Electricity: 1 - Monitoring and measuring AC 2 - Electrical sources and internal resistance

Question 1

What produces D.C. signals?

Answer 1

Batteries

Question 2

How does the current travel in D.C. signals?

Answer 2

In one direction only.

Question 3

What do D.C. signals look like on an oscilloscope?

Answer 3

A straight line.

Question 4

What produces A.C. signals?

Answer 4

Mains supply.

Question 5

What direction does an A.C. signal travel in?

Answer 5

A.C. signals are constantly changing direction.

Question 6

What is the size of an A.C. voltage like?

Answer 6

The size of an A.C. voltage supply is constantly changing.

Question 7

What is the maximum voltage in an A.C. voltage called?

Answer 7

The peak voltage.

Question 8

What is the definition of A.C.?

Answer 8

AC is a current which changes direction and instantaneous value with time.

Question 9

Since the size of an A.C. voltage supply is constantly changing what can we do when working out calculations and what is this called?

Answer 9

We can use an average when doing calculations which is called the “root mean square” or rms voltage.

Question 10

What is the equation for the rms voltage?

Answer 10

Vrms = Vpeak / √2

Question 11

What is the Vrms the equivalent of?

Answer 11

The D.C. voltage.

Question 12

What is the UK mains Vrms quoted value?

Answer 12

230v

Question 13

What is the UK mains frequency quoted value?

Answer 13

50Hz

Question 14

What is the UK mains quoted voltage peak?

Answer 14

325v

Question 15

What is the equation for the rms current?

Answer 15

Irms = Ipeak / √2

Question 16

What is the rms current the equivalent of?

Answer 16

The D.C. current.

Question 17

What equations can you get when the peak or rms values are applied to ohms law?

Answer 17

Vpeak = Ipeak x R and Vrms= Irms x R

Question 18

What is the electromotive force of a source?

Answer 18

The maximum amount of energy that can be transmitted to each coulomb of charge in a circuit by a battery or power supply. It is the ideal voltage of a supply meaning if a supply had a voltage of 1.5v it would supply each coulomb of charge with 1.5J of energy.

Question 19

What is emf measured in?

Answer 19

Volts.

Question 20

Where in a circuit is the emf measured?

Answer 20

Over the power supply.

Question 21

What are the losts volts in a circuit caused by?

Answer 21

The internal resistance.

Question 22

What is the purpose of the internal resistance?

Answer 22

To reduce the potential difference across the external circuit.

Question 23

What causes the volts lost in a power supply?

Answer 23

The internal resistance of the supply causes volts to be lost due to collisions between charged particles and other atoms within the power supply.

Question 24

What is the terminal potential difference?

Answer 24

The voltage output of a device and is measured across its terminals.

Question 25

What is the equation for the EMF?

Answer 25

EMF = TPD + ‘lost’ volts
E = V + Ir
E = IR + Ir
E = I(R+r)

Question 26

What happens to resistance if the battery becomes short circuited?

Answer 26

The load resistance is practically 0. Which is incredibly dangerous.

Question 27

In a short circuited circuit why is it dangerous?

Answer 27

As there is no load resistance emf = I x r which means the current will become incredibly high.

Question 28

Describe an experiment to investigate the emf and internal resistance?

Answer 28

Aim: To calculate the internal resistance of a cell and analyse the graphs of t.p.d. Versus current to obtain relevant info.

Apparatus 1.5v cell, variable resistor (1-100ohms), switch, digital voltmeter and ammeter.

Instruction: Complete a circuit with the variable resistor switch then ammeter in series. Then place the voltmeter in parallel across the variable resistor. Record the reading on the voltmeter which will show the t.p.d. then close the switch record the readings on the ammeter and voltmeter which will show the lost volts. Now change the resistance of the variable resistor and repeat. To calculate the internal resistance take away the lost volts from the t.p.d. and multiply it by the current. Now plot a graph of t.p.d. against current. When the current is 0 the t.p.d. is equal to the emf and when the V.t.p.d. is 0 the current will be equal to the short circuit current. Internal resistance is equal to the gradient m of the graph.