Section 5 - Electricity: 5.1 - Current 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 a resistance of 1Ω 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 of change of current with respect to voltage.

This is not the same as 1/R

Question 8

Does this graph represent an ohmic conductor?
(Insert graph)

Answer 8

Yes, as the line has a
constant gradient and
passes through the origin.
This shows that voltage is
directly proportional to
current.

Question 9

Which graph represents an appliance with higher
resistance? (Insert graphs A and B)

Answer 9

B

A higher voltage is required in
B for the same change in
current in comparison to A.

Question 10

What common appliance could this curve represent?
(Insert graph)

Answer 10

A filament lamp. As the
current increases the the
resistance also increases.
A big increase in the
voltage produces only a
small increase in current

Question 11

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

Answer 11

As current flows 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 12

What is a diode? What does the graph look like?

Answer 12

A diode is a appliance that only allows current to
flow in one direction.
(Insert graph)

Question 13

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

Answer 13

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
through the appliance.

Question 14

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

Answer 14

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

Question 15

What is an Light Dependent Resistor (LDR)?

Answer 15

A semiconductor that is sensitive to light.

As the light intensity increases, its resistance
decreases.

Question 16

How does a thermistor work?

Answer 16

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

Question 17

What is resistivity?

Answer 17

The resistance of a 1m cylinder with a cross
sectional area of 1m2

. Resistivity is an inherent

property of a material.

ρ = RA/L

ρ = resistivity (Ωm) R = resistance (Ω)
A = cross-sectional area (m2

) L = length (m)

Question 18

Describe an experiment to determine the resistivity
of a metal. Draw diagram.

Answer 18

1. Measure the diameter of the wire with a micrometer.
   Then calculate the cross-sectional area = π(d/2)2
2. Set up the circuit as shown.
3. Vary the wire length and record the voltage and
   current for each length.
4. Use R=V/I to work out the resistance.
5. Plot a graph of resistance against wire length.
6. The gradient = resistivity ÷ cross-sectional area
7. So resistivity = gradient x cross-sectional area
   (Insert diagram)

Question 19

What is a superconductor?

Answer 19

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 20

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

Answer 20

RTotal= R1+ R2 + R3 + …
Add the individual resistances of each component

Question 21

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

Answer 21

5V.

Question 22

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

Answer 22

30V

(5V x 6)

Question 23

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

Answer 23

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

Question 24

Is the current in parallel components the same?

Answer 24

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

Question 25

What is Kirchhoff’s first law?

Answer 25

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

Question 26

What is Kirchhoff’s second law?

Answer 26

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

Question 27

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

Answer 27

They will cancel out. The total emf would be equal to:
ε total= ε1 - ε2.

Question 28

What is power?

Answer 28

The rate of energy transfer.

Question 29

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

Answer 29

Power = Current x Voltage
P = IV

Question 30

What is the purpose of a potential divider?

Answer 30

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

Question 31

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

Answer 31

Total resistance = 6 + 7 = 13Ω

(7/13) x 12 = 6.5V

Question 32

What is emf?

Answer 32

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

Question 33

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

Answer 33

Ɛ = IR + Ir
Ɛ = V + Ir

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

Question 34

What are two applications of superconductors?

Answer 34

● 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 35

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

Answer 35

1 / RT = 1⁄3 + 1/7 + 1/9
1 / RT = 37 / 63
RT = 63 / 37 = 1.7Ω