Magnetism And Electromagnetism Flashcards
(85 cards)
1
Q
Which 4 materials can be magnetic?
A
- cobalt
- iron
- nickel
- steel (iron alloy)
2
Q
What is the pneumonic for the type of electromagnet given by iron and steel?
A
Steel
Permanent
Iron
Temporary
3
Q
What are the names of the two poles of a bar magnet?
A
North seeking pole
South seeking pole
4
Q
How do you represent magnetic field on a diagram?
A
By drawing lines of flux / field lines
5
Q
What direction is flux?
A
Isolated North Pole takes north to south
6
Q
What 3 factors affect the strength of an electromagnet
How do each affect strength?
A
- current (more = stronger)
- number of coils (greater no. Of turns per unit length = stronger)
- material of core (iron = best)
7
Q
When conventional current is travelling from:
- into a page
- out of a page
, in what direction does the magnetic field travel?
A
- clockwise (into)
- anti-clockwise (out of)
8
Q
When conventional current is travelling from bottom to top, in what direction does the magnetic field travel?
A
- from a Birds Eye view, anti-clockwise
- from below, clockwise
9
Q
When looking down an electromagnet, how can you tell which is the north seeking pole and which is the south seeking pole?
What are the two pneumonics?
A
- the side where the coil moves up the back of the e.m., conventional current is clockwise
- the side where the coil comes down the front of the e.m., conventional current is anti-clockwise
ClockwiSe - south
ANti-clockwise - North
10
Q
What principal is used to determine the direction of magnetic fields?
A
The right hand grip rule
11
Q
How do lines of flux show the strength of a magnet?
A
The closer the lines, the stronger the magnetic field
12
Q
Where is magnetic field strongest?
A
The poles
13
Q
How do opposite magnetic poles attract?
A
The magnetic fields combine to produce a uniform magnetic field
14
Q
what is a uniform magnetic field?
How is it represented?
A
- A magnetic field with the same strength at every point
- represented with parallel lines of flux
15
Q
How do like magnets repel?
A
The magnetic fields cancel out
16
Q
How can you detect magnetic fields?
A
Using a compass
17
Q
What is used to show the shape of a magnetic field?
A
Iron fillings
18
Q
What is a plotting compass?
A
A small magnetic compass used to show the shape and direction of a magnetic field
19
Q
Where does the Earth’s magnetic field come from?
A
the Earth’s core, which is made from iron and nickel
20
Q
How does a compass work? (2)
A
- The needle of the compass is a small bar magnet
- that points in the direction of the Earth’s magnetic field
21
Q
What is produced around a current carrying wire?
A
A magnetic field
22
Q
How is magnetic field around a current-carrying wire displayed?
A
With circles coming out of the wire
23
Q
How do you find the direction of magnetic field around a current carrying wire?
A
The right hand grip rule
24
Q
What is represented in the right-hand grip rule? (2)
A
- thumb shows the direction of conventional current
- fingers show direction of the field
25
What is the **effect of length of conductor** on the **strength** of a **wire’s magnetic field**?
Greater length of wire = stronger magnetic field
26
What is the **effect of distance** on the **strength** of a **bar magnet’s magnetic field**?
Greater distance from magnet = weaker magnetic field
27
What is a **solenoid**?
What is it used for?
- A wire coiled into a spiral shape
- used to make electromagnets
28
**Describe** *how to represent* the **magnetic field** *around* a **solenoid**
(**2**)
- *Lines of flux* from **north seeking pole to south seeking pole**
- *lines of flux* from **south to north** ***through the solenoid***
29
What are the **properties** of the **magnetic field *inside* a solenoid**?
Strong and uniform
30
What is an **electromagnet** *made of*?
A **solenoid** with an **iron core**
31
What are the **advantages** of **electromagnets** *over* **permanent magnets**?
- they can be switched off
- their strength can be changed
32
What is a **magnetic material**?
Any material that is strongly attracted to a magnet
33
Give **2 examples** of **permanent magnets**
- bar magnet
- horseshoe magnet
34
**Where** is **magnetic force strongest** on a **magnet**?
Near the magnet’s poles
35
What are **attraction and repulsion** *examples of*?
Non-contact forces between magnetic poles
36
What does a **permanent magnet** *do*?
Produces its own magnetic field
37
What is an **induced magnet**?
A **material** that **becomes a magnet** *when* **placed in a magnetic field**
38
What does **induced magnetism** *always do*?
**Always** *causes* a **force of attraction**
39
**What happens** when an **induced magnet** is **removed** from a **magnetic field**?
It loses most or all of its magnetism quickly
40
*How* do you **test** for a **magnetic material**?
See if it is **attracted to both ends** of a **known magnet**
41
*How* do you **test** for a **magnet**?
See *if it can be* **repelled** by **one end of** a **known magnet**
42
Explain the **motor effect** (**2**)
- When a **conductor (wire) carrying current** is *placed in* a **magnetic field of a magnet**
- *causing* the **conductor and magnet** to **exert force on each other**
43
What is **Fleming’s Left Hand Rule** *used to find*?
How?
- The **direction of the force** *on a* **conductor** *in a* **magnetic field**
- *by holding your* **thumb, forefinger and second finger** *at **right angles*** to each other
44
Using the **Left hand rule**, *what* does your **index finger represent**?
The **direction of the magnetic field** from **north to south**
45
Using the **Left hand rule**, *what* does your **middle finger represent**?
The **direction of the current** from **positive to negative**
46
Using the **Left hand rule**, *what* does your **thumb represent**?
The **direction of the force** *on the conductor (wire) carrying the current*
(**up or down**)
47
What **factors** *affect the* **size of the force** *on a* **conductor** *in a* **magnetic field**?
(**3**)
- magnetic flux density
- current through the conductor (wire)
- length of the conductor (wire)
48
What is **Magnetic flux density**?
A measure of the strength of a magnetic field
49
State the **equation** *linking* **current, length, force and magnetic flux density**
State **units**
F=BIL
Force = (Mag Flux dens) x (current) x (length)
N = T x A x m
Newtons = Tesla x amperes x metres
50
**Why** does a **coil** *in a* **magnetic field** *rotate*? (**2**)
- **current** *in one side of the coil* **causes** a **downward force**
- **current** *in the **other side** of the coil* **causes** an **upward force**
51
What is an **electric motor**?
A device that **uses the motor effect** to **produce a turning effect** *in a* **coil of wire**
52
What is a **DC motor**?
An **electrical motor** that **uses direct current** *to produce* a **turning effect**
53
What is **direct current**?
An **electric current** that **flows in one direction** *only*
54
What is a **transformer**?
A device that **can increase or decrease** the **voltage** *of* **alternating current**
55
What does a **step-up transformer** do?
**Increases voltage** *by having* **more turns** on the **secondary coil**
56
What does a **step-down transformer** do?
**Decreases voltage** by having **fewer turns** *on the* **secondary coil**
57
Describe the **structure** of a **basic transformer**
A **primary coil** and a **secondary coil** *wound on* an **iron core**
58
**why** is **laminated soft iron used** for the **core of a transformer**? (**2**)
- Iron is *easily magnetized* and *demagnetized*
- therefore it *provides* a **path for magnetic flux**
59
**Why** are **transformers supplied** *with* **alternating current**?
- So that the **magnetic field** *produced in* the **primary coil** is **continually changing**
- so the **secondary coil cuts these field lines**, even when not moving
60
State the **equation** linking the **ratio of voltages** to the **ratio of the number of turns in each coil**
State what each symbol means
| (*Transformers*)
*V1* / *V2* = *N1* / *N2*
- *V1* = primary coil voltage,
- *V2* = secondary coil voltage,
- *N1* = no of coils on primary coil,
- *N2* = no of coils on secondary coil
61
What is the **other name** for **voltage**?
Potential difference
62
How does **induced potential difference** occur? (**3**)
- When a **conductor** *cuts* **magnetic field lines**
- or if there is **a change in a magnetic field around a conductor**,
- a **potential difference** is **induced across** the **ends of the conductor**
63
How does **induced current** occur? (**2**)
- *If* the **conductor** that *has had* a **potential difference induced across its ends** *in part of a complete circuit*,
- a **current is induced** *in the* **conductor**
64
Explain the **generator effect** (**3**)
- When the a **conductor** *cuts* **magnetic field lines**
- *causes* a **potential difference** *and* a **current** *(if part of a complete circuit)*
- to be **induced in a conductor**
65
What are the **properties** of a **magnetic field** *generated by* **induced current**? (**2**)
- it **opposes** the **original change**
- this can be **movement of the conductor** *or* the **change in magnetic field**
66
**How** do you **reverse** the **direction of induced potential difference** *or* **current**?
**Reverse** the **direction of movement** of the **magnet** *or* **coil**
67
What **factors** *affect the **size*** of **induced potential difference/current**? (**3**)
- **speed of movement** of the coil
- **magnetic field strength** being increased
- **number of turns** on the coil
68
What is a **generator**?
A **device** that **uses** the **generator effect** to **convert kinetic energy** *to* **electrical energy**
69
What is an **alternator**? (**2**)
- An **electrical generator** which **produces alternating current** (A.C. Generator)
- by **rotating a coil of wire** *between* the **poles of a magnet**
70
Under what **principal** does the **a simple generator work**?
How?
- The principal of conservation of energy
- the **force required** for the **bar magnet** to **overcome** the **like electromagnetic pole’s magnetic field** *converts* **kinetic energy** to **electrical energy**
71
*Describe* the **structure** of an **alternator**
A **coil of wire** *rotating* in a **magnetic field** and **two slip rings**
72
What **graph** does **voltage produced by an a.c. Generator** *follow*?
A sine curve
73
What is a **dynamo**? (**2**)
- An **electrical generator** which **produces direct current** *(a d.c. generator)*
- from the **kinetic energy** of a **moving magnet**
74
**Describe** the **structure** of a **dynamo**
A **coil of wire** *rotating* in a **magnetic field** and **a split ring commutator**
75
What is the name the **sensitive ammeter** in a **simple generator**?
What is it attracted to / where does it point?
What does it measure?
- a centre zero ammeter/galvanometer
- toward conventional current
- milliamps or micro-amps
76
Which rules do:
- a.c.
- d.c.
Follow?
- a.c. - right hand grip rule
- d.c. - left hand rule
77
What is **electromagnetic induction** (EMI)? (**3**)
- when the **magnetic field** *near a **conductor*** is changed
- causing the **conductor** to **cut** the **field lines** and **electrical voltage** to be **produced**
- this can be done **by changing the current** in a **neighboring coil** (*thus **changing** the **magnetic field***)
78
In the **motor effect**, how is **force increased**? (**3**)
- increased current
- stronger magnet used
- length of wire exposed to magnetic field is increased
79
In the **Left Hand Rule**, what happens when **current and magnetic field** are **parallel**?
No force is produced
80
What is the **frequency of the mains**?
What does this mean?
- 50 Hz
- it changes direction 100 times per second - 50 complete cycles a second (A.C.)
81
What is the **difference** between **a.c. and d.c.**? (In terms of direction)
a.c. Is bidirectional and d.c. is unidirectional
82
**Why** does **electromagnetic induction** *not work* when using **direct current**? (**2**)
- to **induce current** there must be a **changing magnetic field**
- this is *only produced* using **alternating current**, which produces a **changing magnetic field**
83
What is the **voltage** in UK **households**?
240V
84
What **type of current** is **supplied to/produced by** a *transformer*
A.c.
85
What **effect** do **transformers utilise**?
Electromagnetic Induction (EMI)
