A-LEVEL Physics: 5.1.11, 5.1.12, 5.1.13: Emf & Internal Resistance (Seneca)

Question 1

What is ‘Electromotive Force’?

Answer 1

EMF is the Total Electrical Energy Produced by the Cell, Per Unit Charge.

(V=E/Q)
Not all of this is Seen by the Current in the Circuit.

Question 2

A Cell Produces a Set Amount of ___. This ___ is what Makes the ___ Flow Around the Circuit.

Answer 2

Voltage, Voltage, Current.

Question 3

A Cell Produces a Set Amount of Voltage. This Voltage is what Makes the Current Flow Around the Circuit. Some of this Voltage has Already been Used by the ___ ___ of the Cell Itself.

Answer 3

Internal Resistance.

Question 4

Some of the Voltage Produced by a Cell is Decreased due to its…

Answer 4

Internal Resistance.

Question 5

What is Emf Measured in?

Answer 5

Volts (V).

Question 6

EMF = …

Answer 6

(Internal Resistance of Cell + Circuit Resistance) * Current

EMF = (r + R)*I

(r + R = Rtotal)

Question 7

What does the Internal Resistance of a Cell Cause?

Answer 7

The Internal Resistance of a Cell Causes a Loss of Voltage Before the Circuit Begins.

Question 8

Most Cells have some ___ ___. What do they do?

Answer 8

Internal Resistance.
They Reduce the Voltage Output of the Cell Before it Reaches the Circuit.

Question 9

Why do Cells have Internal Resistance?

Answer 9

Because the Cell is Made up of Chemicals, which Will Resist the Flow of Charge.

Question 10

The Inside of a Cell is very complex, & the different ___ have different ___.

Answer 10

Chemicals, Resistances.

Question 11

A Cell Can be thought of as…

Answer 11

A Swirl of Chemicals which Generate Power, but also Provide Resistance.

Question 12

Explain the Experiment ‘Investigating Internal Resistance (of a Cell in a Circuit)’: (6)

Answer 12

1) Finding the EMF:
-Firstly, Connect the Cell to 2 Wires: 1 to the Positive Terminal, & 1 to the Negative Terminal.
-Use a Voltmeter to Measure the Voltage Between these 2 Wires. This is known as the Open-Circuit Voltage, & is the Same as the EMF (Total Output Voltage = Total Input Voltage).

2) Finding the Current & Voltage:
-Connect a Resistor of Known Resistance Between the 2 Wires.
-Use a Voltmeter to Measure the Voltage Across the Resistor.
-Use an Ammeter to Measure the Current Flowing in the Circuit.

3) Calculation:
- Voltage = emf - (Current * Internal Resistance)
V = emf -Ir

Question 14

What is the PD Across the Terminals of the Source? (eg Battery)

Answer 14

It is the Voltage Delivered by the Source when it is in a Circuit.

Question 15

What is ‘Emf (Electromotive Force)’?

Answer 15

Emf (Electromotive Force, ϵ) is the Total Electrical Energy Provided by the Source, Per Unit Charge.

(V=E/Q)

Question 16

What is the ‘Internal Resistance (r) of a Source’?

Answer 16

The Internal Resistance (r) of a Source is the Loss of PD Per Unit Current in the Source, when Current Passes Through the Source.

Question 17

What is the Role of a Source of Electricity (eg a Battery)?

Answer 17

The Role of a Source of Electricity is to Maintain a Certain PD Across its Terminals.

eg: The Role of a Battery is to Maintain a Certain PD Across its Terminals.

Question 18

ϵ = …
Show how.

Answer 18

I(R+r)

Because:

ϵ = V + Ir
and we know that V=IR
∴ ϵ = IR + Ir

Hence,

ϵ = I(R+r)

Question 19

V = ϵ - Ir

How Can we Use this to Calculate Current?

Answer 19

V = ϵ -Ir
and we know that V=IR
∴ IR = ϵ - Ir
IR + Ir = ϵ
I(R+r) = ϵ

I = ϵ/(R+r)

Hence,

V=IR
= Rϵ / R+r