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

Question 20

___ ___ Counts Towards the Total Resistance of the Ciruict.

Answer 20

Internal Resistance of the Cell.

Question 21

The Total Resistance of a Circuit Includes the…

Answer 21

Internal Resistance of the Cell.

Question 22

Voltage Drop = …

Answer 22

EMF - Vterminal

Question 23

What is ‘Terminal Voltage’?

Answer 23

Terminal Voltage is the Voltage that is Actually Supplied to the Circuit by the Cell, after Considering the Internal Resistance of the Cell.