7.9 : Electromagnetism Electromagnet

•  A wire carrying an electric current produces a magnetic field around it – called as magnetic effect of a current

•  Magnetic effect is good if the current passes through a coil of wire

•  The effect will be even better if a soft iron core is placed inside the coil

•  That piece of soft iron becomes electromagnet

7.9 : Electromagnetism Electromagnet

•  Electromagnet : A magnet due to a current passing through the coil around it

•  Electromagnet loses its magnetism as soon as the current is cut off

•  It has north & south pole

•  Polarity of the electromagnet depends on the direction of the current

7.9 : Electromagnetism Electromagnet

•  To know the poles : –  Look at the end of the

electromagnet –  If the current flows in

an ANTICLOCKWISE direction, that end is NORTH POLE

–  If the current flows in a CLOCKWISE direction, the end is south pole

•  The strength of an electromagnet can be increased by : –  Increasing the current –  Using more turns of

wire round the iron core

–  Using soft iron as the core

7.9 : Electromagnetism Electromagnet

•  The magnetic field around the electromagnet is similar to the field around a bar magnet

•  They are used in – Electric bell – Electric crane – Telephone

earpiece – Certain types of

ammeter

7.9 : Electromagnetism Magnetic Field Around a Straight Wire Carrying a

Current

•  Electromagnetism : Magnetic effect produced by a current in a conductor

•  A straight wire carrying a current has a circular magnetic field around it

•  The direction of the magnetic field depends on the direction of the current

7.9 : Electromagnetism Magnetic Field Around a Straight Wire Carrying a

Current

•  If the current flows from TOP to the LOWER end of the wire, the magnetic field is in a CLOCKWISE direction

•  If the current flows from the BOTTOM to the TOP of the wire, the magnetic field is in an ANTICLOCKWISE direction

•  The magnetic field lines move further apart as they go outwards away from the wire carrying the current

7.9 : Electromagnetism Magnetic Field Around a Straight Wire Carrying a

Current •  In practice,

the direction of the magnetic field is determined by placing a plotting compass on a cardboard near the wire carrying current

7.9 : Electromagnetism Magnetic Field Around a Straight

Wire Carrying a Current •  In theory the direction of the magnetic field can be determined by using the RIGHT HAND GRIP RULE –  Grip the wire

carrying the current with the right hand so that the thumb points in the d direction of the current ( + to – )

–  The four fingers now show the direction of the magnetic field

7.9 : Electromagnetism Magnetic Field Around a Coil of Wire (Solenoid)

•  A coil of wire is called solenoid •  When a current flows through a solenoid,

magnetic field is set up around the solenoid

•  Magnetic field observed around the solenoid is similar to the magnetic field around a bar magnet

7.9 : Electromagnetism Ways for determining the poles

•  Use the same way for an electromagnet –  Look at the end

of the solenoid –  If current flows

in clockwise direction, that end is a south pole

–  If the current flows in an anticlockwise direction, that end is a north pole

7.9 : Electromagnetism Ways for determining the poles

•  Use Right Hand Grip Rule : –  Imagine griping the solenoid with the right hand so

that fingers point in the direction of the current –  Stretch out the thumb; the direction of the thumb

shows the direction and north of the magnetic field

7.9 : Electromagnetism Ways for determining the poles

•  Strength of the magnetic field can be increased by : – Increasing the

current flowing through the solenoid

– Increasing the number of turns of wire for the solenoid

– Inserting a piece of soft iron into the solenoid