magnets and magnetic field

chapter 26 and 27

What is a magnetic field

any region of space where magnetic forces can be felt

what are magnetic poles

the regions at each end of a magnet where the magnetic forces are greatest. magentic poles are always found in pairs

draw the magnetic field around a bar magnet

draw the magnetic field around a u-shaped magnet

give one use of earth’s magnetic field

a magnetic compass shows direction, it is used in each navigation

why does a magnet that is free to rotate point towards the north

because the earth has a magnetic field that exerts a force on the magnet

Describe an experiment to demonstrate the magnetic effect of an electric current

Apparatus- compass, battery(dc power source) wires and switch

procedure- 1.set up equipment with the wire lined up north-south. the plotting compass also lines up North-South 2. close the switch,sending direct current through the wire.the compass needle will deflect. 3.reverse the direction of the current and the needle deflects in the opposite direction. 4. open the switch no current flows, the magnetic field disaoppears and the ompass again lines up N-S

Conclusion- every current carrying conductor has a magnetic field around it caused by the current

draw a sketch of the magnetic field due to a long straight current-carrying conductor

give one use of an electromagnet

Scrapyard cranes; electric motors; electric bell; doorbell

sketch the magnetic field due to a current in a solenoid

describe an experiment to show that a current-carrying conductor in a magnetic field experiences a force

Apparatus- aluminum foil, strong magnet, d.c power source,wire

procedure-switch on current

observation- the foil will be seen to move forwards and backwards depending on which direction the current is flowing

conclusion- a current-carrying conductor in a magnetic field experiences a force

list two devices based on the fact that a current-carrying conductor in a magnetic field experiences a force

an electric motor, a moving coil meter, a loudspeaker

list three factors that affect the force on the current carrying conductor in the magnetic field.

the size of the current

the length of the wire

the magnetic flux density

the sine of the angle between the conductor and the magnetic field

what happens if the conductor is placed parallel to the magnetic field

there will be no force on the conductor

define magnetic flux density

At a point in a magnetic field the magnetic flux density is a vector whose: direction is the direction of the force on a north pole placed at that point and whose magnitude is the value of B from the equation F=IlB, where F is the force on the conductor of length l carrying a current I perpindicular to the magnetic field

Describe an experiment to show the forces on a current carrying coil in a magnetic field.

Set up the equipment as shown above. the coil is free to rotate about the axis shown. When the current is switched on, the coil begins to rotate in an anti-clockwise direction due to the forces acting on it

what is the direction of the forxeoon a current-carrying conductor in a magnetic field

the force is always perpendicular to the current and perpendicular to the magnetic field

si unit of magnetic flux density

the tesla T

give two ways of deflecting a beam of electrons

it can be deflected in an electric field or in a magnetic field

what is electric current

An electric current is a flow of electric charge

deine the ampere . what is the si unit

the ampere is that current which, if maintained in two infinitely long parallel conductors of negligable cross-section placed 1 metre apart in a vacuum,would produce a force on each wire of 2×10^-7 newtons per meter length

state the principle on which the ampere is based

two current carrying conductors exert a force on each other due to their magnetic fields

define the coulomb

the amount of charge that passes any point in a circuit when a current of 1 ampere flows for 1 second

describe the experiment to demonstrate the principle on which the definition of the ampere is based

apparatus - parallel strips of alluminium foil connect to a wire circuit, ampere d.c power supply

procedure-1. send a current through the parallel strips of aluminium foil

observation the foil strips will be seen to move away from eachother

conclusion-there is a force between current-carrying conductors due to their magnetic fields