EMF & Internal Resistance

Question 1

What is the internal resistance caused by?

Answer 1

• Internal resistance of a source of electricity is due to opposition to the flow of charge through the source.
• This causes electrical energy produced by the source to be dissipated inside the source when charge flows through it.

Question 2

What is the emf?

Answer 2

Electrical energy per unit charge produced by the source, measured in volts.

Question 3

What is the formula for emf?

Answer 3

• e = E ( energy ) / Q ( charge )
• e = I ( R + r )
• e = V + v

Question 4

What is the pd across the terminals of the source?

Answer 4

• Electrical energy per unit charge delivered the source when it is in a circuit.
• Always less than the emf whenever current passes through the source, due to internal resistance.

Question 5

What is the internal resistance defined as?

Answer 5

Loss of potential difference per unit current in the source when current passes through the source.

Question 6

What is the formula for current passing through a cell?

Answer 6

I = e / ( r + R )
where r = internal resistance
R = resistance
e = emf

Question 7

What is the formula for the total resistance of a circuit?

Answer 7

r + R

Question 8

What is the formula for the cell emf?

Answer 8

• e = I ( R + r )

* e = IR + Ir

Question 9

What is the formula for lost potential difference?

Answer 9

Potential difference lost inside the cell = cell emf - potential difference across components

Question 10

What is the formula for power supplied by the cell?

Answer 10

• Power, I x e = ( I^2 x R ) + ( I^2 x r )

* Power supplied by cell = power delivered to R + power wasted in cell due to internal resistance

Question 11

What is the formula for power to R?

Answer 11

• I^R = ( e^2 / ( R + r )^2 ) x R

* Since I = e / R + r

Question 12

When is maximum power delivered to the load?

Answer 12

• Maximum power is delivered to the load when the load resistance is equal to the internal resistance of the source.
• Load is then said to be matched to the source.

Question 13

How can the pd across the terminals of a cell be measured?

Answer 13

The pd across the terminals of a cell when the cell in is in a circuit can be measured by:
• Connecting a high resistance voltmeter directly across the terminals of the cell.
• Current altered via a variable resistor.
• Bulb limits the maximum current that pass through the cell
• Ammeter measures cell current.

Question 14

How does terminal pd vary with current?

Answer 14

Terminal pd decreases as the current increases.
Why:
• Terminal pd is equal to the cell emf at zero current, this is because lost pd is zero at zero current.
• Graph, straight line with negative gradient, Voltage = e - I x r

Question 15

What is the formula for voltage?

Answer 15

• V = e - I x r
• V = e - v
v = lost volts
e = emf

Question 16

What is the e.m.f. of multiple cells in series?

Answer 16

e, total = e1 + e2 +e3 + …

Question 17

What is the e.m.f. of multiple cells in parallel?

Answer 17

• e, total = e1 = e2 = e3
• Current will be split equally between identical cells, charge only gains e.m.f. from the cells it travels through, so overall e.m.f. doesn’t increase.

Question 18

What is the formula for internal resistance of two cells?

Answer 18

r = V1 - V2 / I2 - I1