8/10/2019 Electricity Lecture 1 Annotated

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In response to a stimulus, an electrical pulse is

excited in a nerve cell, with 0.5 nC of charge inthe form of Na+ ions moving across the cellmembrane.How man Na+ ions does this corres ond to? 

eg: Nerve impulse

7         ...

Static Electricity 

[http://www.amasci.com/emotor/vdg.html]

Can create charged objects from neutral objects –

Use friction to cause charge separation

Rub two objects togethereg: fur on amber 

feet on carpet 

tyres on road 

8

Van de Graaff Generator 

.

        ...

93%

Two charges are observed to repel each other.They must be …

1. both positive

2. both negative

3. the same (both positive or both negative)

9

    b  o   t   h 

  p  o  s   i   t   i  v

  e

    b  o   t   h 

  n  e  g   a   t   i  v

  e

    t   h  e  s  a

  m  e   (    b  o

   t   h . . .

   o  n  e

   p  o  s   i   t   i

  v  e   a . . .

1%  6%

1%

4. one positive and one negative

        ... 10         ...

The electrical force between two point chargesvaries as ...

88%

1.inverse of distance between charges

2.inverse distance squared

11    i  n  v  e

  r  s  e  o  f

   d   i  s

...

    i  n  v  e

  r  s  e  d   i  s  t  a  n

...

    i  n  v  e

  r  s  e  d   i  s  t  a  n

...

7% 4%

3.inverse distance cubed

        ...

Forces on Charges

Charges exert electrical forces on each other.

•  Like charges (+ & + or - & -) repel.

•  Opposite charges (+ & -) attract.

The force between two point charges Q and q,

12

  , ,

Coulomb’s Law

where k = 9.0 x 109 N m2 /C2

  Direction: along the line between the charges.

Forces from multiple charges add as vectors.

2

QqF k 

r =

        ...

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50%

A 1.0 C charge is found to exert a force with amagnitude of 1.0 N on a 2.0 C charge.The magnitude of the force exerted by the 2.0 C

charge on the 1.0 C charge is:

1. 0.5N

2. 1.0 N

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   0.  5   N

   1.  0    N

   2.  0    N

    N  e  e

  d   t  o

    k  n  o

  w   t...

    N  e  e

  d   t  o

    k  n  o

  w   t...

13%  16%

1%

20%

. .

4. Need to know their masses

5. Need to know their separation

        ...

eg: Bee

 A honey bee in active flight can acquire anelectrostatic charge of up to 93 pC.

(a) What is the force between two charged beesseparated by 1.20 cm? 

14         ...

eg: Bee

(b) Compare this force to the gravitational force,assuming the mass of a bee is 0.140 g.

15         ...

In the diagram below, is there a position where acharge can be placed so that the nett force on itis zero?

+2 C+10 C

16    Y e  s,

   i  f   p o  s  i  t  i  v e

    Y e  s,

   i  f   n e g

 a  t  i  v

 e

    Y e  s

,   f o  r

   p o  s  i  t  i

..   N o

4%

11%

32%

53%

1. Yes, if positive

2. Yes, if negative3. Yes, for positive or negative

4. No

        ...

Electric Field

A test charge will experience a force that depends on

• its position, compared to other fixed charges,

• their charges &

• its own charge.Forces that act across space

(without physical contact)

exert their influence via fields.

17

 Electric field maps how a test charge of +1 C would

respond to the presence of other fixed charges.

• at a particular position, the force per unit charge on

test charge (q) placed at that position:F 

 E q

=  

        ...

Field lines indicate direction of field.

Electric field lines:

  start at +ve charge

• end at –ve charge.

Closeness of field lines indicate

18

+

+

+

+

+

-

-

-

-

-

eg: Different charge concentrations

inside & outside biological cell

magnitude (strength) of field.

cell

membrane         ...

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A charge at a particular location in an electric field

has a related potential energy, and hence potential

(potential energy per unit charge)

Electric potential energy increases as charge is

Electric Field, Energy & Potential Difference

19

moved opposite to electric force acting on it

⇔ must do work to achieve this.

 “Potential difference” or “voltage” , V 

between 2 points is the change in electric potential

between those points. Units: Volts (V)         ...

Summing Up

 After this lecture you should be able to:

• Understand charge as originating from subatomic particles

• Use inverse square law for force between two charges

• Understand the idea of a field

• Conceptualise the energy of a charge in a field in terms of the electric

20

potential

Supplementary questions:

• Biological Physics (2010) Problems

• 24.5 [F = 2.5×1015 N]

• 25.8 (a) [+1.12×10−20 J]

        ...