Electromagnetism

Question 1

How does electromagnetic induction work?

Answer 1

• as you move magnet into of coil, number of field lines cutting through coil increases
• this induces a voltage, and if circuit complete, a current
• if magnet still, no change ∴ no voltage
• if take magnet out, voltage, but in opp direction
• if you reverse the magnet, voltage in other direction
• KE turned into electrical energy

Question 2

How do you increase the voltage/current?

Answer 2

• stronger magnet ∴ greater change in field lines
• more turns of wire ∴ greater change in field lines
• moves magnet/coil quicker

Question 3

How does an AC generator work?

Answer 3

• as coil turns the field lines cutting through it change continuously ∴ voltage, and if complete circuit, current
• graphite brushes from slip rings to wires conduct electricity

Question 4

Why do we have transformers?

Answer 4

• energy from power stations have v high voltage ∵ at high current things lose lots of heat to atmos (t inefficient). As power stations cant change their power outage, use a high V to reduce the current
• ∵ different users need different supplies of voltage, e.g. Factories/homes, need a device to change the voltage of electricity from power stations

Question 5

What links the coils on each side and the voltage on each side of a transformer?

Answer 5

Ration left turns : right turns is same ratio as voltage in : voltage out
(If 100% efficient)
Power in = power out
Vp/Vs = num coils prim/num coils sec

Question 6

How does a transformer work?

Answer 6

Electric current in a conductor produces magnetic field around it
∴ when no current, no field lines cutting through 2ndary coil
When there is a large current, large number of field lines cutting through secondary coil. the v changes as you can inc/decr V by having more turns of wire
Because AC goes in both directions, continuous change in num field lines ∴ continuous voltage induced, and if complete circuit, continuous current induced
DC only goes in 1 direction ∴ mag field constant ∴ no change in num of field lines cutting through 2nd coil ∴ no V

Question 7

How are electromagnets made?

Answer 7

• piece of current carrying wire wrapped around a soft iron core∴ when current in wire magnetic field induced in the metal

Question 8

When and why is there a force on a charged particle as it moves across a magnetic field?

Answer 8

• force exerted when moves across field lines, as like crossing speed bumps ∴ small force exerted affects its movement
• if moves parallel to field lines doesn’t cross any ∴ no force

Question 9

What is flemings left hand rule?

Answer 9

Thumb is direction of force/movement
Index finger is direction of magnetic field
Middle finger is direction of pos ⇒ neg current

Question 10

How does strength of field and current affect movement of electromagnetic conductor?

Answer 10

If you increase the current of the magnetic field the force on the conductor will increase

Question 11

How does an electric motor work?

Answer 11

• coil of wire attached to armature which can rotate
• current flow in opp directions on each side ∴ one pushed up and the other pushed down (Flemings rule)∴ it spins
• when it rotates to vertical one side at top and other at bottom ∴ it cant spin any more
∴ to spin in same direction the current must be reversed every half turn (again Flemings rule), tf up side will go down and down side up, so it moves in a circle. This done using split ring commutator, made of Cu
• current always comes in on LHS and out on RHS ∴ LHS upwards and RHS downwards
• two graphic brushes push against armature to conduct electricity

Question 12

How does a loudspeaker work?

Answer 12

• coil slides backwards and forwards over a perm bar magnet
• attached to paper cone by brown bars
• wire carries AC from amplifier ∴ making coil and cone move back and forth at the same frequency as the changing current
∴ air moved back and forth to create the sound which will have the same frequency as the current ∴ it will have the same freq as the original noise