Electricity

Question 1

What is electric current? State its units.

Answer 1

The rate of flow of charge. It is measured in amperes (amps).

Question 2

What is potential difference?

Answer 2

The work done moving a unit charge between 2 points in a circuit.

V = W/Q

Question 3

What is resistance?

Answer 3

How difficult it is for current to flow through an appliance.

A component has resistance of 1ohm if 1A flows through it when a p.d. Of 1V is applied across it.

R = V / I

Question 4

What is meant by an ohmic conductor?

Answer 4

A conductor that obeys OHm’s law, meaning that current is directly proportional to potential difference providing physical conditions (such as temperature) remain constant.

Question 5

How can you measure the current in a circuit?

Answer 5

You can measure the current in a circuit with an ammeter connected in series with the component.

Question 6

How do you measure potential difference across a component?

Answer 6

Using a voltmeter, connected in parallel across the component being measured.

Question 7

What does the gradient of a current-potential difference graph represent?

Answer 7

Rate fo change of current with respect to voltage.

This is not the same as 1/R

Question 8

Why does the current increasing on a filament lamp cause an increase in the resistance?

Answer 8

As current flow through the lamp, electrical energy is converted to heat energy so the metal ions vibrate with increased amplitude. This impedes the movement of electrons through the lamp as they collide with the ions (resistance has increased)

Question 9

What is a diode?

Answer 9

A diode is an appliance that only allows current to flow in one direction.

Question 10

Unless stated in the question, should you assume that voltmeters to have zero resistance or infinite resistance?

Answer 10

You should assume they have infinite resistance. Current takes the path of least resistance so, if the voltmeter has infinite resistance, when applied in parallel to the appliance, no current will flow through it and all the current will flow though the appliance.

Question 11

Why should you assume that an ammeter has zero restante unless stated otherwise?

Answer 11

This assumption meant that there would be 0 potential difference across the ammeter and no energy lost across it; it does not affect the circuit.

Question 12

What is a light dependent resistor (LDR)?

Answer 12

A semiconductor that is sensitive to light.

As the light intensity increases, its resistance decreases.

Question 13

How does a thermistor work?

Answer 13

Similar to an LDR but, as the temperature increases, the resistance decreases (This means thermistors have a negative temperature coefficient).

Question 14

What is resistivity?

Answer 14

The resistance of a 1m cylinder with a cross sectional area of 1m^2. Resistivity is an inherent property of a material.

p = RA/L

p = resistivity (Ohm meters) R = resistance (ohms)
A = cross-sectional area (m^2) L = length (m)

Question 15

What is a superconductor?

Answer 15

A material that has a resistivity of zero at or below a critical temperature. The critical temperature is an inherent property of the material.

Question 16

How do you find the total resistance in a series circuit?

Answer 16

R(total) = R(1) + R(2) + R(3) + ………..

Add the individual resistances of each component

Question 17

If 6 cells, each of voltage 5V, are arranged in parallel what is the voltage in the circuit?

Answer 17

5V

Question 18

If 6 cells, each of voltage, are arranged in series what voltage is provided to the circuit?

Answer 18

30V

(5V x 6)

Question 19

How does the current vary between each component of a series circuit?

Answer 19

The current through all of the components in the same so the current does not vary.

Question 20

Is the current in parallel components the same?

Answer 20

No, each branch of a parallel circuit can have different currents through them according to Kirchoff’s first law.

Question 21

What is Kirchhoff’s first law?

Answer 21

All of the current going into a junction is equal to the current leaving the junction.

Question 22

What is Kirchhoff’s second law?

Answer 22

For any path (loop) of a circuit, the sum of all of the potential differences must equal to total emf off the circuit.

Question 23

In a series circuit, if two cells are connected negative to negative, would their emf add up of cancel out?

Answer 23

They will cancel out. The total emf would be equal to:

E total = E1 - E2

Question 24

What is power?

Answer 24

The rate of energy transfer.

Question 25

Give an equation for power in terms of current and voltage.

Answer 25

Power = current x voltage

P = IV

Question 26

What is the purpose of a potential divider?

Answer 26

• To provide variable potential difference, or
• To provide a constant specific potential difference

Question 27

A circuit is set up with a cell providing a voltage of 12V to 2 resistors of 6ohms and 7ohms respectively, in series. What is the voltage across the 7ohm resistor.

Answer 27

Total resistance = 6 + 7 = 13ohm

7/13 * 12 = 6.5V

Question 28

What is emf?

Answer 28

Electromotive force: the electrical energy transferred by a power supply per unit charge.

Question 29

Rearrange the equation E = I(R +r) into the form y = mx + c

Answer 29

E = IR + Ir
E = V + Ir

So for a graph of V against I
V = -rl + E
Gradient = -r and y-intercept = E

Question 30

What are two applications of superconductors?

Answer 30

• Power cables, which would reduce energy loss through heating to zero during transmission.
• Strong magnetic fields, which would not require a constant power source. These could be used in maglev trains, where there would be no friction between the train and rail, and in certain medical applications.

Question 31

What is the total resistance of a parallel arrangement of 3ohms, 7ohms and 9 ohm resistors?

Answer 31

1 / Rt = 1/3 + 1/7 + 1/9
1/Rt = 37/63
Rt = 63/37=1.7 Ohms