses of different capacity are used for different purposes.
Q14 What are the reasons for excessive current flowing through a circuit?
Ans--Reasons for excessive current :
Short circuit – When live and neutral wire touch each other due to faulty insulation.
Overloading – due to connection of many devices to a single socket.
Q15 What are MCBs?
Ans--These days Miniature circuit breakers (MCBs) are increasingly being used in place of
fuses. These are switches which automatically turn off when current in a circuit exceeds the safe
limit. You turn them on and the circuit is once again complete.
Q16 What do mean by magnetic effect of current?
Ans--MAGNETIC EFFECT OF CURRENT –
When electric current passes through a wire it behaves like a magnet. This is the magnetic effect
of electric current.
Q17 Name the scientist who discovered the magnetic effect of current.
Ans--Hans Christian Oersted for the first time observed that a magnetic compass kept close to
a wire carrying current showed deflection. Thus he concluded that a wire carrying current
behaves like a magnet.
Q18 What is an electromagnet?
Electromagnet - A current carrying coil of a wire wrapped around a piece of iron is called an
electromagnet. It is a temporary magnet. As electricity is switched off, electromagnet loose its
magnetism.
Q19 What are the factors affecting strength of an electromagnet?
Ans--Its strength depends on –
(a) Current flowing the coil of wire
(b) Number of turns of wire
Q20 What are the uses of an electromagnet?
Ans--Uses – It is used in cranes to lift heavy iron objects
Used to separate magnetic material from junk.
Electric Bell –It uses an electromagnet.