Topic 10 - Electricity & Circuits

Question 1

Describe the positions, masses and charges of protons, neutrons and electrons

Answer 1

Protons are in the nucleus, have a mass of 1 and a charge of +1
Neutrons are in the nucleus, have a mass of 1 and a charge of 0
Electrons are on the shells, have a mass of 1/2000 and a charge of -1

Question 2

On an electric circuit diagram, how are the positive and negative terminals represented

Answer 2

Positive - thinner vertical line of the cell/battery
Negative - thicker vertical line of the cell/battery

Question 3

On an electric circuit diagram, how is a cell represented

Answer 3

Two parallel lines with the right one thicker than the other
A horizontal line runs through them but is not visible bewteen them

Question 4

On an electric circuit diagram, how is a battery represented

Answer 4

Two cells back to back with the horizontal line between the first negative terminal and second positive terminal being dotted

Question 5

On an electric circuit diagram, how is a open/closed switch represented

Answer 5

Closed - continuous horizontal line between two points
Open - disrupted horizontal line between two points where the line points above the second point

Question 6

On an electric circuit diagram, how is an ammeter represented

Answer 6

A circle with an A in the middle

Question 7

On an electric circuit diagram, how is a voltmeter represented

Answer 7

A circle with a V in the middle

Question 8

On an electric circuit diagram, how is a motor represented

Answer 8

A circle with an M in the middle

Question 9

On an electric circuit diagram, how is a resistor represented

Answer 9

A blank rectangle

Question 10

On an electric circuit diagram, how is a thermistor represented

Answer 10

A rectangle with an arrow that begins parallel to and below the rectangle that begins at the bottom left corner and cuts to the top right diagonally

Question 11

On an electric circuit diagram, how is a fuse represented

Answer 11

A rectangle with the horizontal circuit line running through it

Question 12

On an electric circuit diagram, how is a diode represented

Answer 12

A circle with a triangle facing right and meeting a vertical line, with the horizontal circuit line running through it

Question 13

On an electric circuit diagram, how is a LED (light emitting diode) represented

Answer 13

A diode with two arrows pointing out from the top right of the circle

Question 14

On an electric circuit diagram, how is a LDR (light-dependent resistor) represented

Answer 14

A circle with a blank rectangle in the middle and two arrows pointing towards the top left of the circle

Question 15

On an electric circuit diagram, how is a filament lamp or bulb represented

Answer 15

A circle with a cross in it

Question 16

On an electric circuit diagram, how is a variable resistor represented

Answer 16

A rectangle with an arrow cutting through it diagonally from bottom left to top right

Question 17

On an electric circuit diagram, how is a d.c. supply represented

Answer 17

Two unconnected, small circles with a - and + above the left and right respectively

Question 18

On an electric circuit diagram, how is an a.c. supply represented

Answer 18

Two small circles with a wavy line between them

Question 19

Describe components in a series circuit

Answer 19

Components are connected in a line, end to end, between the positive and negative terminals of the power supply

Question 20

Describe current in series

Answer 20

Current is the same at each component

Question 21

Describe potential difference in series

Answer 21

Potential difference increases with the number of components and is shared to equal that of the sum of the cells (it’s conserved)

Question 22

Describe resistance in series

Answer 22

Net resistance is the sum of the resistances at each component. This is because by adding resistors in series, they have to share the total p.d. - reducing the p.d. across each resistor and thus the current through them. As a series circuit require the current to be the same everywhere so the total current reduces when a resistor is added and the total resisteance increase

Question 23

Describe components in a parallel circuit

Answer 23

Components are on branches connected to the positive and negative terminals of the power supply

Question 24

Describe current in parallel

Answer 24

The total current flowing around the circuit is equal to the total of all the currents through the seperate branches
It is conserved at a junction

Question 25

Describe potential difference in parallel

Answer 25

Potential difference is the same at every component

Question 26

Describe resistance in parallel

Answer 26

The total resistance is less than the resistance of the smallest resistor. This is because the resistors have the same p.d. across them so the pushing force making the current flow is the same as the p.d. for each resistor added. With more branches, the total current around the circuit increases - reducing the net resistance

Question 27

How does a voltmeter work

Answer 27

It is connected in parallel and measures potential difference across the component in volts (V)

Question 28

What is potential difference

Answer 28

It is the energy transferred (work done) per unit of charge passed. It is the amount of energy each coulomb of charge transfers as it passes between two points in a circuit

Question 29

What equation links energy, charge and potential difference

Answer 29

Energy transferred=charge moved x potential difference (E = Q x V)

Question 30

What are the units for charge, energy, p.d., current and resistance

Answer 30

Charge - Coulomb (C) Energy - Joule (J) P.d. - Volt (V) Current - Ampere/Amp (A) Resistance - Ohm (Ω) Time - Second (s)

Question 31

How does an ammeter work

Answer 31

It is connected in series with a component to measure its current

Question 32

What is current

Answer 32

It is the rate of flow of charge It is the flow of electrons in metals

Question 33

What equation links charge, current and time

Answer 33

Charge = Current x Time (Q = I x t)

Question 34

How does a variable resistor change current and potential difference

Answer 34

It increases resistance, which in turn reduces current and reduces p.d.

Question 35

What equation links p.d., current and resistance

Answer 35

Ohm's Law: Potential difference = current x reistence (V = I x R)

Question 36

What is resistance

Answer 36

Anything in the circuit which reduces the flow of current.

Question 37

How does current vary with potential difference at a filament lamp

Answer 37

When an electrical charge flows through a filament lamp, it transfers some energy to the thermal energy store of the filament, which heats up and glows. Resistance increases with temperature, so as more current flows through the lamp, it heats up more and the resistance increases. This means less current can flow per unit of potential difference, so the graph gets shallower - hence the curves at the end of the I-V graph's line

Question 38

How does current vary with potential difference at a diode

Answer 38

A diode only lets current pass through it in one direction, so the resistence depends on the direction of the current: current will be able to flow through one end, but resistance will be too high at the other. This results in the I-V graph's line being along the x-axis before rising diagonally

Question 39

How does current vary with potential difference at a fixed resistor

Answer 39

The current is directly proportional to the p.d. - resulting in a linear I-V graph

Question 40

How does the resistence of a LDR vary with light intensity

Answer 40

The resistence is directly proportional to the light intensity - producing a linear I-V graph with constant light intensity

Question 41

How does the resistence of a thermistor vary with temperature

Answer 41

As current flows through the thermistor, it heats up and causes the resitance to decrease - resulting in the I-V graph being steeper as the current increases, producing a wavy line

Question 42

What devices are used to measure the resistance of a component

Answer 42

An ammeter on the branch/series circuit and a voltmeter across the component

Question 43

How does current relate to temperature and how does this relate to resistance

Answer 43

When an electrical charge flows through a component, there will be a transfer of electrical energy. Some is transferred usefully, but work done against resistance causes energy to be transferred and dissipated to the thermal energy stores of the component and surroundings - causing the resistor to heat up. This is due to the free electrons colliding with the ions making up the lattice - transferring energy from the electrons to the ions, making the ions vibrate and heat up the resistor. The more the ions vibrate, the harder it is for the electrons to pass through the resistor.