5.1 Current Electricity

Question 1

What’s current and it’s units

Answer 1

Rate of flow of charge

Amperes (Amps)

Question 2

What’s potential difference

Answer 2

Work done moving a unit charge between 2 points in a circuit

Question 3

What’s resistance

Answer 3

How difficult it is for current to flow through a component

Component has resistance of 1 Ohm if 1A flows through it when a P.D of 1V is applied across it

Question 4

What’s ohmic conductor

Answer 4

Obeys Ohms Law

Current is directly proportional to potential difference as temperature is constant

Question 5

How do you measure the current

Answer 5

Ammeter in series to test component

Question 6

How do you measure potential difference

Answer 6

Voltmeter in parallel to test component

Question 7

What does the gradient of a I-V graph represent

Answer 7

Rate of change of current with correlation to voltage

Question 8

How does a Ohmic conductor graph look like

Answer 8

Line has a constant gradient and passes through origin, showing that voltage is directly proportional to current

Question 9

How does an I-V graph represent a component with a higher resistance

Answer 9

A steeper line = resistor has low resistance

A shallow line = resistor has high resistance

Question 10

What’s the name of the component that has a curve gradient passing through origin

Answer 10

Filament lamp

As current increases the resistance also increases

Greater increase in voltage produces only a low increase in current

Question 11

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

Answer 11

Electrons flowing in wire start to collide with metallic ions within metallic lattice structure more often

Since it affects flow of electrons as that attempt to flow through conductor and effective resistance will increase

Question 12

What’s a diode

Answer 12

Allows current to flow through one direction

Question 13

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

Answer 13

Assume infinite resistance

Current takes path of least resistance so voltmeter has infinite resistance when applied in parallel to component no current will flow through it and all current will flow through appliance

Question 14

Should you assume that an ammeter has zero resistance unless stated otherwise

Answer 14

Means that there would be 0 P.D across ammeter and no energy is lost across it; doesn’t affect circuit

Question 15

What’s LDR

Answer 15

Semiconductor that’s sensitive to light

As light intensity increases, resistance decreases

Question 16

How does thermistor work

Answer 16

As temperature increases, resistance decreases

Means thermistors have a negative temperature coefficient

Question 17

What’s resistivity

Answer 17

Resistance of a 1m cylinder with a cross sectional strap of 1m^2

Resistivity is an inherent property of a material

Question 18

Describe an experiment to determine resistivity of a metal

Answer 18

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

Question 19

What’s a superconductor

Answer 19

Material that has a resistivity of 0 at or below critical temperature

Critical temperature is an inherent property of material

Question 20

How do you find total resistance in a series circuit

Answer 20

Rtotal = R1 + R2

Add individual resistance of each component

Question 21

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

Answer 21

30V = 5V x 6

Question 22

How does current vary between each component of a series circuit

Answer 22

Current is same through all components so it does not vary

Question 23

Is current same in parallel components

Answer 23

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

Question 24

What is Kirchhoff’s first Law

Answer 24

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

Question 25

What’s Kirchhoff’s second Law

Answer 25

For any path (loop) of a circuit, sum of all of P.Ds must equal to total emf of circuit

Question 26

In series circuit, if 2 cells are connected negative to negative, would their emf add up or cancel out

Answer 26

Cancel out

Question 27

What’s power

Answer 27

Rate of energy power

Question 28

Give an equation for power in terms of current and voltage

Answer 28

Power = Current x Voltage

P = IV

Question 29

What’s purpose of a potential divider

Answer 29

• To provide variable P.D
• To provide constant specific potential difference

Question 30

Circuit is set up with cell (12V) to 2 resistors (6 and 7 Ohms) in series.

What’s voltage across 7 Ohms resistor

Answer 30

Total R = 6+7 = 13

(7/13) x 12= 6.5V

Question 31

What’s emf

Answer 31

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

Question 32

2 applications of superconductors

Answer 32

• power cables which reduce energy loss through heating to 0 during transmission
• strong magnetic fields, which wouldn’t require a constant power source

Question 33

Total resistance of parallel arrangement of 3 Ohms, 7 Ohms, and 9 Ohms resistors

Answer 33

1/Rt= 1/3+1/7+1/9

1/Rt= 37/63

Rt= 63/73 = 1.7 Ohms