# * is the opposition to charge moving through a source of electricity - PowerPoint PPT Presentation

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5.3 Emf and internal resistance. Internal resistance. * is the opposition to charge moving through a source of electricity * causes electrical energy to be dissipated inside the source as charge moves through it. Producing heat !. Electromotive force of a source.

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* is the opposition to charge moving through a source of electricity

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5.3 Emf and internal resistance

Internal resistance

* is the opposition to charge moving through a source of electricity

* causes electrical energy to be dissipated inside the source as charge

moves through it

Producing heat !

Electromotive force of a source

* is the potential (chemical) energy transferred to electrical energy when

one coulomb of charge passes through it

E = W

Q

5.3 Emf and internal resistance

E

E

V

r

r

A high resistance voltmeter measures terminal Pd.

Open circuit Pd of a cell equals the EMF in volts,

because the current through the cell is negligible.

Energy supplied

per coulomb

by cell

Energy changed

per coulomb

by external circuit

Energy wasted

per coulomb

by internal resistance

=

+

I

EMF

=

Pd across R

+

Pd across r

R

E

=

V

+

v

=

IR

+

Ir

E

=

I ( R + r)

E

5.3 Emf and internal resistance

E

E

V

r

r

Q. Find the internal resistance of a cell if its emf is 3V

and the pd across an external resistance is 2.5 volts

when 0.5 A flows.

EMF

=

Pd across R

+

Pd across r

E

=

V

+

v

“terminal pd”

“lost volts”

I

=

IR

+

Ir

E

=

+

0.5 r

2.5

3

R

=

0.5

0.5 r

r

=

1.0 ohm

5.3 Emf and internal resistance

Power

Power supplied by cell = I E

= I(IR +Ir)

= I2 R + I2r

Useful power delivered to R

Maximum power delivered to R

Power

delivered

to R

* occurs when the load resistance R

matches the internal resistance r

(R+r)I = E

I2 R = E2 R

(R+r)2

I = E

(R+r)

r

5.3 Emf and internal resistance

Measurement of emf and internal resistance r

* adjust the variable resistor for

different values of current and

record the terminal pd across the cell

r

R limits the

current to

safe levels !

Terminal

Pd /V

2

Emf = 1.5V

1

Circuit current / A

0.5

1.0

5.3 Emf and internal resistance

Measurement of emf and internal resistance r

EMF

=

Pd across R

+

Pd across r

E

=

V

+

v

=

IR

+

Ir

E

-

Ir

=

E

IR

Terminal

Pd /V

rI

-

=

E

V

Compare with straight line equation:

-

2

=

c

mX

Y

Emf = 1.5V

(0, 1.5)

m = Y

m = -0.5 = 0.42

1.2

(1.2 , 1)

1

Int resistance r =

Circuit current / A

0.5

1.0