Electricity

Question 1

Define electric current. State its units.

Answer 1

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

Question 2

Equation for Current

Answer 2

Current = change in charge/ change in time

I = Q/T

Question 3

What is the unit for electric current (I) ?

Answer 3

Ampere (A)

Question 4

What is the unit for charge (Q) ?

Answer 4

Coulomb (C)

Question 5

What is the unit for time (t) ?

Answer 5

Seconds (s)

Question 6

Define Potential difference (V)

Answer 6

The energy transferred per unit charge between two points in a circuit

Question 7

Give the equation for potential difference (V)

Answer 7

V = W/Q

Potential difference = energy transferred/ Charge

Question 8

What is the unit for potential difference (V) ?

Answer 8

Volt (V)

Question 9

What is the unit for energy transferred?

Answer 9

Joules (J)

Question 10

Define resistance (R)

Answer 10

A measure of how difficult it is for charge carriers to pass through a component,

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

Question 11

Equation for resistance

Answer 11

R= V/I

Resistance = potential difference/ current

Question 12

Unit for resistance

Answer 12

Ohm

Question 13

what is meant by an ohmic conductor?

Answer 13

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 14

How can you measure the current in a circuit?

Answer 14

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

Question 15

How do you measure potential difference across a component?

Answer 15

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

Question 16

What does the gradient of a currentpotential difference graph represent?

Answer 16

Rate of change of current with respect to voltage.
This is not the same as 1/R

Question 17

what graph represents an ohmic conductor?

Answer 17

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

Question 18

On a graph, does a steeper line represent a higher or lower resistance?

(current=y axis voltage= x axis)

Answer 18

lower resistance

Question 19

What common appliance could a curve represent?

Answer 19

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 20

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

Answer 20

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 21

What is a diode?

Answer 21

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

Question 22

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

Answer 22

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 23

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

Answer 23

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 24

What is an Light Dependent Resistor (LDR) ?

Answer 24

A semiconductor that is sensitive to light.

As the light intensity increases, its resistance decreases.

Question 25

How does a thermistor work?

Answer 25

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

Question 26

What is resistivity?

Answer 26

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 27

Describe an experiment to determine the resistivity of a metal.

Answer 27

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

Question 28

What is a superconductor?

Answer 28

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 29

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

Answer 29

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

Question 30

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

Answer 30

5V

Question 31

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

Answer 31

30V

Question 32

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

Answer 32

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

Question 33

Is the current in parallel components the same?

Answer 33

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

Question 34

What is Kirchhoff’s first law?

Answer 34

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

Question 35

What is Kirchhoff's second law?

Answer 35

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

Question 36

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

Answer 36

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

Question 37

What is power?

Answer 37

The rate of energy transfer.

Question 38

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

Answer 38

Power = Current x Voltage P = IV

Question 39

What is the purpose of a potential divider?

Answer 39

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

Question 40

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 40

Total resistance = 6 + 7 = 13Ω (7/13) x 12 = 6.5V

Question 41

What is emf?

Answer 41

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

Question 42

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

Answer 42

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

Question 43

What are two applications of superconductors?

Answer 43

● 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 44

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

Answer 44

1 / RT = ⅓ + 1/7 + 1/9 1 / RT = 37 / 63 RT= 63 / 37 = 1.7Ω