4.5 electromagnetic effects

Question 1

how can an emf in the conductor be induced?

Answer 1

by moving a conductor across a magnetic field or changing magnetic field linking with a conductor

Question 2

factors that affect the magnitude of an induced emf

Answer 2

1. the speed at which the wire, coil or magnet is moved
2. the number of turns on the coils of the wire
3. the size of the coils (bigger area = bigger field)
4. the strength of the magnetic field

Question 3

construction of a simple transformer

Answer 3

soft iron core used for voltage transformations

Question 4

step up transformer

Answer 4

increases the potential difference of a power source
- has more turns on secondary than primary coil

Question 5

step down transformer

Answer 5

decreases the potential difference of a power source
- fewer turns on secondary coil than primary

Question 6

transformer roles in high voltage transmission

Answer 6

1. used to increase the voltage of electricity before it is transmitted across the national grid
2. used to lower the high voltage electricity used in power lines to lower voltages used in houses
3. used in adapters to lower mains voltage to lower voltages used by many electronic devices

Question 7

advantages of high voltage transmission

Answer 7

When electricity is transmitted over large distances the current in the wires heat them, resulting in energy loss
To transmit the same amount of power as the input power the voltage should be increased
- results in a smaller current being transmitted bcs P = IV
Smaller current flowing through the power lines result in less heat being produced

Question 8

simple d.c motor consists of:

Answer 8

a coil of wire (that is free to rotate) positioned in a uniform magnetic field

Question 9

how does a d.c motor work? (without split ring first)

Answer 9

When a d.c current is passed through the wires it creates a magnetic field.
This mag field interacts with the mag field of the magnets, resulting in a force being exerted on the coil (direction determined by flemings right hand rule)
As current flows in opposite direction on each side of coil the force will be exerted on different sides (up/down) causing the coil to rotate.
The coil will continue rotating until the vertical position.

Question 10

what does a split ring commutator do in a d.c motor?

Answer 10

reverses the direction of the current in the coil every half turn
- keeping the coil rotating continuously as long as current is flowing

Question 11

how to increase the speed the coil rotates (d.c motor)

Answer 11

• increasing the current
• using a stronger magnet

Question 12

how to change the direction of rotation of coil (d.c motor)

Answer 12

• reversing the direction of current
• reversing the poles of the magnet

Question 13

how to increase the force supplied (d.c motor)

Answer 13

• increasing the current in the coil
• increasing the strength of the mag field
• adding more turns to the coil

Question 14

transformer

Answer 14

an electrical device that can be used to incresae or decrease the pd of an ac
- by the generator effect

Question 15

what does a basic transformer consist of

Answer 15

primary coil
secondary coil
soft iron core

Question 16

why is a iron used in transformers

Answer 16

because iron is easily magnetised (its a soft magnetic material)

Question 17

operation of a transformer

Answer 17

1. ac supplied to PC
2. ac produces a changing mag field around PC
3. mag field passes through iron core
4. changing mag field inside SC
5. induces an alternating pd on the SC
6. (if SC is part of complete circuit an ac will flow)

Question 18

generator effect

Answer 18

the induction of EMF, therefore producing an induced current when a wire experiences a changing magnetic field

Question 19

equation for 100% efficiency in a transformer

Answer 19

IpVp = IsVs
p (primary coil)
s (secondary coil)

Question 20

Power equation with current and resistance

Answer 20

current^2 x resistance

Question 21

why is power loss in cables smaller when the voltage is greater?

Answer 21

the higher the voltage, the lower the current (P=IV) through the wire. since I^2 is directly proportional to power, for a small current power loss is also small.