e.m.f and internal resistance - Physics form 5

e.m.f. and internal resistance

Physics form 5

What to expect from this lesson?

• I can state what is electromotive force, Ɛ

• I can state what is internal resistance, r

• I can calculate questions using Ɛ = V + Ir

e.m.f.

• e.m.f., Ɛ is the energy transferred or work done by an electrical source to move one coulomb of charge in a complete circuit.

• For a dry cell with 1.5 V, the e.m.f. for the cell is 1.5 V or 1.5 J C^-1.

electromotive force vs potential difference

• Both e.m.f. and potential difference have the same S.I. unit which is V (volt).

• For a dry cell, the value of potential difference is slightly less than the value of e.m.f..

Internal resistance?

• Internal resistance, r is the resistance caused by electrolyte in the dry cell.

• The unit for internal resistance is Ω (ohm).

Ɛ = V + Ir

• Electromotive force = Potential difference produced by cell + Voltage drop

• 1.5 V = 1.48 V + 0.02 V

Quiz time!

There are 6 questions.

Multiple Choice

Electromotive force (e.m.f.) and potential difference have the same S.I. unit which is V.

True of false?

• True

• False

Multiple Choice

What is the correct symbol for internal resistance?

• R

• R

• r

• r

Multiple Choice

Which one is the formula for voltage drop?

• IR

• IR

• Ir

• Ir

Multiple Choice

A dry cell with 1.5 V is connected to a bulb. A voltmeter is connected in parallel to the dry cell show reading of 1.47 V when the circuit is closed.

What is the value of voltage drop?

• 0.01 V

• 0.02 V

• 0.03 V

• 0.04 V

Multiple Choice

If Ɛ = 1.5 V, R = 0.74 Ω and r = 0.01 Ω, find the current flow in the closed circuit?

• 1.0 A

• 2.0 A

• 3.0 A

• 4.0 A

Multiple Choice

If Ɛ = 1.5 V, I = 4.0 A and R = 0.37 Ω, find the internal resistance of the dry cell?

• 0.005 Ω

• 0.05 Ω

• 0.5 Ω

• 5.0 Ω

Graph of V against I

• Based on experiment 3.5 (text book page 118), the voltage drop increases as the current, I increases.

• It is impossible to get I = 0 A, but we can extrapolate the graph.

• The value of Ɛ is obtained from the y-intercept.

Graph of V against I

• Note that a straight line is produced in the graph of V against I.

• The gradient of the straight line is represent the value of internal resistance, r of the dry cell.

The equation

• From the graph, we can have the equation which is V = −r + Ɛ

• The equation is actually resemble y = mx + c

• The value of r is negative since the gradient of the straight line is negative.

Discussion time!

Dry cells in Series or Parallel arrangement? Which one have higher e.m.f.?

Multiple Choice

Which arrangement of dry cells produce more e.m.f.?

• Series

• Parallel

Multiple Choice

If you are supplied with two dry cells, what type of arrangement can reduce the effective internal resistance of the dry cells?

• Series

• Parallel

Congratulations!

You completed this lesson.