Physics 212 Section Group Force on a Point Charge Due to an Electric Field I Name 1 Signature Grade After this activity you should know: ο· know the basic relation for the force on a point charge due to a electric field and the direction and magnitude of the force. β know the difference between electric field and electric force. 1. The electric field at a point P is βπ¬. What is the electric force if a new charge q is placed at point P? Answer in β . This is the basic relation you will use in this activity. Be careful with notation – make sure terms of π and π¬ to put arrows on quantities that are vectors. β of 120,000 π/πΆ 2. A region has a uniform electric field π¬ π directed at an angle 120 ccw from +π₯. Here uniform means constant in space. The electric field is caused by charges not shown. a. In the left diagram, draw an arrow showing the direction of the electric force on a new charge of +4 ππΆ placed at the point marked x. Hint: what is the direction when you multiply a vector by a positive scalar? +y +x βπ¬ β × βπ¬ β × b. In the right diagram, draw an arrow showing the direction of the electric force on a new charge of −4 ππΆ placed at x. Hint: what is the direction when a vector is multiplied by a negative scalar? c. What is the magnitude of the electric force that the electric field exerts on the −4 ππΆ charge? d. What is the direction of the electric force this electric field exerts on the −4ππΆ charge? Give angle counterclockwise from +x between 0π and 360π . Physics 212 Force on a Point Charge Due to an Electric Field I 2 3. A pith ball (a small object that picks up charge easily) of mass 2.5 grams sits in an electric field of magnitude 300,000 N/C pointing straight downwards. The pith ball is negatively charged with a charge of −0.15 ππΆ (micro-coulomb). π e. Draw the free body diagram showing the force on the pith ball. Include gravity in your analysis. Label the electric force πΉπΈ (not πΈ) to avoid confusion between the electric force and electric field. f. πΈ Apply Newton’s 2nd law to determine the magnitude and direction (up/down) of the acceleration of the pith ball. 4. A pith ball (a small object that picks up charge easily) of mass π is tied to a string. The pith ball sits in a uniform electric field of magnitude πΈπ (created by other charges) directed as shown. The string is at an angle π when equilibrium is reached. Use the free body diagram method to determine the charge π of the pith ball. g. Draw a label free body diagram showing the forces acting on the pith ball. (Remember that the labels in the FBD are the magnitudes of the forces. Label the electric force as πΉπΈ – labeling the electric force as πΈ will confuse the force with the electric field.) π πΈπ π +π¦ +π₯ h. Use Newton’s 2nd law in the π₯ and π¦ directions to determine the charge π in terms of π, πΈπ , π, and/or π.