Electric Current and Its Effects

How Steady is Your Hand? – Circuit Representation

You might remember the game “How Steady is Your Hand?” from Class VI, Chapter 12. In this game, players try to move a loop along a wire without touching it, testing hand steadiness.

Paheli and Boojho set up the game by connecting an electric circuit, just as suggested in Class VI. They had fun playing with family and friends, and they wanted to share it with a cousin in another town.

Paheli made a neat drawing showing how the electric components were connected (see Fig. 14.1). This helped them understand the connections better.

Question for Students:

• Can you draw this circuit neatly on your own?

• Boojho wondered: Is there an easier way to represent electric components?
  
  

Key Concept:

• Instead of drawing the actual shapes of bulbs, batteries, and wires, scientists and engineers use symbols to represent components in a circuit diagram.

• This makes it easier to draw, read, and understand complex circuits without confusion.

14.1 Symbols of Electric Components

Use of Symbols

Some common electric components are represented by symbols.
These symbols help us draw electric circuits easily and neatly.
Different books may use different symbols, but we will use only the symbols given in this book.

Symbol of an Electric Cell

The symbol of an electric cell has:

• One long line

• One short but thicker line
  
  

An electric cell has two terminals:

• The longer line represents the positive terminal

• The shorter, thicker line represents the negative terminal
  
  

Symbol of a Switch and Wires

• The ON and OFF positions of a switch are shown using symbols.

• Wires used to connect components in a circuit are represented by straight lines.
  
  

Battery and Its Symbol

A battery is a combination of two or more electric cells.

To make a battery:

• The positive terminal of one cell is connected to the negative terminal of the next cell.
  
  

The symbol of a battery shows:

• Several pairs of long and short parallel lines

• Each pair represents one cell
  
  

Arrangement of Cells in Devices

In many devices such as:

• Torches

• Toys

• Transistors

• TV remote controls
  
  

Cells are not always placed in a straight line.Sometimes, cells are placed side by side.

Inside the battery compartment:

• A metal strip or thick wire connects the positive terminal of one cell to the negative terminal of the next

• ‘+’ and ‘–’ symbols are printed to help place cells correctly
  
  

Cell Holders

Cells can be connected using:

• A cell holder made at home using a wooden block, iron strips, and rubber bands

• Ready-made cell holders bought from the market
  
  

The cells must be placed so that:

• Positive terminal of one cell connects to negative terminal of the next

Circuit Diagram

A circuit diagram is a drawing of an electric circuit using symbols instead of actual components. It is much easier to draw a circuit diagram using symbols. Therefore Electric circuits are generally represented using circuit diagrams

Activity 14.1

• Make the electric circuit shown in Fig. 14.7

• Copy the circuit in your notebook

• Draw its circuit diagram using symbols
  
  

Observation:

• The bulb glows only when the switch is in the ON position
  
  

Closed and Open Circuits

• When the switch is ON, the circuit is closed

  • Current flows through the circuit

• When the switch is OFF, the circuit is open

  • No current flows

The switch can be placed anywhere in the circuit.

Bulb and Filament

Inside the bulb:

• There is a thin wire called the filament

• The filament glows when electric current passes through it
  
  

If the bulb is fused:

• The filament is broken

• The circuit becomes incomplete

• The bulb does not glow
  
  

CAUTION

• Never touch a glowing bulb connected to the mains

• Do not experiment with:

  • Electric supply from mains

  • Generator

  • Inverter

• Use only electric cells for activities
  
  

14.2 Heating Effect of Electric Current (Elaborated)

What is Heating Effect of Electric Current?

When an electric current passes through a wire, the wire resists the flow of current. Due to this resistance, electrical energy is converted into heat energy. This production of heat in a conductor is called the heating effect of electric current.

Why Does a Bulb Become Hot?

• When the switch is ON, current flows through the bulb

• The bulb has a thin filament

• The filament offers resistance to current

• Due to this resistance, the filament gets heated

• At very high temperature, it starts glowing
  
  

This is why a glowing bulb becomes warm or hot.

Activity 14.2 – Explanation

• When the switch is OFF, the circuit is open

• No current flows → bulb does not glow

• When the switch is ON, the circuit is closed

• Current flows → bulb glows → heat is produced

• After switching OFF, the bulb slowly cools down
  
  

Conclusion:
Electric current produces heat when it flows through a conductor.

Activity 14.3 – Explanation

• Nichrome wire is used because it has high resistance

• When current flows:

  • Wire resists current

  • Heat is produced

• When switched OFF:

  • No current flows

  • Wire cools down
    
    

Conclusion:
Heat is produced only when current flows.

Uses of Heating Effect

The heating effect is useful in many appliances:

Electric iron - Pressing clothes

Electric heater - Heating rooms

Electric cooker - Cooking food

Electric kettle - Heating water

Heating Element

• The coil of wire used in heating appliances is called an element

• It is usually made of nichrome

• Nichrome is used because:

  • It has high resistance

  • It does not burn easily

  • It can withstand high temperature
    
    

Why Do All Wires Not Become Hot?

• Connecting wires are:

  • Thick

  • Made of materials with low resistance

• Hence, they do not produce much heat

On the other hand:

• Thin wires with high resistance produce more heat
  
  

Factors Affecting Heat Produced

The heat produced in a wire depends on:

1. Material of the wire

2. Length of the wire

3. Thickness of the wire

Longer and thinner wires produce more heat

Why Does a Wire Melt?

• If very large current flows through a thin wire:

  • Excess heat is produced

  • Wire may melt and break

This can be dangerous and may cause fire.

Electric Fuse – Detailed Explanation

• A fuse is a thin wire made of special material

• It melts easily when excess current flows

• When it melts:

  • Circuit breaks

  • Current stops flowing
    
    

Role of Fuse:

• Prevents:

  • Overheating of wires

  • Fire accidents

  • Damage to appliances

Therefore, a fuse is a safety device.

Causes of Excess Current

1. Short Circuit

   • Happens when:

     • Insulation of wires gets damaged

     • Live wires touch each other
       
       

2. Overloading

   • Too many appliances connected to one socket
     
     

Both can cause fires.

Energy Wastage in Bulbs

• Electric bulbs give:

  • Light ✔️

  • Heat ❌ (wasted energy)
    
    

• This wastage can be reduced by using:

  • Tube lights

  • CFLs
    
    

These use less electricity.

ISI Mark – Importance

• ISI mark ensures:

  • Safety

  • Energy efficiency

  • Good quality
    
    

Always check the ISI mark before buying electrical appliances.

14.3 Magnetic Effect of Electric Current

Activity 14.5 – Step-by-step Explanation

Materials used:

• Cardboard tray from a matchbox

• Insulated electric wire

• Compass needle

• Electric cell

• Switch
  
  

Procedure:

1. Wrap the electric wire a few times around the cardboard tray.

2. Place a compass needle inside the tray.

3. Connect the free ends of the wire to an electric cell through a switch.

4. Observe the direction of the compass needle when the switch is OFF.

5. Bring a bar magnet near the compass needle and observe the deflection.

6. Now remove the magnet.

7. Switch ON the circuit and observe the compass needle carefully.

8. Switch OFF the circuit and observe again.

9. Repeat the experiment a few times.
   
   

Observations

• When the switch is OFF:

  • No current flows

  • Compass needle points in the north–south direction

• When a bar magnet is brought near:

  • Compass needle deflects

• When the switch is ON:

  • Current flows through the wire

  • Compass needle deflects

• When the switch is switched OFF again:

  • Compass needle returns to its original position
    
    

What Does This Experiment Indicate?

• A compass needle is a tiny magnet

• It gets deflected:

  • Near a magnet

  • Near a current-carrying wire

• This shows that a current-carrying wire produces a magnetic effect
  
  

Important Conclusion

When electric current flows through a wire, the wire behaves like a magnet.
This effect is called the magnetic effect of electric current.

Scientist Behind the Discovery

• Hans Christian Oersted was the first scientist to observe this effect.

• He noticed that: A compass needle deflects whenever electric current flows through a nearby wire.

14.4 Electromagnet (Activity 14.6)

What is done in the activity

• An insulated wire is tightly wound around an iron nail.

• The wire is connected to a cell through a switch.

• Small iron pins are placed near the nail.
  
  

Observations

1. When the switch is ON, current flows through the coil.

2. The iron nail attracts the pins.

3. When the switch is OFF, current stops.

4. The pins fall off the nail.

What does this indicate?

• The nail becomes magnetic only when current flows.

• When the current is switched off, magnetism disappears.
  
  

What is an Electromagnet?

• An electromagnet is a temporary magnet produced by passing electric current through a coil of wire wound around a soft iron core.
  
  

Important characteristics of electromagnets

• Magnetism exists only when current flows.

• Strength can be increased by:

  • Increasing number of wire turns

  • Increasing current

  • Using a soft iron core

• Can be switched ON and OFF easily.
  
  

Uses of electromagnets

1. Cranes to lift heavy iron and steel objects.

2. Separating magnetic materials from junk.

3. Used by doctors to remove tiny iron particles from the eye.

4. Used in electric bells, toys, motors, relays, etc.
   
   

Difference between magnet and electromagnet (quick idea)

Magnet     Electromagnet

Permanent Temporary

Cannot be switched off Can be switched ON/OFF

Fixed strength Adjustable strength

Key takeaway 

• Electric current produces magnetism.

• Electromagnets are temporary but very powerful magnets.

14.5 ELECTRIC BELL

What is an electric bell?

• An electric bell is a device that produces sound using electric current.

• It works on the principle of an electromagnet.

Main parts of an electric bell

1. Battery (cell) – provides electric current

2. Switch – completes or breaks the circuit

3. Electromagnet – a coil of wire wound around an iron core

4. Iron strip (armature) – gets attracted by the electromagnet

5. Hammer (striker) – attached to the iron strip

6. Gong (bell) – produces sound when struck

7. Contact screw – makes and breaks the circuit

Working of an Electric Bell (Step-by-Step)

Step 1: Circuit is completed

• When the switch is ON, current flows through the coil.

• The coil becomes an electromagnet.

Step 2: Attraction of iron strip

• The electromagnet attracts the iron strip.

• The hammer attached to the strip hits the gong, producing sound.

Step 3: Circuit breaks

• When the iron strip moves, it loses contact with the contact screw.

• The circuit breaks and current stops flowing.

Step 4: Electromagnet loses magnetism

• Since current stops, the coil is no longer an electromagnet.

• It cannot attract the iron strip anymore.

Step 5: Iron strip returns to original position

• The iron strip comes back to its original position due to its elasticity.

• It again touches the contact screw, completing the circuit.

Step 6: Repetition of process

• Current flows again → electromagnet forms → hammer strikes gong.

• This process repeats very fast.

• Continuous striking produces a ringing sound.

Why does the bell ring continuously?

• The circuit keeps making and breaking automatically.

• This rapid action causes the hammer to strike the gong repeatedly.

Important points to remember

• The electric bell works due to the magnetic effect of electric current.

• The electromagnet is temporary.

• The bell rings only when current flows.

Flow summary (easy to remember)

Current ON → Electromagnet forms → Iron strip attracted → Hammer hits gong → Circuit breaks → Magnetism lost → Strip returns → Circuit completes → Bell rings

Keywords – Meanings

1. Battery - A battery is a combination of two or more cells that provides electrical energy to a circuit.

2. Electric components - Electric components are the parts used to make an electric circuit, such as a cell, bulb, switch, wire, and battery.

3. Circuit diagram - A circuit diagram is a drawing that uses symbols to show how electric components are connected in a circuit.

4. Electric bell - An electric bell is a device that produces sound using an electromagnet when electric current flows through it.

5. Electromagnet - An electromagnet is a temporary magnet formed when electric current flows through a coil of wire wrapped around an iron core.

6. Fuse - A fuse is a safety device made of a thin wire that melts and breaks the circuit when excessive current flows, preventing damage.

7. Heating effect of current - The heating effect of current is the production of heat when electric current flows through a wire.

8. Magnetic effect of current - The magnetic effect of current is the property by which a current-carrying wire behaves like a magnet.

Exercise

1. Symbols of electrical components

(Students should draw these in notebook)

• Connecting wire – straight line

• Switch (OFF) – break in the line

• Switch (ON) – closed line

• Bulb – circle with a filament inside

• Cell – one long line and one short line

• Battery – combination of two or more cells

2. Circuit diagram for Fig. 14.21

Draw a circuit diagram using symbols showing:

• Cell / battery

• Switch

• Bulb

• Connecting wires
  (All connected in a closed circuit)

3. Making a battery of four cells (Fig. 14.22)

Connect:

• Positive terminal of one cell to the negative terminal of the next cell

• Continue till all four cells are connected in series

4. Bulb does not glow (Fig. 14.23)

Problem:

• Circuit is incomplete / wrong connections / bulb fused

Correction:

• Ensure switch is ON

• Connect positive terminal of cell to bulb properly

• Complete the circuit

5. Two effects of electric current

1. Heating effect of electric current

2. Magnetic effect of electric current

6. Compass needle deflects – Explain

When electric current flows through a wire, it produces a magnetic field. The compass needle (a tiny magnet) gets deflected due to this magnetic field.

7. Compass deflection in Fig. 14.24

Yes, the compass needle will show deflection because current flows when the switch is closed.

8. Fill in the blanks

(a) Longer line in the symbol for a cell represents its positive terminal.
(b) The combination of two or more cells is called a battery.
(c) When current is switched ‘on’ in a room heater, it produces heat.
(d) The safety device based on heating effect is called a fuse.

9. True (T) or False (F)

(a) F
(b) T
(c) F
(d) T

10. Can an electromagnet separate plastic bags?

No. Plastic is non-magnetic, so an electromagnet cannot attract it.

11. Replacing a fuse with a wire – Agree?

No, I would not agree.
A fuse protects appliances by melting during excess current.
A wire will not melt easily and may cause fire or damage.

12. Bulb does not glow – Possible defects

• Loose connections

• Switch not working

• Bulb fused

• Cell exhausted

• Wrong terminal connections

13. Circuit in Fig. 14.25

(i) No bulb will glow when switch is OFF.
(ii) When switch is ON, all bulbs (A, B, C) glow together (same order).

Download the Goa Board Std 7 Science Part II Textbook PDF. Includes all lessons from the official syllabus.