Chapter 4: Electric circuits

Question 1

Electrical charges and fields

Answer 1

• Some insulating materials become charged when rubbed together
• Electrons are transferred when objects become charged:

  o	Insulating materials that become positively charged lose electrons 
  
  o	Insulating materials that become negatively charged gain electrons

• Like charges repel. Unlike charges attract
• The force between two charged objects is a non-contact force

Question 2

Current and charge

Answer 2

• Every component has its own agreed symbol. A circuit diagram shows how components are connected together
• A battery consists of two or more cells connected together
• The size of an electric current is the rate of flow of charge
• The equation for the electric current of a circuit is: I = Q/t

• The above equation can be rearranged to find charge flow or time:
Charge flow, Q (coulombs, C) = current, I (amperes, A) x time taken, t (secs, s)

Question 3

Potential difference and resistance

Answer 3

• Potential difference, V is voltage
• Potential difference across a component is V = energy /charge
• Resistance, R = potential difference, V / current, I
• Ohms law says that the current through a resistor at constant temperature is directly proportional to potential difference across the resistor
• Reversing the potential difference across a resistor reverses the current though it

Question 4

Component characteristics

Answer 4

• A filament lamp’s resistance increases as its temperature increases
• Diode: forward resistance low; reverse resistance high. Resistance when current flowing the correct direction is low but when current flows other way it is high
• Thermistor’s resistance decreases if its temperature increases
• And LDRs resistance decreases as light intensity increases

Question 5

Series circuits

Answer 5

• For components in series:

  o	The current is the same in each component 

  o	The total potential difference is shared between the components

  o	Adding their resistances gives the total resistance 

• For cells in series, acting in the same direction, the total potential difference is the sum of their individual potential differences
• Total resistance Rtotal = R1 + R2
• Adding more resistors in series increases the total resistance because the current through the resistors is reduced and the total potential difference across the is unchanged

Question 6

Parallel circuits

Answer 6

• For components in parallel:

  o	The total current is the sum of all the currents through the separate components

  o	The potential difference across each component is the same 

• The bigger the resistance of a component, the smaller the current that will pass through that component
• The current through a resistor in a parallel circuit is I = V/R
• Adding more resistors in parallel decreases the total resistance because the total current through the resistors is increased and the total potential difference across them is unchanged