7.9 : electromagnetism
TRANSCRIPT
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